PASTO - Particle Acceleration in Astrophysical Objects

5 Sept 2022, 07:00 → 7 Sept 2022, 18:50 Europe/Rome

Astronomical Observatory of Rome

LOC PASTO workshop

Conference poster

Monday 5 September

08:00 → 08:30 Meeting point in Frascati and bus trip

If you have your own transportation (your car), you can go directly to the venue OAR after 8.30 and ask the guards to open you the gate (only registered participants are allowed to enter).

08:30 → 09:20 Registration

Registration at the venue.

09:20 → 09:30 Welcome by the organizers

LucaFoffano_PASTO2022_intro_slides.pdf

09:30 → 11:00 Theory of Particle Acceleration in Astrophysical Environments

Convener: Giovanni Morlino (Istituto Nazionale di Astrofisica (INAF))

09:30 Review talk: The surprising effectiveness of cosmic ray acceleration

Cosmic rays are accelerated to a few PeV, and maybe 100PeV, in the Milky Way Galaxy. They are accelerated beyond 100EeV elsewhere. If I did not know better, I would say this could not happen, especially with such high efficiency. In this talk I discuss how acceleration to these energies pushes against the limits of theoretical credibility and how the underlying plasma physics has to be finely tuned to reach observed cosmic ray energies.

Speaker: Tony Bell (Rutherford Appleton Laboratory)

PASTO2022 Tony Bell.pptx

10:25 Thematic talk: Particle acceleration at nonrelativistic astrophysical shocks: eligibility to participate in the diffusive shock acceleration

Cosmic rays that span many decades in energy are one of the main contributors to the non-thermal energies in the universe. Although the diffusive shock acceleration (DSA) is the promising mechanism for particle acceleration at shocks, whether or not the mechanisms that promote to the DSA act similarly for electrons and protons are still not well understood. Since the energy gained by a particle depends on the non-linear interactions with the electromagnetic fields involving different length and time scales for electrons and protons, a self-consistent investigation is unavoidable to find a meaningful answer. In this talk, I will review the ongoing efforts to solve this puzzle. I will focus on electron acceleration at nonrelativistic shocks and discuss the results from our investigations using kinetic particle-in-cell simulations. I will demonstrate these results with the help of test-particle analysis and compare theoretical predictions with observations of non-thermal emissions such as gamma-rays, X-rays, and radio found in most astrophysical systems.

Speaker: Dr Siddhartha Gupta (University of Chicago)

Gupta_PASTO2022.pdf

11:00 → 11:30 Coffee Break

11:30 → 12:25 Theory of Particle Acceleration in Astrophysical Environments

Convener: Giovanni Morlino (Istituto Nazionale di Astrofisica (INAF))

11:30 Thematic talk: From Turbulence to Reconnection to Particle Acceleration: Connecting the Dots

Magnetized turbulence and magnetic reconnection are often invoked to explain the generation of high energy particles in astrophysics. Originally, these two routes for particle acceleration were treated as distinct plasma processes. However, with the rapid advances in computing power and theory, they are converging towards a unified domain. In this talk, I will outline recent developments in this fast-growing front exploiting the results of first-principles kinetic (PIC) simulations. I will also highlight their direct implications for some astrophysical sources.

Speaker: Luca Comisso (Columbia University)

Comisso_Pasto2022.pdf

12:05 Particle acceleration and plasma reconfiguration by magnetic reconnection in laboratory and astrophysical plasmas

