Magnetars are amongst the most extreme astrophysical sources in the universe. With their high densities and magnetic field strengths, they are an ideal test case for fundamental physical processes. However, with only 29 known magnetars, the mechanisms for their gamma-ray emission is not well understood. The Fermi Gamma-ray Burst Monitor (GBM) is an ideal instrument in detecting outbursts of...
Dwarf novae are close binary systems where one of the components is a white dwarf that appears to exhibit abrupt and recurrent changes in viscosity in its accretion disk, causing material to fall towards the surface of the star with the consequent release of energy. As a result, these cataclysmic systems intermittently change brightness at different wavelengths. During outbursts, the increased...
The observation and modelling of the gamma-ray emission from molecular clouds (MCs) is currently the best tool to study Galactic cosmic rays. The highest energy that Galactic cosmic rays achieve is believed to be ∼ 10^15 eV, this is 1 PeV, that way a PeVatron is a source accelerating protons up to this energy. The quest for PeVatron sources aims to identify the astrophysical sources of...
A significant fraction of currently known gamma-ray binaries have bright optical counterparts that were classified as luminous early-type stars in objective prism catalogues compiled more than half a century ago. Representative examples of this statement include, in particular, LS I 61303, LS 5039 and also MWC 148. In previous years, we explored several of these historical catalogues by...
Recently, massive stars have been suggested as important sources of Galactic cosmic rays. These stars have intense stellar winds that can accelerate particles up to relativistic energies. It is expected that these stellar winds make a substantial contribution to the production of galactic cosmic rays, albeit less than that from supernova remnants. In 2019, the first detection of non-thermal...
Electrospheres are the environments of magnetized and rapidly rotating neutron stars acting like particle accelerators. With their central star slightly less energetic than the one of a pulsar, they do not host electron-positron pair creation processes. They consist of a low density plasma made of primary high energy particles. Even if we do not know their number, they are supposed to be...
Magnetar flares are one of the possible explanations for Fast Radio Bursts (FRBs). The first evidence for the FRB-magnetar connection was provided in April 2020, when the hard X-ray and soft gamma-ray bursts emitted by SGR 1935+2154 (detected by INTEGRAL, AGILE, Insight-HXMT, Konus-Wind) were observed to be associated with FRB emission. SGR 1935+2154 is a Soft Gamma Repeater, i.e. a source of...
This work investigates Cherenkov radiation originating from the quantum vacuum in pulsars. These compact astronomical objects, characterized by extreme magnetic fields, emit radiation as they rotate. By analyzing the vacuum polarization near the pulsar’s surface, we explore the contribution of Cherenkov radiation to the pulsar’s electromagnetic spectrum. Preliminary results, based on the...
The search of gamma-ray emission from accreting pulsars in X-ray binaries (XRBs) has been ongoing for some time. Recent marginal detections in high-mass X-ray binaries (HMXBs) have sparked renewed interest in this area. Anticipating future advances in gamma-ray telescopes like the Cherenkov Telescope Array (CTA), we investigate the expected emission above 10 GeV from XRBs using an enhanced...
Galactic cosmic rays may be accelerated up to PeV energies due to collective stellar winds surrounding stellar clusters. Further particle acceleration may occur due to supernova remnants within the wind-blown bubble. We apply a model of particle acceleration accounting for the stellar cluster wind termination shock and supernova remnant shocks to young and massive stellar clusters catalogued...
A significant fraction of the highest energy gamma-ray astrophysical sources observed are associated with Pulsar Wind Nebulae (PWNe). Given recent observations, the postulated, but unverified, hadronic component from PWNe requires renewed attention. We estimate possible ranges for the average pulsar pair production multiplicity on 29 sources in the Australia Telescope National Facility (ATNF)...
Cosmic rays (CRs) are high-energy charged particles ($10^{9} \sim 10^{20}$ eV) originating from the universe, yet their exact sources remain unknown. CRs are categorized into Galactic CRs (lower energy, $ \lesssim 10^{15}$ eV) and extragalactic CRs (highest energy, $\sim 10^{20}$ eV). As their names imply, Galactic CRs are accelerated within our Galaxy, while extragalactic CRs originate from...
Measuring pulsar spectra in the tens of GeV range is essential for constraining their high-energy emission models. However, this task is complicated by the fact that, in this energy domain, Fermi-LAT suffers from limited statistics, and the sensitivity of ground-based telescopes is restricted.
In this study, we combine data obtained with the largest H.E.S.S. telescope (CT-5) and...
Magnetic reconnection is ubiquitous in Astrophysics, from the Earth's magnetotail to the solar and black hole coronae, and becomes an effective mechanism for converting magnetic to kinetic and thermal energy in turbulent environments. We study turbulence's effect on magnetic reconnection rate via high-resolution 3D MHD simulations across an extensive parametric space. With an initial...
1ES2344+514 is a nearby HBL (Extreme high-frequency peaked) BL Lac object, located at a redshift of z=0.044. This source was discovered in VHE by the Whipple 10m telescope during a bright flare in December 1995. Following the event, few multi wavelength (MWL) campaigns have been organised to obtain a better understanding of the source. The extreme nature of the 1ES2344+514 in the X-ray band...
High-redshift ($z > 2$) blazars have spectral energy distributions whose inverse Compton peak usually lies in the MeV-GeV energy range. In particular, the AGILE satellite investigated 4C $+$71.07 and PKS 1830$-$211 triggering multi-wavelengths observations from the radio to the gamma-ray energy bands, in response to gamma-ray flares. We report on the multi-wavelength observations, discussing...
Lorentz invariance violation (LIV) in gamma rays can have multiple consequences, such as energy-dependent photon group velocity, photon instability, vacuum birefringence, and modified electromagnetic interaction. Depending on how LIV is introduced, several of these effects can occur simultaneously. Nevertheless, in experimental tests of LIV, each effect is tested separately and independently....
3C 216 is an extra-galactic radio source classified as a Compact-Steep Spectrum. The source is known to have extended radio structure on kpc scale and a blazar core on pc scale. In general high energy emission is more easily observed in blazars, whose jets are closely aligned with the line of sight. Starting from November 2022 Fermi-LAT observed an enhancement in the gamma-ray activity of 3C...
The interaction of cosmic rays with the cosmic microwave background (CMB) has been the subject of extensive research in the past 50 years or so. These studies have concentrated on the impact of such interactions on cosmic ray physics while neglecting the potential influence on the CMB itself due to its presumed minimal amplitude. However, the prospects of ultra-high-precision measurements of...
Star-forming galaxies, although low in gamma-ray luminosity, offer insights into cosmic ray processes. While gamma-ray emission in the GeV range is closely linked to their star formation rate, the origins of their higher-energy emissions remain unclear due to limited observations.
We gathered a comprehensive sample of galaxies, including those observed by Fermi-LAT in the GeV range and...
The origin of high-energy emission in blazars, a subclass of active galactic nuclei known for their variable, non-thermal emission across the electromagnetic spectrum, remains highly debated. Traditional one-zone models have struggled to explain the dynamic nature of these emissions, prompting more sophisticated approaches. The recent availability of X-ray polarization observations offers a...
Rapid flux variability over a large range of wavelengths is a well-known signature of blazar emission, with variability time scales of the order of a few days and below frequently observed at high energies.
Radiative models with varying degrees of complexity are generally successful in reproducing individual flare events or overall statistical behaviour, but the physical origin of blazar...
Two stages of non-thermal emission from relativistic jets in active galaxies can be distinguished: a low level persistent emission and a short period flaring emission. It has been recently proposed that both stages are produced in the inner (parsec scale) jet region when electrons are expected to be accelerated to TeV energies. The low level persistent emission originate in the extended,...
The reclassification of approximately 3000 gamma-ray extragalactic sources from the Fourth Fermi LAT Catalog resulted in an increased number of gamma-ray emitting Seyfert galaxies, including Narrow-Line Seyfert 1, misaligned active galactic nuclei (AGN), changing-look AGN, and several ambiguous objects (Foschini et al. 2022). To confirm or reject the new classifications and resolve the...
Long-term periodicity in Blazar light curves could be linked to the innermost zone of the complex structure of AGN, such as possible presence of binary system of supermassive black holes.
We analysed 16 years light curves of 1525 gamma-ray sources coming from the Fermi-LAT Light Curve Repository (LCR). First, we set up a periodicity analysis pipeline using Lomb-Scargle Periodogram (LSP) and...
PG 1553+113 is a high-frequency peaked BL Lac object (HBL), with redshift 0.433, detected with the current generation of IACTs (Imaging Atmospheric Cherenkov Telescopes) up to ~1 TeV. Interestingly, the continuous gamma-ray lightcurve collected by Fermi-LAT since 2008 showed a signature of a periodic modulation of 2.18 ± 0.08 years at energies above 100 MeV and 1 GeV. In addition, the source...
We present the results of a comparison between different methods to estimate the power of relativistic jets from active galactic nuclei (AGN). We selected a sample of 32 objects (21 flat-spectrum radio quasars, 7 BL Lacertae objects, 2 misaligned AGN, and 2 changing-look AGN) from the very large baseline array (VLBA) observations at 43~GHz of the Boston University blazar program. We then...
