Conveners
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- Fabrizio Tavecchio (Istituto Nazionale di Astrofisica (INAF))
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- Luigi Costamante (Former ASI)
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- Andreas Zech (LUTH, Observatoire de Paris - PSL, CNRS, 5 pl. Jules Janssen, 92195 Meudon)
Parallel 2
- Catherine Boisson
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- Antonio Stamerra (Istituto Nazionale di Astrofisica (INAF))
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- Wystan Benbow (Center for Astrophysics | Harvard & Smithsonian)
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- Annalisa Celotti (Istituto Nazionale di Astrofisica (INAF))
Parallel 2
- Gianpiero Tagliaferri (Istituto Nazionale di Astrofisica (INAF))
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...
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...
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...
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...
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....
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,...
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...
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,...
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...
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 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....
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...
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...
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...
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...
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....
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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,...
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...
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...
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...
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...
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...
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 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...
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...
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 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 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...