Magnetic reconnection is a powerful acceleration mechanism since it can impulsively deliver energy that was accumulated for long times and in large volumes. The promptly available energy is stored in magnetic fields produced by current sheets, i.e. antiparallel fields at the two sides of the sheet, with a null in between.
The sudden onset of reconnection events poses the so-called trigger problem: most reconnection theories consider strongly unstable plasmas, but how can such plasmas survive along the path to strong instability conditions? Laboratory experiments indicate that weakly unstable plasmas evolve to fast reconnection by super-exponential growth [1], due to non-linear instability evolution or to detonation effects.
The key condition for acceleration is reconnection efficiency, i.e. the plasma inflow (towards the current sheet) velocity normalized to the Alfvén velocity. The scaling problem deals with efficiency dependence on plasma characteristics. Efficiencies in the 0.03-0.1 range have been found for collisionless plasmas, and theoretical explanations for these results have recently emerged [2].
How delivered energy is distributed among bulk plasma acceleration, thermal heating and acceleration of high-energy tails is sometimes called the equipartition problem. It has been shown that the presence of a “guide” magnetic field along perpendicular to the reconnecting field can impede tail acceleration as it modifies particle orbits.
Another important aspect of magnetic reconnection is plasma reconfiguration, in particular the generation of toroidal currents from driven axial currents, in the absence of any toroidal electric field. This aspect is presently being investigated in the PROTO-SPHERA experiment.
The main concepts and problems of magnetic reconnection will be illustrated in the talk, and recent progress will be reviewed.

[1] P Buratti et al 2003 Plasma Phys. Control. Fusion 45 L9
[2] YH Liu et al 2022 Commun Phys 5, 97

Speaker: Dr Paolo Buratti (Istituto Nazionale di Astrofisica (INAF))

12:25 → 13:40 Lunch Break

13:40 → 15:30 Theory of Particle Acceleration in Astrophysical Environments

Convener: Frank Jenko

13:40 Reconnected Astrophysical Plasma results from PIC code simulations

Non-thermal acceleration is one of the most challenging problems in theoretical plasma astrophysics. There is an enormous number of astrophysical objects that emits powerful outbursts of high-energy radiation with non-thermal spectrum like Pulsar Wind Nebulae (PWNe), and Blazars. There is a strict relation between the emitting particle’s energy and the resulting photon energy. The ideal Magnetic Hydrodynamics is not always able to explain recent observations on Crab Nebula and Blazars. Magnetic Reconnection (MR) is one of the candidates to explain the most explosive phenomena seen in our Universe, from the topological point of view, MR mechanism is a change in the plasma magnetic fields lines which causes the conversion on site of magnetic energy into kinetic energy. The most energetic flares can be caused by the so-called “fast reconnection”, which implies that plasma is very diluted and collisionless. I will present some results from simulations obtained from PIC model code (Zeltron) to explain some transient phenomena such as those observed in the Crab Nebula and Blazars. Zeltron code is based on PIC (Particle-in-Cells) models, in which the Vlasov equation is solved indirectly. Zeltron code does not integrate on single particles but on their trajectories. PIC type models allow to overcome limits of the MHD codes by identifying the Magnetic Reconnection process as the cause of the observation of synchrotron radiation above the Burn-off limit. I will present simulations using different initial conditions by using Zeltron code, for example, results from 2-antiparallel current sheets for the magnetic field with periodic conditions.

Speaker: Dr Eloisa Menegoni (Istituto Nazionale di Astrofisica (INAF))

PASTO_2022_Menegoni_new_v6.pdf

14:00 Thematic talk: The current state of the electron injection problem

Supernova remnants (SNRs) are believed to produce the most part of the galactic cosmic rays (CRs). SNR non-relativistic collisionless shocks responsible for acceleration of CRs via diffusive shock acceleration. This process involves pre-existing mildly energetic particles, a means of pre-acceleration is required, especially for electrons. Electron injection remains one of the most troublesome and still unresolved issues and our physical understanding of it is essential to fully comprehend the physics of SNRs. To study any electron-scale phenomena responsible for pre-acceleration, we require a method capable of resolving these small kinetic scales and Particle-in-Cell simulations fulfill this criterion. Here I report about the latest achievements on kinetic simulations of non-relativistic high Mach number shocks. I discuss how the physics of SNR shocks depends on the shock parameters (e.g., the shock obliquity, Mach numbers, the ion-to-electron mass ratio), which processes are responsible for the electron pre-acceleration and how these shocks can be studied using in-situ satellite measurements. Finally, I outline future perspectives of the electron injection problem.