Narrow-line Seyfert 1 (NLS1) are a subset of AGN that accrete matter near or above the Eddington limit. Some of these galaxies can generate relativistic jets and emit gamma rays, as demonstrated by PMN J0948+0022, the first identified gamma-ray NLS1. In the early 2000s, its optical spectrum showed weak forbidden lines and a narrow H$\beta$ line with a Lorentzian profile, indicating turbulent...
Understanding the structure of active galactic nucleus (AGN) jets is still an open question. Relativistic magnetohydrodynamical (RMHD) simulations help study these jets' dynamics and emission. Recent research focuses on instabilities downstream of recollimation shocks, using 3D simulations to show their complex dynamics and effects on jet structures. Turbulence in these regions can accelerate...
Axion-like particles (ALPs) are pseudo-Nambu-Goldstone bosons predicted as an extension of the Standard Model of elementary particles, also considered as viable candidates for Dark Matter. When propagating through astronomical environments embedded with magnetic fields, very-high-energy (VHE) gamma rays may undergo conversion into ALPs, thereby altering the spectral energy distribution (SED)...
The First G-APD Cherenkov Telescope (FACT) has been monitoring blazars at TeV energies since October 2011. Within a decade of operation, it collected more than 15000 hours of physics data.
Designed for remote and automatic operation and using semiconductor photosensors, the duty cycle of the instrument is maximized and the gaps in the light curves are minimized. Thanks to the unbiased...
The Fermi-LAT detected more than 7000 γ-ray sources in 14 years of operation. Many of these sources are still unassociated with counterparts in other wavelengths, others are associated to generic classes, but their classification is still unclear. I present a Machine Learning approach to the classification of Fermi-LAT Unidentified Sources and Blazars of Unknown Type using multiwavelength...
In the last 15 years narrow-line Seyfert 1 galaxies (NLS1) have been investigated mainly in the radio, optical, UV and X-ray energy bands. In 2008, the detection of PMN J0948+0022 by Fermi-LAT allowed us to extend their spectral energy distribution to the gamma-ray energy band, paving the way to include gamma-ray NLS1 galaxies into the class of extra-galactic jetted sources. Indeed, their...
1ES 0647+250 is a seemingly distant high-frequency-peaked BL Lac (HBL) object. Its redshift is uncertain but a recent 2023 estimate from the MAGIC Collaboration places 1ES 0647+250 at a redshift of 0.45 +/– 0.05, which is in agreement with most estimates and lower limits in the literature. A spectrum taken with the Keck Echellette Spectrograph and Imager on 2022/12/24 shows no spectral lines...
PG 1553+113 is a BL Lac object located at redshift z=0.433. It is one of the brightest and most observed extragalactic sources in the very-high-energy (VHE, E>100 GeV) gamma-ray band. One of its characteristics is the evidence of quasi-periodic modulation in high-energy (HE, >100 MeV) gamma-rays detected by Fermi-LAT, with a period of about 2.2 years.
In this contribution, we present the...
AGN jets are sprawling entities, with structures of differing size and magnetic field strength extending from very close to supermassive black holes up to over a hundred parsecs away. However, it has long been expedient to calculate their emission in one-zone models, which are effectively spheres. This is optimal for variability studies but also sidesteps the issue of irresolvable structure in...
In September 2017, the IceCube Neutrino Observatory detected a high-energy neutrino event, IceCube-170922A, associated with a gamma-ray flare from the blazar TXS 0506+056, with a probability of chance coincidence rejected at the 3σ level. This remains the most significant photon-neutrino correlation observed to date. Here, we present results from the long-term monitoring of TXS 0506+056...
X-ray observations by Chandra reveal the presence of bright spherical regions within the kilo parsec-scale jet of Centaurus A. While several models have attempted to explain such high-energy emissions at this distance, a promising scenario involves interactions between the jet and AGB stars.
I will present my recent work on jet-star interactions using 2D/3D RHD and RMHD simulations. Our...
We present a comprehensive investigation into the optical-to-X-rays properties of the gamma-ray emitting jetted Narrow-Line Seyfert 1 galaxy 1H 0323+342 ($z=0.063$). Our study spans across the years 2006 to 2023, incorporating a rich dataset with 170 $Swift$ observations in the optical, UV, and X-ray bands, integrated with $Fermi$ LAT observations from 2008 to 2023. Our key findings include...
The spectral energy distributions (SEDs) of the high energy neutrino emitted from the accretion flows are still highly uncertain, because the global structure of the accretion flow can affect the neutrino SEDs. We have calculated very high energy neutrino SEDs by using three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations data of a magnetized accretion flow around a...
The flat-spectrum radio quasar 3C 279 is well known for its prolific emission
of rapid flares. One particular event occurred on December 20th, 2013, exhibiting
a large flux increase with a doubling time scale of a few hours, a very hard γ-ray
spectrum, and a time asymmetry with a slow decay, while no significant variations
in the optical range were detected.
We propose a novel scenario...
Fermi-LAT has accumulated continuous, high signal-to-noise, flux monitoring of bright blazars for over a decade in the gamma-ray band, measuring the dynamics of the particle acceleration and radiation zone in the blazar jet. The statistical methods often used to characterize the measured time variability, such as techniques based on the Fourier transform, rely on the underlying assumption that...
A ubiquitous feature of accreting black hole systems is their hard X-ray emission which is thought to be produced through Comptonization of soft photons by electrons and positrons (pairs) in the vicinity of the black hole. The origin and composition of this hot plasma source, known as the corona, is a matter open for debate.
In this contribution we investigate the role of relativistic protons...
Featuring two peaks in the spectral energy distribution, blazars show a high variability in X-rays and very-high-energy gamma rays. A harder-when-brighter behaviour is found in many studies of the spectral index in correlation with the flux.
Within the FACT monitoring program, more than 3200 hours of physics data have been taken on the blazar Mrk 421 at TeV energies. Thanks to an unbiased...
The Spectral Energy Distribution (SED) of blazars consists of two components. The low-energy bump is interpreted as synchrotron radiation emitted by accelerated electrons while the high-energy one is produced via inverse Compton scattering of the electrons by low-energy photons. In the leptonic interpretation, the latter photon field can be provided either by the synchrotron radiation of the...
The flat spectrum radio quasar OP 313 showed extremely intense $\gamma$-ray activity from November 2023 to March 2024, as observed by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. This initiated a large number of follow-up campaigns at all wavelengths, resulting in a confirmation of the increase of the source activity from the radio to very high energy (VHE) bands....
The origin of fast flux variability in blazars is a long-standing problem, with many theoretical models proposed to explain it. In this study, we focus on BL Lacertae to model its spectral energy distribution (SED) and broadband light curves using a diffusive shock acceleration process involving multiple mildly relativistic shocks, coupled with a time-dependent radiation transfer code. BL...
Correlations between various multiwavelength bands are an intermittent feature in blazar light curves; that is, they are observed in some instances but not in others. In order to understand the cause of this intermittency, high-cadence observations are required in as many bands as possible. In turn, correlations have been studied predominantly during flaring states. However, with the CTAO we...
I will review our recent advances in the study of the origin and development of plasma instabilities, namely Kelvin-Helmholtz and Current-Driven instabilities, at the base of quasar jets such as 3C 279, 3C 273 or 3C 111, which are variable or continuous gamma-ray emitters. We study the evolution of perturbed jets via relativistic (ideal) magnetohydrodynamical numerical simulations. An...
Nowadays, we are witnessing spectacular discoveries of ultra-high energy (E > 100 TeV) gamma-ray sources in the Milky Way, revealing the sites of PeVatrons - Cosmic Ray Factories accelerating protons and electrons to energies of 1 PeV and higher. These perfectly designed by nature particle accelerators represent at least three galactic source populations - pulsars, microquasars, and stellar...
Observations at gamma-ray energies over the last ten to fifteen years have revolutionized the study of rotation-powered pulsars. The Fermi Gamma-Ray Space Telescope has now discovered over 300 gamma-ray pulsars at energies above 100 MeV, over half not previously known at other wavelengths. Pulsars were detected for the first time in very-high-energy gamma rays by MAGIC, VERITAS and H.E.S.S....
Blazars are among the most luminous objects in the γ-ray sky, but the mechanisms behind their emission are still far from understood. In 2022, IXPE reported the first detection of X-ray polarization of blazars, opening a new window for testing acceleration and radiation models.
In this contribution, we present the insights gained on the two archetypal TeV blazars Mrk 421 and Mrk 501 exploring...
Recent measurements revealed the presence of several features in the cosmic ray spectrum. In particular, the proton and helium spectra exhibit a spectral hardening at ~ 300 GV and a spectral steeping at ~ 15 TV, followed by the well known knee-like feature at ~ 3 PV. The spectra of heavier nuclei also harden at ~ 300 GV, while no claim can be currently done about the presence of the ~15 TV...