Speaker: Artem Bohdan (Deutsches Elektronen-Synchrotron DESY)

2022. PASTO2022 (A. Bohdan).pptx

14:35 Model of Gamma-ray Bubbles in the Galaxy and Relevant Mode Particle Interactions

A plasma outflow coming from the center of “Milk Way” Galaxy is simulated by an ion “beam” reaching a nearly stationary magnetic field “arch” with dimensions of the same order of magnitude as that of the Galaxy [1]. Then waves can be excited efficiently in the rarefied plasma (10-2-10-4 cm-3) permeating the relevant magnetic field configuration. These are electrostatic “Lower Hybrid” modes driven to instability via Cerenkov interaction [2]. By using a fluid model the relevant dispersion relation is derived and the growth rate is evaluated both analytically and numerically. Then efficient energy transfer from the perpendicular ion “beam” to the electron population, via Landau damping, can be expected accelerating fast electrons, or heating the overall electron population. The radiation emission due to the energetic electron component could explain the observed X and gamma-ray spectra characterizing the “Fermi Bubbles” of the Galaxy.
[1] H.-Y. K. Yang, M. Ruszkowski, E.G. Zweibel, Galaxies 6, 29 (2018).
[2] T. Chang and B. Coppi, Geophys. Res. Lett. 8, 1253 (1981).

*Sponsored in part by the U.S. Department of Energy and by C.N.R of Italy.

Speakers: Alessandro Cardinali (Istituto Nazionale di Astrofisica (INAF)), Prof. Bruno Coppi (MIT Kavli Institute for Astrophysics and Space Research)

Presentation_OAR_MontePorzio.pptx.pdf

14:55 Thematic talk: Advances in particle acceleration at non-relativistic shocks: from plasma instabilities to entire supernova remnants

Cosmic ray driven instabilities play a crucial role during particle acceleration at shocks and during the propagation of the accelerated cosmic rays in astrophysical systems. The instabilities amplify magnetic fields and modulate cosmic ray transport so that the intrinsically collisionless cosmic ray population is coupled to the thermal plasma and provides important dynamical feedback. We found a new cosmic ray driven instability (called intermediate-scale instability) that excites comoving ion-cyclotron electromagnetic waves at sub-ion skin-depth scale. The instability operates if cosmic rays with a finite pitch angle drift with moderately super-Alfvénic speeds. Its linear growth rate is typically about 100 times faster in comparison to that at the ion gyro-scale, making it a crucial player in both acceleration and transport of charged particles in galactic and stellar environments. I will demonstrate that that this new instability provides the only known mechanism to date for efficient electron acceleration at parallel electron-ion shocks, thus solving the long-standing electron-injection problem at these shocks. We apply this physics to 3D magnetohydrodynamics simulations of the evolution of an entire supernova remnant, where we self-consistently include cosmic ray protons and follow the time evolution of the cosmic ray electron spectrum. By matching the observed morphology and non-thermal spectra of shell-type supernova remnants (SN 1006, RXJ 1713, and Vela Jr.) in radio, X-rays and gamma-rays, I demonstrate how we gain insight into the following topics: 1) leptonic vs. hadronic model of the gamma-ray emission, 2) origin of patchiness of TeV gamma-ray maps and how this relates to interstellar magnetic turbulence, 3) quasi-parallel vs. quasi-perpendicular acceleration of cosmic ray electrons, 4) nature of magnetic field amplification and damping in supernova remnants (turbulently vs. Bell-amplified magnetic field).

Speaker: Christoph Pfrommer (Leibniz Institute for Astrophysics Potsdam)

Pfrommer_Rome22.pdf

15:30 → 16:00 Coffee Break

16:00 → 18:05 Modeling the interactions of particles

Convener: Alessandra Lamastra (Istituto Nazionale di Astrofisica (INAF))

16:00 Review talk: Modeling the interactions of particles

This talk reviews the basic physics of particle interaction in astrohysical environments. It will provide an overview of leptonic emission mechanisms and hadronic interactions. The latter will include a review of basic kinematics of hadron-hadron and hadron-photon interactions as well as electromagnetic and neutrino emission from the decay products of such interactions.