During April 2013, the archetypal TeV blazar Mrk421 underwent a very bright outburst. The flare was observed over nine consecutive days from radio to very-high-energy (VHE; E>100GeV). In particular, MAGIC, VERITAS and NuSTAR exposures provided the most extensive simultaneous X-ray/VHE coverage to date during a blazar flare. The flux reached 15 times that of the Crab Nebula at VHE, and the...
Blazar variability, specifically in the very-high-energy (VHE; E>100 GeV) regime, can powerfully probe the inner workings of jet dynamics that drive the emission we observe. In late Fall 2022, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) observed VHE flaring activity from BL Lacertae on a much longer timescale than ever seen before. On October 15, 2022, the Fermi-Large...
The Galactic center region is one of the richest in the Milky Way, hosting a large variety of sources such as supernova remnants, pulsar wind nebulae and stellar clusters. The supermassive black hole Sgr A* is surrounded by dense molecular cloud complexes that shape the so-called central molecular zone (CMZ). Very-high-energy gamma-ray emission has been reported towards this inner 200 pc...
Most known Galactic supernova remnants do not appear to contribute significantly to the cosmic-ray (CR) flux at PeV energies at their current evolutionary stage. However, supernovae (SNe) could still be major contributors to the “knee” of the CR spectrum if their shocks are efficient hadronic accelerators at earlier times – within days or weeks after the collapse of their progenitor's core....
Flat Spectrum Radio Quasars (FSRQs) are some of the most powerful and dynamic sources in the Universe, exhibiting emission across the entire electromagnetic spectrum, from radio to gamma rays. Despite their intense luminosity, detecting FSRQs at very-high-energy (VHE) gamma rays (E>100 GeV) remains a challenge, largely due to internal absorption of these photons within the source itself. To...
Cosmic rays (CRs) interact with turbulent magnetic fields in the interstellar medium (ISM), generating non-thermal emission. After many decades of studies, the theoretical understanding of their diffusion in the ISM continues to pose a challenge. This study numerically explores a recent prediction termed "mirror diffusion” and its synergy with traditional diffusion mechanism based on ...
We report the detection of a TeV blazar PKS 0346-27 at redshift 0.99 by the High Energy Stereoscopic System (H.E.S.S.) on 3rd November, 2021 with a significance above 5 σ. The spectral energy distribution (SED) consists of the simultaneous observations by Fermi-LAT, Swift XRT and UVOT during the H.E.S.S detection period. We show that a hadronic one-zone model (modified by strong EBL...
The blazar PKS 0903-57 underwent a huge gamma-ray outburst in early 2020, where the gamma-ray flux increased by more than an order of magnitude and the gamma-ray peaked energy increased by almost a factor 100. Follow-up observations with H.E.S.S. over 6 nights reveal a complex evolution of the gamma-ray component suggesting time- and energy-dependent acceleration and cooling processes. The...
Cosmic rays (CRs) in the GeV-PeV range are believed to originate mainly from sources in the Milky Way (MW) disk, propagate diffusively in the interstellar medium, and eventually escape into the circumgalactic medium (CGM) that permeates the MW halo. Circumgalactic CRs (CGCRs) may play important thermal and dynamical roles for the formation and evolution of the MW, but observational evidence...
High-redshift blazars ($z>3$) allow us to probe their jets at radio frequencies down to the central black hole due to reduced opacity in the rest frame of the source and to study the accretion processes and black hole growth in the early Universe. However, the detection of gamma-ray emission from these distant sources is difficult–only about a dozen have been detected by Fermi-LAT $> 100$ MeV....
Gamma-ray observations provide important information on cosmic-ray (CR) propagation in our Galaxy.
First, we discuss detected TeV halos. We show that current gamma-ray measurements place interesting constraints on the turbulent magnetic fields around these pulsars, and we examine the implications for CR transport. Also, we suggest that extended gamma-ray sources of a hadronic origin should...
The Milky Way, with its distinctive observational features, is a unique laboratory to constrain physical parameters and test various theories of galaxy evolution ranging from star formation, to the formation of gaseous structures and galactic outflows, such as the Fermi and eRosita bubbles. A particularly important ingredient in the interstellar medium are cosmic rays (CRs), which reveal their...
Blazars are a highly energetic subclass of jetted active galactic nuclei, which show a broad band spectral energy distribution (SED) composed of two bumps. They are interpreted as the result of non-thermal emission from the relativistic particles forming the jet.
In 1998, a phenomenological population study – which was later confirmed in 2017 – showed an anticorrelation between the SED...
Assuming galactic cosmic rays originate in supernovae and the winds of massive stars, starburst galaxies should produce VHE gamma-ray emission via the interaction of their copious quantities of cosmic rays with their large reservoirs of dense gas. Such VHE emission was detected by VERITAS from the starburst galaxy M82 in 2008-09. An extensive campaign followed these initial observations,...
The center of our Milky Way galaxy hosts a series of energetic outbursts, including the well-known Fermi and eROSITA bubbles, Galactic center chimneys, the inner 15-pc Sgr A lobes. Are they long-lasting or fast evolving explosive events? What causes these structures? Are they PeVatrons related to ultra high energy gamma ray emissions from the central molecular zone and the Galactic center? The...
Cosmic rays are a crucial component of the interstellar medium. Measurements of cosmic ray composition in our Galaxy has shown that they are primarily composed of relativistic protons, with only a subdominant contribution from leptons to the total cosmic ray energy budget. Although the precise origins of cosmic rays are still uncertain, it is widely believed that the interaction of massive...
The large-scale structures such as Fermi Bubbles and eROSITA Bubbles provide a unique opportunity to study our Milky Way. However, the nature and origin of these large structures are still under debate. In this talk, I will present the identification of several kpc-scale magnetised structures based on their polarized radio emission and their gamma-ray counterparts, which can be interpreted as...
Compact symmetric objects (CSOs) are sources with radio lobe emission on both sides of an active nucleus and an overall size of less than one kpc.
From the detection of 3 CSOs by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope, we know that the emission from these objects can extend into the GeV band. Surprisingly, the first LHAASO catalog reported a TeV source,...
In 2010, the Fermi Gamma-ray Space Telescope observed two gamma-ray emitting structures, the Fermi Bubbles (FBs), that extend up to 55° above and below the Galactic plane and that seem to emanate from the Galactic center region. Although the spectrum at latitudes |b| > 10° has a softening or a cutoff around 100 GeV, the one at the base of the FBs, |b| <10°, extends up to about 1 TeV without a...
The fast variability of sources such as pulsars, gamma-rays bursts (GRB) or flares of active galactic nuclei (AGN) can be used to detect or constrain Lorentz invariance violation (LIV) by measuring time lags in time of flight of high-energy photons. However, an important source of uncertainty arises from the intrinsic processes within the source. Combining observations of different sources...
Our Galaxy is known to be a strong source of gamma-ray emission. Besides the active objects liable for the persistent and variable high and very-high energy photons, the interplay between the interstellar matter and cosmic rays is of crucial relevance, as it assumes the role of a passive source of gamma rays. Indeed, the collisions of extremely energetic charged particles with the gaseous...
Axion-like particles (ALPs) are neutral, spin zero bosons primarily interacting with photons, are predicted by String Theory and are among the best candidates for dark matter. Due to their low coupling with photons, ALPs are difficult to detect in laboratory experiments, but they produce observable effects in the astrophysical environment, which represents the best opportunity to study ALP...
The Galactic Centre (GC) is a complex region with several sources emitting very-high and ultra-high energy photons. It is also a unique place for Dark Matter (DM) searches under the assumption of cuspi profile in the WIMP theory. The Southern Wide field-of-view Gamma-ray Observatory (SWGO) is currently under development aiming to observe the GC region among other interesting regions in the...
LHAASO keeps operating with high duty cycle and providing unique observations in gamma-ray sources and diffuse charged cosmic rays. Gamma-rays from the BOAT GRB, blazars, near-by AGNs and PeVatrons in our own galaxy are well detected. Physics associated with EBL, new physics searches, radiation mechanism of various sources are discussed. Particle acceleration in the galactic sources and...
The High-Altitude Water Cherenkov (HAWC) Observatory in Puebla, Mexico, is a wide field of view (FoV) gamma-ray survey instrument. It effectively covers nearly two-thirds of the entire sky, spanning declinations from $-31^{\circ}$ to $+69^{\circ}$. HAWC’s wide FoV and high-duty cycle offer advantages for the continuous observation of astrophysical objects such as like pulsar wind nebulae...
The array of four Large-Sized Telescopes is under construction at La Palma, Spain, under the framework of the Cherenkov Telescope Array Observatory. It will be completed by the end of 2025. The LST consists of a 23m diameter primary reflector with 198 high precision segmented mirrors, a high-resolution camera comprising 1855 channel high Q.E. PMTs and GHz sampling fast readout electronics, and...
The instrumentation for gamma-ray astronomy has advanced tremendously during the last two decades. The study of the most violent environments in the Universe has opened a new window to understand the frontier of physics, exploring processes that are beyond the capabilities of Earth-based laboratories to replicate. One of the instruments at the forefront of ground-based gamma-ray astronomy is...
VERITAS is one of the world’s most sensitive detectors of astrophysical very high energy (E> 100 GeV) gamma rays. The array is located in southern Arizona, USA and is made up of four 12-m imaging atmospheric Cherenkov telescopes (IACTs). With nearly 20 years of operation since the first telescope’s installation was complete, the instrument has been able to study Galactic sources such as...
Firmamento is a new-concept platform dedicated to multi-messenger astrophysics that combines imaging, spectral, and time-domain data with specific algorithms, machine learning tools, and AI. Firmamento can identify counterparts within the uncertainty regions of sources detected by instruments with non-negligible localization areas, including X-ray, gamma-ray, and astrophysical neutrinos....
Three pulsars, namely Crab, Vela and Geminga, have been detected in gamma rays above 20 GeV using Imaging Cherenkov Atmospheric Telescopes (IACT). Detecting new very-high-energy (VHE) pulsars is challenging due to the limited sensitivity of the current IACTs. The Cherenkov Telescope Array Observatory (CTAO) will be the next generation of Cherenkov telescopes, comprised of dozens of telescopes...
The Imaging X-ray Polarimetry Explorer (IXPE) measures fluxes, spectra, and polarization properties of blazars at X-ray energies (2-8 keV). In its first two years of operation, IXPE has detected X-ray polarization from seven TeV-emitting blazars, constraining the geometry of the magnetic field in the X-ray emitting region and refining models for particle acceleration within relativistic jets....
The Crab, Geminga and Vela pulsars are among the brightest gamma-ray pulsars detected so far. Fermi-LAT observations revealed that each of these three pulsars has its GeV pulse profile consisting of two peaks and a bridge between them. There are a number of candidate emission regions in a pulsar magnetosphere, and various leptonic mechanisms were proposed to explain pulsars’ gamma-ray...
Blazars are a special type of AGN, with jets that happen to point very close to the direction of Earth. The powerful gamma-ray beam from distant blazars represents a unique tool to explore the environment along its path and allows us to probe opacity both inside the source and in the intergalactic medium. Internally, gamma rays experience attenuation due to photon-photon absorption, a result...
Recent very-high energy (VHE) observational studies have indicated the presence of the so-called TeV halos around several nearby middle-aged pulsars. Follow-up theoretical studies point out the possible general existence of the TeV halos, although under debate. If they generally exist as suggested, they would contribute significantly to cosmic positrons/electrons in our Galaxy. The full...
Black hole-driven relativistic astrophysical jets, such as blazars and gamma-ray bursts (GRBs), are powerful sources of electromagnetic radiation. When these jets are directed toward observers, they intensify emitted radiation, condensing the entire output into a focused, point-like object. Blazars exhibit emissions across the electromagnetic spectrum, spanning from radio frequencies to very...
In this talk I will present the current status of our development of an effective radiative model versatile enough to be applied to hundreds of
pulsars. The model follows the dynamics of charged particles being accelerated in the magnetosphere of a pulsar and computes their emission via synchro-curvature radiation, with only three free effective parameters involved. The model has succeeded in...
VERITAS began full-scale operations in 2007 and it remains one of the world’s most sensitive very-high-energy (VHE; E > 100 GeV) gamma-ray observatories. More than 8,300 hours (~50%) of its good-weather data were targeted on active galactic nuclei (AGN). Many of these observations were taken as part of an ongoing, comprehensive program to discover new VHE AGN. Upon discovery, the VERITAS...
We report on the discovery of multi-TeV gamma-ray pulsationsfrom PSR J1509-5850 with the H.E.S.S. array of imaging atmospheric Cherenkov telescopes. The light curve above 500 GeV is similar to the one obtained in the multi-GeV range with Fermi-LAT with no significant evolution with increasing energy.
The pulsed spectrum, as measured in the 500 GeV-10 TeV range, displays a hard index and is...
Based on a 10 years sample of gamma-ray flares of FSRQs collected with Fermi and AGILE, I will report on a statistical study of variability for a sample of more than 300 FSRQs.
I will focus on waiting time between flares (defined as the time intervals between consecutive activity peaks; published paper: L. Pacciani, A&A, 2022, 658, 164). The investigation revealed that gamma-ray activity...
‘Spider’ binary systems – black widow and redback compact binaries differentiated by their companion’s mass and nature – are an important type of pulsar system exhibiting a rich empirical phenomenology, including radio eclipses, optical light curves from the heated companion, as well as non-thermal X-ray and GeV light curves and spectra. Multi-wavelength observations have now resulted in the...
Blazars, a type of active galactic nuclei (AGN) with relativistic jets aimed at the observer, display flux variability across the electromagnetic spectrum due to particle acceleration within their jets. The power spectral density (PSD) of blazars reveals breaks at specific frequencies, especially in X-rays, which correlate with the accretion regime and the mass of the central black hole....
Pulsars are the main population of gamma-ray emitters in the Galaxy. To date, the Fermi Large Area Telescope (LAT) has detected about 300 gamma-ray pulsars, some young and energetic and others belonging to the family of older and fast-rotating millisecond pulsars. Because of their much broader gamma-ray beams, a significant number of LAT pulsars discovered in gamma rays are undetectable in...
Star-forming galaxies (SFGs) have been considered to be important contributors to the extragalactic gamma-ray background. Their high energy emission is usually considered to be driven predominantly by hadronic processes, and is regulated by the properties of the underlying galaxy populations — in particular their star-formation rates. In these galaxies, cosmic ray protons are accelerated and...
Radio pulsars are quintessential high-energy astrophysics objects. They emit most of
their energy in gamma-rays, accelerate particles to very high energies and produce
dense relativistic plasma. Despite decades-long efforts, we still do not have a
consistent model of radio pulsars, though significant progress has been achieved in
the last two decades thanks to advances in numerical models...
We report the first observational evidence for magnetic field amplification in the north-east/south-west (NE/SW) shells of supernova remnant SN 1006. SN1006 is one of the most promising sites of the production of galactic cosmic ray (CRs) through diffusive shock acceleration (DSA), although the detailed process of DSA is not well understood. Particularly, the magnetic field strength and...
3D Magnetohydrodynamic (MHD) resistive simulations have highlighted the unequivocal significance of widespread turbulence to drive fast reconnection. Moreover, it has been demonstrated that particle acceleration via reconnection in 3D magnetized flows, where turbulence is embedded within large-scale magnetic fields such as in relativistic jets and accretion flows around compact sources, is...
G284.3$-$1.8, also known as MSH 10-53, is a supernova remnant (SNR) with a radio shell (e.g., Milne et al. 1989) and thermal X-ray emission (e.g., Williams et al. 2015). Near the center of the SNR is the gamma-ray binary 1FGL J1018.6$-$5856, which was discovered in high-energy gamma rays by the Fermi Large Area Telescope (Fermi LAT Collaboration 2012). Follow-up observations found X-ray...
The Fanaroff-Riley (FR) classification system provides a fundamental framework for understanding the morphological dichotomy observed in some radio galaxies. Jet properties, acceleration mechanisms, and environmental interactions of extragalactic jets are often discussed to understand FR I/FR II galaxies.
Recent numerical works show the role of mass loading from stellar winds in decelerating...
Wide field-of-view survey instruments with good sensitivity at the highest energies have recently revealed an unexpectedly large population of galactic gamma-ray sources reaching ultra-high-energies (>100 TeV). Several of these have no known counterpart accelerator. However, the highest energy cosmic rays accelerated by supernova remnants will escape from the shock region at early times in the...
Active Galactic Nuclei (AGN) are the most powerful persistent sources in the Universe. Blazars, AGN whose jet is pointed towards the Earth, present the most energetic emission. Lately a specific kind of blazar drew the attention of the gamma ray astronomy community: the Extreme TeV Bl Lacs. These sources exhibit a peak of radiation at TeV energies and a hard intrinsic spectrum at sub-TeV...
In recent years the number of known sources emitting in the TeV-PeV regime has increased significantly thanks to facilities like LHAASO and HAWC. These observations could change our understanding of high-energy processes in our Galaxy. However, many of the observed sources are still unidentified or poorly constrained due to the limited angular resolution of these instruments, and most of the...
Recent progress in numerical simulations of magnetically arrested accretion onto supermassive black holes has provided significant insights into the formation and dynamics of magnetospheric current sheets near the black hole horizon. Focusing on M87* and by treating the pair magnetization in the upstream region and the mass accretion rate as free parameters, we estimate the magnetic field...
The quest for PeVatrons, sources of galactic cosmic rays accelerated up to PeV energies, has lived exciting developments in the last years, thanks to the many gamma-ray sources detected by ground array experiments at energies above 100 TeV. Among these, the supernova remnant SNR G106.3+2.7 appears to be one of the most promising hadronic PeVatron candidates, for which the ultra-high energy...
Jets of active galactic nuclei (AGN) are observed from sub-parsec (pc) to megaparsec (Mpc) scales. They are powerful particle accelerators, producing emissions ranging from radio to gamma rays. In this talk, I will present our analytical and numerical work on particle acceleration in jets. Multi-wavelength (MWL) observations, such as those for Cen A and M87, suggest continuous particle...
Investigating the molecular clouds surrounding a supernova remnant (SNR) is essential in understanding the acceleration and diffusion processes of cosmic rays because the clouds act as targets for cosmic-ray protons to produce gamma-rays via the hadronic processes. We present new observations of TeV gamma-ray SNR W28 with the Atacama Large Millimeter/submillimeter Array (ALMA) in $^{12}$CO(J...
Blazars and radio galaxies are famously known to be variable sources across the entire electromagnetic spectrum due to rather close alignment of their jet with our line of sight and relativistic jet speeds. In the very-high-energy (VHE, E > 100 GeV) gamma rays, the fastest flares reach hour-to-minute timescales that cannot be explained by the typical shock acceleration scenario. Magnetic...
Although significantly fainter than the Galactic Magnetic Field (GMF), the Intergalactic Magnetic Field (IGMF) is believed to pervade the vast Cosmic voids. The IGMF was lately constrained by novel upper and lower experimental limits which motivated us to investigate the scenario in which Magnetic Monopoles (MMs) are accelerated in the IGMF and GMF. We found that IGMF acceleration demands an...
Some classes of transient sources (e.g. Novae or Kilonovae) can be at first approximation well modeled by quasi-spherical explosions in which central hot objects are surrounded by fast expanding shells. We assume that processes in the shells can turn to the acceleration of relativistic electrons which are able to comptonize soft radiation from the central object. We calculate the time...
The concomitant observation of the gravitational wave signal from a binary neutron star (BNS) merger and its electromagnetic counterparts in 2017 confirmed that these events can produce relativistic jets responsible for short Gamma-Ray Bursts (sGRBs). The complex interaction between the jet and the surrounding post-merger environment shape the angular structure of the outflow, that is then...
Fast blue optical transients (FBOTs) are extreme instances of interaction-powered supernovae which exhibit high bolometric luminosities ($\sim$ 10$^{43}$ erg s$^{-1}$) with exceptionally fast rise and decay timescales ($<$ days). Since the discovery of AT2018cow, the landmark FBOT, by the Zwicky Transient Factory (ZTF), these transients have gained increasing attention as potential particle...
Recent TeV detections of gamma-ray burst afterglows offer new insights into particle acceleration at relativistic shocks. Kinetic simulations have improved our understanding of shock microphysics, enhancing models of particle acceleration relevant to afterglows. We explore scenarios for determining the maximum achievable energy, comparing our findings with data from GRB 190829A. This...
Ultra-relativistic plasma outflows are intrinsically connected with gamma-ray bursts. Over the years, a large number of analytical and numerical works has been devoted to understanding the intricacies of their complex dynamics, with most of these past studies performed in the ideal MHD regime. We propose a self-similar formalism, based on the expansion of the equations of resistive...
The promise of multi-messenger astrophysics was clearly shown through coordinated observation campaigns of TXS 0506+056 and GW170817 in 2017. These led to the detection of a flaring gamma-ray blazar that was potentially associated with a high-energy neutrino event, and the first detection of gravitational waves from a neutron star merger by LIGO/Virgo. The multi-messenger group in VERITAS has...
Gamma-ray bursts (GRBs) exhibit a rich variety of X-ray lightcurve behaviours, including the presence or absence of plateau/shallow decay phases. There is not yet an agreed consensus about the interpretation of these findings. In this study, we analyse the properties of GRB afterglow X-ray lightcurves, focusing on two distinct populations based on their detection at high and very high energies...
The AGILE space mission, with its unique features (two coaligned imaging X-and gamma-ray detectors, a non-imaging calorimeter, and the observing capability to cover about 80 %if the sky in 7minutes) makes it very suitable in seatch for high-energy counterpar of transient of various nature. AGILE participated in all the recent campaign to search for electromagnetic (e.m.) counterparts to...
Indirect dark matter searches with gamma rays involve looking for spectral signatures that could be associated to either annihilation or decay of dark matter particles in space. In this work we present the results of a search for line-like and box-like features in the gamma-ray spectra in five sky regions centered on the Galactic Center, optimized for different DM density profiles and...
Since the detection of the binary neutron star merger (GW170817) in coincidence with the short gamma ray burst (GRB 170817A), the search for electromagnetic emission from similar events has been an imperative part of multi-messenger astronomy. The LIGO\Virgo\Kagra Collaboration (LVK) is currently in the middle of their fourth observing run (O4), which began in May 2023. Of is the most...
X-ray flares are characterized by a sudden rebrightening of X-ray radiation during the afterglow phase of gamma-ray bursts (GRBs). While the majority of GRBs exhibit a flux temporal decay consistent with the standard afterglow model, approximately one-third of GRBs observed by the Swift X-ray Telescope (XRT) display these flares. These flares, which occur primarily within a thousand seconds...
The gamma-ray burst GRB 221009A stands out as an exceptional event for its intensity, spectral evolution, and duration. We investigate the early afterglow emission of this burst, especially focusing on the unique set of simultaneous GeV-TeV spectral and intensity data obtained by AGILE and LHAASO.
We present the results of a relativistic fireball model with a set of physical parameters that...
The measurement of the flux of cosmic rays in the past could give some important information about the sources of cosmic rays, the evolution of the neighbourhood of the Solar System in the Galaxy, and the Galaxy itself. It could also inform our understanding of key events in the Earth’s history such as mass extinctions.
The paleo-detector technique consists of looking for damages inside of...
We study the supersymmetric Q balls which decay at present and find that they create a distinctive spectrum of gamma rays at around O(10) MeV. The charge of the Q ball is lepton numbers in order for the lifetime to be as long as the present age of the universe, and the main decay products are light leptons. However, as the charge of the Q ball decreases, the decay channel into pions becomes...
Gamma-ray counterparts to astrophysical neutrino sources is a topic of big interest, being the contemporary observation of both these messengers a smoking gun for cosmic-ray production.
The Cherenkov Telescope Array Observatory (CTAO) will be the next major observatory in the Very High Energy gamma-ray band. Based on the imaging atmospheric Cherenkov technique, it will reach unprecedented...
Prompt emission of GRB is believed to be produced from electrons accelerated up to non thermal energies in the internal shocks. This emission peaks in the keV-MeV energy band, but a high energy (HE; 0.1-100 GeV) component is theoretically expected. While photons in the very high energy (VHE; E>20GeV) domain have been detected by Imaging Atmospheric Cherenkov Telescopes in recent years,...
Instruments such as the ROTSE, TORTORA, Pi of the Sky, MASTER-net, and others have recorded single-band optical flux measurements of gamma-ray bursts starting as early as ∼ 10 seconds after gamma-ray trigger. The earliest measurements of optical spectral shape have been made only much later, typically on hour time scales, never starting less than a minute after trigger, until now. We designed...
The recent detections of Very High Energy (VHE) emission from GRB afterglows by the MAGIC and H.E.S.S. telescopes has opened new prospects for observing these energetic transients. Just before these detections, the seminal event GRB170817A, associated with the gravitational wave (GW) signal from a binary neutron star (BNS) merger, marked a new era in multi-messenger astronomy, providing...
The 9.7m aperture Schwarzschild-Couder Telescope (SCT) is being developed as an alternative advanced design of the medium-sized telescope for the Cherenkov Telescope Array Observatory (CTAO), which covers the CTAO’s core energy range, from about 150 GeV to 5 TeV. The novel aplanatic dual-mirror optics of the SCT makes it possible to simultaneously achieve a wide 8-degree field of view and...
In this work, it is demonstrated the potential of modern and future air shower arrays with water-Cherenkov stations for detecting upward-going neutrino events from tens of GeV to hundreds of TeV. The detection methodology employs a novel machine learning-based analysis of the signal time traces of individual stations with multiple photosensors. This enables the reconstruction of the neutrino's...
Gamma-ray astrophysics increasingly focuses on time-domain studies of variable sources like GRBs and AGNs. As the foundation for CTAO's science analysis tools, the open Python analysis library Gammapy must adapt to support these advancements. This contribution highlights the recent expansion of Gammapy's time-domain capabilities and outlines its near-future plans, particularly regarding power...
We present preliminary results on likely associations between the latest Fermi catalogue (4FGL DR3) and all INTEGRAL surveys.
The cross correlation analysis (up to a distance of 3 arcmin) finds ~90 associations of which 5+/-2 could be by chance and thus false.
Some interesting findings emerge from the analysis:
1) on the Galactic Plane we find ~20 associations where the emission is likely...
ALPACA (Andes Large-area PArticle detector for Cosmic-ray physics and Astronomy) is a new air shower array observatory to be constructed in Mt. Chacaltaya, Bolivia in 2025. The experiment will mainly be dedicated to the search for Galactic PeVatrons in the yet-unexplored southern sky through the observation of sub-PeV gamma rays. The observatory consists of two parts: a surface air shower...
Identifying extended degree-scale $\gamma$-ray structures is a challenging task for imaging atmosperic cherenkov telescopes (IACTs). This is primarily due to their comparatively small field-of-view (FOV) of around 3.5° - 5°, and a large background induced by cosmic-rays. In order to estimate this background, many approaches depend on the existence of a $\gamma$-ray free region in each...
We developed deep learning enhancements for the real-time analysis of Cherenkov telescopes data, applicable to the context of the Cherenkov Telescope Array Observatory (CTAO). The CTAO will have a Science Alert Generation (SAG) system tasked with real-time reconstruction and analysis of data, as part of the Array Control and Data Acquisition (ACADA) system. We developed two applications of...
In addition to capturing the Cherenkov signal triggered by extensive atmospheric showers, cameras from imaging atmospheric Cherenkov telescopes are also subject to signals from the night sky background and electronic noise. Image cleaning methods are employed to eliminate noise-contaminated pixels which do not have information regarding the shower. If not effectively removed, these noisy...
The ASTRI Mini-Array is an international project led by the Italian National Institute for Astrophysics (INAF) aimed at operating an array of nine small-sized (4-m diameter) Imaging Atmospheric Cherenkov Telescopes (IACTs). This array will conduct extensive galactic and extragalactic gamma-ray sky observations in the 1–200 TeV energy band, and it will be located at the Observatorio del Teide...
The array layout design of an ultra-high-energy gamma rays water Cherenkov detector represents a big challenge at the time to reach a sensitivity in the PeV energy scale. This is the current phase where the Southern Wide-field Gamma-ray Observatory (SWGO) collaboration is. In this work we address the array layout problem building a continuous model whose parameters are the primary particle...
The small-size single mirror telescopes, SST-1Ms, are two Cherenkov telescope prototypes developed by a consortium of Czech, Polish, and Swiss institutions. Featuring a 9.42 m² multi-segment mirror and a 5.6 m focal length, the SST-1Ms offer a broad 9-degree field of view and have proven capable of detecting gamma rays with energies starting from several hundred GeV. The innovative cameras...
At the Politecnico di Milano (campus Bovisa) we are installing a facility to test various possible sensors detecting the Cherenkov light emitted in water by the charged particles of the extensive air showers produced by ultra-high energy gamma rays (> 100 GeV), within the framework of the SWGO collaboration. We realized a metallic cylindrical tank (diameter 3.36 m, height 3.12 m) containing a...
High-energy neutrinos are generated in particularly energetic astrophysical environments. However, it is not easy to identify neutrino sources, given the low spatial resolution of the most sensitive current telescopes, such as IceCube or KM3NeT. The search for electromagnetic counterparts is therefore essential to identify the source and thus to study the physical conditions that lead to...
We describe the design and the expected performances of the X/Gamma-ray Imaging Spectrometer (XGIS), a GRBs and transients monitor being developed for the THESEUS mission now in Phase A as ESA M7 candidate. XGIS is capable of covering an unprecedented wide energy band (2 keV – 10 MeV), with imaging capabilities and location accuracy <15 arcmin up to 150 keV over a Field of View of 2 sr, a few...
For detailed studies of Pulsar Wind Nebulae (PWNe), objects that show photon emission across the entire electromagnetic spectrum, multiwavelength analyses are crucial. The comparison of especially X-ray and gamma-ray emission and their angular sizes can help us to constrain the properties of PWNe, such as their particle transport mechanism or their potential for the acceleration of hadronic...
GRBs are gigantic extragalactic explosions, known to release 10$^{51}$-10$^{54}$ ergs of isotropic energy, which outshine all radiation in the sky when they occur. GRBs are thought to arise in dissipation processes in which the energy of the relativistic jet is converted into non-thermal radiation. GRBs are unpredictable events both in time and place and one needs to search for them...
Authors: T. Mineo, D. Mollica, A.Compagnino, G.Leto, P.Bruno, G.Contino, S.Crestan, S.Iovenitti
The ASTRI Mini-Array is an INAF project devoted to study gamma-ray sources emitting at very high-energy in the TeV spectral band. It consists of an array of nine innovative Imaging Atmospheric Cherenkov Telescopes, that are an evolution of the double-mirror ASTRI-Horn telescope operating at the...
The Tibet AS𝛾 experiment, which observes gamma-ray/cosmic ray air showers above a few TeV, is located 4,300 m above sea level in Tibet, China. The experiment is composed of a surface air shower array (Tibet-III) and underground water Cherenkov muon detectors (MD).
The surface air shower array is used for reconstructing the primary particle energy and direction, while the underground muon...
This presentation introduces GammaBayes, https://github.com/lpin0002/GammaBayes, a Bayesian Python package designed for dark matter detection using the Cherenkov Telescope Array Observatory (CTAO). GammaBayes processes CTAO gamma-ray measurements alongside user-defined dark matter particle models, providing the posterior distribution for dark matter parameters such as the dark matter mass and...
The First G-APD Cherenkov Telescope (FACT) is observing gamma-ray sources at TeV energies. Thanks to its unbiased observation strategy, silicon-based photosensors and remote and automatic operation, it features ideal conditions both for monitoring and target-of-opportunity observations. This results in an unprecedented data sample of more than 15000 hours of physics data including monitoring...
Lorentz Invariance Violation (LIV) effects could be spotted by Imaging Atmospheric Cherenkov Telescopes (IACTs) by searching for energy-dependent time delays in the gamma-ray photons coming from distant and highly variable astrophysical sources. As part of its scientific program, The Cherenkov Telescope Array Observatory (CTAO) will explore problems in fundamental physics, including studying...
Diffuse gamma-ray emission is thought to be primarily produced by interactions between cosmic-ray protons and interstellar protons via hadronic processes. Therefore, it provides a valuable opportunity to obtain a comprehensive understanding of cosmic-ray distribution and the interstellar gaseous medium. We present the first analysis of the spatial distributions of GeV and TeV gamma-rays, hard...
Fukui et al. quantified the hadronic and leptonic gamma-rays in the young TeV gamma-ray shell-type supernova remnant (SNR) RX J1713.7$-$3946 (RX J1713), and demonstrated that gamma rays are a combination of hadronic and leptonic gamma-ray components with a ratio of $\sim$6:4 in gamma-ray counts $N$g. This discovery, which adopted a new methodology of multi-linear gamma-ray decomposition, was...
Below the geographic South Pole, the IceCube project has transformed one cubic kilometer of natural Antarctic ice into a neutrino detector. IceCube detects more than 100,000 neutrinos per year in the GeV to 10 PeV energy range. Among those, we have isolated a flux of high-energy neutrinos originating beyond our Galaxy, with an energy flux that is comparable to that of the extragalactic...
The study of GRBs at gamma-ray energies has been recently boosted by the detection of a handful of events at energies above 100 GeV. These observations are revealing the existence of a surprisingly bright TeV component present at least in some of the most energetics GRBs during their afterglow phase.
I this talk I will review the observations and the theoretical interpretations put forward,...
In this talk I will provide highlights on the current status of the field of gravitational waves and their connection with high-energy transient phenomena, through the lens of multi-messenger astrophysics.
After briefly reviewing the astrophysical and cosmological relevance of the diffuse extragalactic background light in the infrared and the impossibility to measure it with current instrumentation, I will discuss the possibility to constrain it effectively from measurements of the photon-photon opacity in the direction of local VHE gamma-ray sources.
An important aspect of this project is the...
At first glance, the question of what the universe is made of may seem unrelated to astroparticle physics. It is in fact optical and microwave observations that have revealed that the energy balance of the universe is dominated by the density of dark energy (70%) and dark matter (25%). The nature of these two entities is elusive to this day. The remaining 5% is sometimes erroneously excluded...
Space-based gamma-ray telescopes provide insight into energetic astrophysical processes from 100s of keV to 100s of GeV. A rich array of instruments in flight are gathering observations of outbursts from black holes, neutron stars, binary systems and other dynamic environments and mapping the traces of extreme activity in the past. I will talk about ways that space-based observations have...
The field of ground-based gamma-ray astronomy is flourishing, with the current generation of instruments providing a rich and complex picture of non-thermal astrophysics in the TeV-PeV domain. At the same time a new generation of instruments is planned or already in construction, promising a fundamental step forward in our understanding of many astrophysical systems as well as deep searches...
Observation with an arc-minute scale resolution of dense molecular clouds located at pulsar-wind nebulae (PWNe) is important to study the interaction between the interstellar gas and PWN and the evolution of the complex morphology of PWNe, as noted in some previous literature. Our study performs the first dedicated research of the molecular clouds located at Vela X, the large PWN around the...
Imaging Atmospheric Cherenkov Telescopes (IACTs), the most sensitive astronomical instruments in the VHE band, rely on the Earth’s atmosphere as part of the detector. Therefore the presence of clouds affects observations and introduces biases which need to be corrected. Typical correction methods require knowledge of the instantaneous atmospheric profile, that is usually measured with external...
Approximately 300 stable elements exist in nature. However, the origin of r-process elements such as Pt, Au, and other rare-earth elements is still under debate. They may have originated in an explosive event, such as binary neutron star mergers called “kilonova”, through a rapid neutron-capture process. In the case of a kilonova, 20–50% of the total radioactive energy can be released in the...
RS Ophiuchi is a recurrent nova which explodes on average every 15 years. These explosions result in nova shocks from which non-thermal particles and radiation are produced. In fact, the most recent outburst of RS Ophiuchi in 2021 has been observed by a few different gamma-ray instruments including FERMI-LAT, HESS and MAGIC. Interestingly, the highest TeV gamma rays are only detected about two...
The Single-Mirror Small Size Cherenkov Telescope (SST-1M) was developed by a consortium of institutes in Switzerland, Poland, and the Czech Republic. The SST-1M design is based on the Davies-Cotton concept, featuring a 4-meter mirror and an innovative SiPM-based camera. It is most sensitive to gamma rays in the TeV and multi-TeV energy bands. Since 2021, two SST-1M prototypes have been...
Two recent classical novae, V1723 Sco (2024) and V6598 Sgr (2023), were detected by the Fermi-LAT. V1723 Sco is one of the brightest novae observed to date by the LAT, providing a two-week window for detection. The extensive Fermi observations of V1723 Sco, complemented by a rich multi-wavelength dataset including NuSTAR and VLA, enable precise constraints on various parameters of the emission...
The number of young star clusters identified as sources of gamma-ray emission has been increasing in recent years, hinting at ongoing particle acceleration within these regions. The interaction between winds from massive stars can create collective shocks and lead to the formation of superbubbles (SBs), whose interior is filled with tenuous, hot, turbulent plasma. Characterising these...
The recent discovery of several ultra high-energy gamma-ray emitters in our Galaxy represents a significant advancement towards the characterization of its most powerful accelerators. Nonetheless, in order to unambiguously locate the regions where the highest energy particles are produced and understand the responsible physical mechanisms, detailed spectral and morphological studies are...
Young massive stellar clusters have recently brought attention as PeVatrons candidates, to explain the knee of the cosmic ray spectrum and how protons can be accelerated to such energy scale in galactic sources. Since direct verification is not possible because of the diffusion of cosmic rays in the interstellar medium, one can use photons that are produced when cosmic rays interact with...
The ASTRI-Horn telescope is a prototype of a compact aplanatic dual-mirror (4 m diameter) Imaging Atmospheric Cherenkov Telescope developed under the leadership of the Italian Istituto Nazionale di Astrofisica (INAF). It is the pathfinder of the small-sized telescopes adopted for both the ASTRI Mini-Array (Tenerife, Canary Islands) and the SST/CTA array (Paranal, Chile) for gamma-ray astronomy...
Dozens of gamma-ray sources are now observed to extend their emission up to Ultra-High Energies (UHE, E > 100 TeV). Most of these sources are located along the Galactic Plane and appear largely extended to ground detectors. Imaging Atmospheric Cherenkov Telescopes (IACTs) provide excellent angular resolution and a large effective area, but to build an array which is sensitive to UHE emission...
PeVatrons constitute a fascinating class of astrophysical objects capable of accelerating particles up to PeV energies (1 PeV = 1015 eV). However, their nature and their acceleration mechanisms are still uncertain. The accelerated particles interact with the surrounding interstellar medium and background radiation fields to produce secondary ultra-high-energy gamma rays (>100 TeV), which are...
Young massive stellar clusters (YMSCs) have emerged as a potential
gamma-ray sources, after the recent association of several YMSCs with extended gamma-ray emission. The large size of the detected halos, comparable to that of the wind-blown bubble expected around YMSCs, makes the detection of individual YMSCs rather challenging. As a result, the gamma-ray emission from most of the Galactic...
Atmospheric Cherenkov telescopes observe cosmic gamma rays with energies
upwards of twenty giga electronvolt in collecting areas which exceed the
collecting areas of satellites by orders of magnitude. However, as we further
push the concept of the Cherenkov telescope array, the intrinsic limitations of
imaging itself become more evident. Aberrations limit our field of view and the
angular...
The Transient High-Energy Sky and Early Universe Surveyor (THESEUS) is a mission concept aimed at fully exploiting Gamma-Ray Bursts (GRB) for
investigating the early Universe and as key phenomena for multi messenger astrophysics. Developed by a large European collaboration coordinated by INAF and under study by ESA since 2018, THESEUS is currently one of the three candidate M7 missions for a...
Massive Star Clusters (SCs) have been proposed as additional contributors to Galactic Cosmic rays (CRs), to overcome the limitations of supernova remnants (SNR) to reach the highest energy end of the CR spectrum. Thanks to fast mass losses due to the collective stellar winds, the environment around SCs is potentially suitable for particle acceleration up to PeV energies and the energetics is...
The Cygnus X region has become a source of great interest since its detection in gamma-rays by Fermi, HAWC and recently LHAASO, the latter having measured photon energies above 1 PeV. This likely indicates the presence of a hadronic source of PeV cosmic rays in the region, although the accelerator has not been yet identified. The emission is coincident with the stellar association Cygnus OB2,...
Recent breakthroughs in neutrino astronomy indicate that the majority of cosmic-ray accelerators responsible for hadronic emission are not associated with Fermi-LAT’s bright sources at GeV energies. Instead, they suggest that the solution to the century-old cosmic-ray mystery may lie at MeV energies. Additionally, precision anti-deuteron measurements provide a background-free indirect dark...
High-density and high-Z crystals are a key element of most of the $\gamma$-ray telescopes operating at the GeV energy scale (such as Fermi-LAT). The lattice structure of these materials is usually ignored for all practical purposes, such as instrument calibration or simulation. However, recent studies performed by the STORM-OREO collaboration have shown that this is a rough approximation,...
We present the observational capabilities of the ASTRI mini-array, an array of nine small-sized Cherenkov telescopes deployed at the Teide Observatory (Tenerife, Spain), by focusing on the state of the art of TeV emitting sources in the Cygnus region (along the 60-80 longitudinal range of the Galactic plane). This is currently the richest known region of Galactic sources emitting above 1 TeV,...
The Small-Sized Telescopes (SSTs) are the smallest of the three different sizes of Cherenkov telescopes that will be part of the Cherenkov Telescope Array Observatory (CTAO). Based on a Schwarzschild-Couder-like dual-mirror optical configuration, they have a primary mirror of ~4-m diameter and are equipped with a focal plane camera based on SiPM detectors covering a field of view of ~9°. They...
Recent gamma-ray observations detect photons up to energies of a few PeV. These highly energetic gamma rays are emitted by the most powerful sources in our galaxy. Propagating over astrophysical distances, gamma rays might interact with background photons producing electron-positron pairs, then deflected by astrophysical magnetic fields. In turn, these charged particles might scatter through...
In high energy Gamma-Ray Astronomy with shower arrays the most discriminating signature of the photon-induced showers against hadron-induced cosmic-ray ones is the content of muons in the observed events.
In the electromagnetic $\gamma$-showers the muon production is due to the dominant channels: photo-production of pions followed by the decay $\pi\to\mu\nu$, prompt leptonic decay of charmed...
ASTRI is an Italian project aimed at the study of the cosmic very high-energy gamma radiation. In the past decade, ASTRI has developed a new kind of Cherenkov telescope, based on a dual mirror Schwarzschild-Couder optical configuration and miniaturized silicon photomultiplier sensors. Nowadays, the realization of a nine-telescopes array of the ASTRI kind working in stereoscopic mode, the...
Detection of gamma rays and cosmic rays from the annihilation or decay of dark matter particles is a promising method for identifying dark matter, understanding its intrinsic properties, and mapping its distribution in the universe. I will review rthe current status and discuss the prospects for indirect searches to robustly identify or exclude a dark matter signal using upcoming experiments...
The Extragalactic Background Light (EBL) is the accumulated light emitted throughout the history of the universe, spanning the UV, optical, and IR spectral ranges.
Stars and dust in galaxies are expected to be the main source of the EBL. However, recent direct measurements performed beyond Pluto's orbit (less affected by foregrounds than those performed from the Earth) hint at an EBL level in...
The Extragalactic Background Light (EBL) is the aggregate of all photon emissions in the universe since the cosmic dark ages, dominated by the optical and infrared emissions from thermal processes. Using the EBL absorption imprint on the $\gamma$-ray spectra of extragalactic sources, we study the decade-old tension between EBL intensities inferred from galaxy counts (IGL) and from direct...
I review the arguments for interpretations of the Galactic Center gamma ray excess, the only significant detection to date of a possible indirect dark matter annihilation signal. I describe current search implications for complementary targets, most notably including nearby dwarf spheroidal galaxies, and the prospects for future improvements in gamma ray probes of DM.
The launch of the Compton Gamma-Ray Observatory (CGRO) ushered in a new era in our understanding of the phenomenon of relativistic jets produced by compact objects. AGN with radio jets pointed towards us (blazars) turned out to be unexpectedly powerful and rapidly variable gamma-ray sources, while gamma-ray bursts were shown to be distributed isotropically on the sky, significantly increasing...
A central team in Europe for the development of high-energy astronomy was the group in Milan that had formed around Bruno Rossi, Giuseppe (Beppo) Occhialini, and since 1966 Giovanni (Nanni) Bignami. They were in contact with groups with similar research interests in Great Britain, France, Germany, and the Netherlands and established a collaboration, named the CARAVANE collaboration, to build...
I had the privilege to work with Prof. Giovanni Fabrizio Bignami (he was ‘Nanni' for me, and I was 'a Piè' for him) since the end of the 70’s on several space programs, and scientific endeavours, as well dealing with personal life’s challenges. In particular, our first contact happened during the proposal phase of the high energy Gamma-ray camera to be selected aboard the newly approved NASA...
Since the first detection of Gamma-ray Bursts (GRBs) in 1967, GRBs have been an active subject of study with many questions still left unanswered. In particular, the dominant radiation mechanism responsible for the prompt emission of GRBs remains an open question. As the host of possible GRB prompt emission models grows it has become clear that relying on spectral information alone to discern...
In the recent (sub-)PeV gamma-ray astronomy, we can pinpoint locations of Galactic PeV cosmic-ray accelerators, known as PeVatrons, as LHAASO has detected 43 sources with E>100 TeV. Such ultra-high-energy (UHE) photons likely originate from protons, since the Klein-Nishina effect would suppress leptonic gamma-ray emission. Some of these UHE sources are poorly explored in other wavelengths,...
We study the effects of magnetic acceleration on GRB afterglows by our implemented 1D special relativistic MHD simulation code with adaptive mesh refinement. Our simulation can treat magnetization more than 1, which is an efficient condition for magnetic acceleration. We simulate the interaction between a strongly magnetized thin/thick shell jet and an ambient medium. The Lorentz factor...
The first LHAASO catalogue has provided many new VHE/UHE sources and potentially new classes of sources that are primed for exploration with IACTs. The improved sensitivity at low energies ( E < ~1 TeV) and angular resolution of IACTs make them the ideal instruments to help associate and perform spectro-morphological studies of these newly discovered sources. Most of these sources...
Gamma-ray bursts (GRBs) are the brightest, yet among the most obscure, explosions in the Universe. Their temporal and spectral properties keep eluding our attempts at understanding them in a systematic way, and surprising events with unprecedented features are observed every year. A recent example is GRB 221009A, the "brightest of all times" (BOAT). The occurrence rate of such an event, based...
On October 9th, 2022, the brightest gamma-ray burst (GRB) ever recorded (GRB 221009A) was initially detected by the Fermi-GBM and Swift-BAT telescopes and subsequently by other satellite and ground-based instruments. Its remarkably bright emission, partially due to its close distance to Earth (z=0.151), makes this GRB a unique event. The outstanding characteristics of GRB 221009A, including...
The unassociated population represents about 30% of the sources in the latest 4FGL-DR4 release, with about one half lying at low (|b|<10°) Galactic latitudes. Many of these low-latitude sources exhibit properties that set them apart from established classes of Galactic gamma-ray emitters, in particular very soft spectra. The latter feature earned them the denomination "soft Galactic...
The observation of delayed GeV emission after a Gamma Ray Burst (GRB) detected at the very-high energies (VHE) beyond 100 GeV could indicate a non-zero magnetic field in the intergalactic medium. Indeed, VHE photons interact with the Extragalactic Background Light (EBL) to produce electron-positron pairs, which in turn can initiate electromagnetic cascades. An intergalactic magnetic field...
Microquasars present a remarkable opportunity to study particle acceleration in astrophysical jets due to their relative proximity to Earth compared to their extra-galactic counterparts. Yet this opportunity is rather limited since TeV emission has only been firmly associated with the jets of one microquasar so far. Fortunately, this might be about to change: in this contribution we present...
The occurrence of long-duration gamma-ray bursts (GRBs) is linked to ultra-relativistic jets formed soon after the collapse of massive stars. Initially, a highly variable radiation in the MeV range is detected, lasting for a few minutes, which is a result of internal dissipation within the jet. This is followed by afterglow radiation lasting for even several days, originating from non-thermal...
Very recently, gamma-ray emission detected by Fermi-LAT was reported from a star forming region NGC 2071. The high-energy radiation was claimed to be associated with occasional mega-flares thought to occur in T Tauri stars. The source, detected at energies $E \sim 100$\,GeV, appears to be transient, and was only detectable during the first two years of observation. In this work, we...
The presence of pair echo GeV emission after a Gamma Ray Burst (GRB) detected in the very-high energy band (VHE, E>100 GeV) can be the signature of the existence of a non zero magnetic field in the intergalactic medium. Indeed, VHE photons interact with the Extragalactic Background Light (EBL) to produce electron-positron pairs, which in turn can initiate electromagnetic cascades. In presence...
In the last 20 years, very-high energy gamma rays have been detected in different types of binary systems, including gamma-ray binaries, X-ray binaries, colliding-wind binaries and novae. Owing to the nature of the two components in these binaries, different scenarios have been considered for particle acceleration, gamma-ray emission and absorption processes. In this talk I will review our...
New instruments in gamma-ray astronomy like Fermi and the next generation IACTs have boosted our understanding of the high-energy Universe and its non-thermal processes. In fact, with the improved spatial and temporal resolutions together with real-time multimessenger astronomy, the instruments now provide us with information that cannot be modeled with the simplifying 1D approaches of...
In recent years, associations of tidal disruption events (TDEs) with astrophysical neutrinos detected by IceCube indicate that these transient phenomena could be responsible for accelerating cosmic rays up to PeV energies. These energetic relativistic particles can potentially also give rise to high- and very-high-energy (VHE) gamma-ray components. Although over 100 events have been identified...
The Gammapy library is an open-source framework designed for gamma-ray astronomy data analysis. Built on scientific Python ecosystem and leveraging open data formats, Gammapy offers a uniform platform for reducing and modeling data from different gamma-ray instruments. It greatly facilitates interoperability between observatories, enabling comprehensive joint analyses. Initiated in 2014 as a...
In recent years there have been numerous efforts to build a constellation of small satellites which would provide an all-sky coverage and quick localization of gamma-ray bursts (GRBs). One of the mission proposals is the CAMELOT constellation with a newly developed gamma-ray detector composed of a CsI(Tl) scintillator coupled with silicon photomultipliers (SiPMs). The prototype of this...
The Fermi Large Area Telescope (Fermi-LAT) has been continuously observing the sky from 20 MeV to 1 TeV for more than 15 years. Although Fermi-LAT’s sensitivity reaches down to 20 MeV, its low-energy range has been largely left under-explored. As we await an all-sky MeV mission such as COSI, it is now the prime time to capitalize on the full capabilities of Fermi-LAT. To complement and improve...
The High Energy Stereoscopic System HESS continues to operate with very
high efficiency and a high rate of five-telescope stereoscopic data taken
every year. The facility is mature and operates in a stable instrumental
configuration. This allows for extensive observations of homogeneous data
quality - either in deep observations or homogeneous surveys and
facilitates high up-time as...
The Pierre Auger Observatory is the largest detector in the world for ultra-high-energy cosmic rays. After 20 years of operation, many important results about the most energetic particles in the universe have been obtained. For example, the observation of a dipole in the distribution of the cosmic rays above $8\times10^{18}$ eV pointing away from the Galactic Center, indicates an extragalactic...
SWGO is an international proposal for the construction of a wide-field observatory to explore the Southern Hemisphere sky in the energy range from a few hundred GeV to the PeV. It aims to open a new observational window in astronomy by being the first wide-field ground-based gamma-ray instrument to survey the austral sky in the very- to ultra-high-energy range, where observational coverage is...
ASTRI is an INAF (Italian National Institute for Astrophysics) project for the construction and operation of an array of innovative IACT telescopes for ground-based gamma-ray astronomy in the energy range ~1 TeV to ~200 TeV.
Such an array, called ASTRI Mini-Array, currently under construction at the Teide Observatory in Tenerife, Spain, consists of nine telescopes of small size (~4m...
LHAASO has found more than 40 UHE cosmic accelerators within the Milky Way, with the highest energy photon reaching 1.4 quadrillion electron-volts, the highest energy photon ever observed. Most of these sources are extended sources that require telescopes with higher angular resolution and sensitivity to observe and study their morphology. Therefore, we propose a new project: Large Array of...
I will summarize and comment on key results in the galactic science reported during the meeting."
I will summarize and comment on key results in the extragalactic context reported during the meeting.
Astrophysics, and specifically gamma-ray astronomy, are transitioning into an era characterized by vast and complex datasets. The ability to make new discoveries depends on the efficient and accurate analysis of this data, necessitating the adoption of innovative methodologies. Deep Learning has become increasingly vital in addressing astrophysical challenges, with its application expanding...
The prompt emission in gamma-ray bursts (GRBs) has been a subject of debate for half a century. Photospheric radiation emitted when the jet transitions from the optically thick to the optically thin regime is a promising candidate. To account for the observations, subphotospheric dissipation should occur before the photons decouple from the plasma. Due to the high radiation pressure, shocks...