Speaker: Prof. Markus Boettcher (North-West University)

Boettcher_PASTO2022.pdf

Boettcher_PASTO2022.pptx

16:55 Thematic talk: Leptonic and hadronic photon emission models of high-energy sources

This talk provides an overview of emission models for non-thermal photon production, with focus on jetted cosmic-ray sources. I will discuss the application of these models to non-extended and extended emission from extragalactic jets, in the framework of different environments and configurations.

Speaker: Anita Reimer

PASTO_JetPhotonModels_AReimer.pdf

17:30 Thematic talk: The gamma-ray emission of Galaxy Outflows

Galaxies display a variety of outflows, which are detected from near the central super-massive black hole to the entire host. These outflows are often powerful enough to unbind, if sustained over time, the gas of these galaxies. Propagating through the galaxy, the outflows should interact with the interstellar medium creating a strong shock, similar to those observed in supernovae explosions, which is able to accelerate charged particles to high energies. Here we report the {\it Fermi} Large Area Telescope detection of gamma-ray emission from galaxies with two different types of outflows: ultrafast and molecular outflows. In this talk I will review our findings and discuss them in terms of particle acceleration at the shock front.

Speaker: Marco Ajello (Clemson University)

Outflows_Rome.pdf

Outflows_Rome.pptx

Tuesday 6 September

08:15 → 08:30 Meeting point in Frascati and bus trip

08:30 → 09:00 Bus trip

09:00 → 10:30 Galactic Astrophysical Accelerators

Convener: Barbara Olmi (Istituto Nazionale di Astrofisica (INAF))

09:00 Review talk: Galactic astrophysical accelerators: an overview

In this talk I will summarize the status of the century long quest for the sources of Galactic Cosmic Rays. I will discuss recent challenges to the paradigm associating Galactic Cosmic Rays to acceleration in Supernova Remnants, and then introduce alternative classes of sources, such as star forming regions and Pulsar Wind Nebulae, that are gaining increasing attention thanks to highlights from high energy astrophysical observations. I will conclude by assessing what directions of investigation appear as the most promising to make progress.

Speaker: Elena Amato (Istituto Nazionale di Astrofisica (INAF))

Amato_PASTO.pdf

09:55 Thematic - Phenomenology (S. Gabici)

gabici.pdf

10:30 → 10:40 Group picture

10:40 → 11:10 Coffee Break

11:10 → 12:25 Galactic Astrophysical Accelerators

Convener: Barbara Olmi (Istituto Nazionale di Astrofisica (INAF))

11:10 Thematic talk: Theoretical studies of gamma-ray binaries as powerful galactic accelerators

Gamma-ray binaries, which consist of a compact object and a star, are galactic non-thermal sources that present radiation from radio to gamma rays, and a rich phenomenology largely affected by the system binarity. Of particular interest are gamma-ray binaries hosting massive stars, as they are among the most powerful and efficient galactic persistent accelerators. The most common scenario for these sources is that with a non-accreting pulsar powering the emitting particles, although accretion (and hybrid) scenarios have been also proposed. In this talk, I will provide context for gamma-ray binaries, and use results obtained from semi-analytical and numerical modelling to discuss the phenomenology at very high energies and the potential sites of particle acceleration of these sources.

Speaker: Valenti Bosch-Ramon (Universitat de Barcelona/ICCUB)

vbosch.pdf

11:45 Leptonic Nonthermal Emission from Supernova Remnants Evolving in the Circumstellar Magnetic Field

The very-high-energy gamma-ray emission observed from a number of Supernova remnants (SNRs) indicates particle acceleration to high energies at the shock of the remnants and a potentially significant contribution to Galactic cosmic rays. It is extremely difficult to determine whether protons (through hadronic interactions and subsequent pion decay) or electrons (through inverse Compton scattering on ambient photon fields) are responsible for this emission. For a successful diagnostic, a good understanding of the spatial and energy distribution of the underlying particle population is crucial. Most SNRs are created in core-collapse explosions and expand into the wind bubble of their progenitor stars. This circumstellar medium features a complex spatial distribution of gas and magnetic field which naturally strongly affects the resulting particle population. In this work, we conduct a detailed study of the spectro-spatial evolution of the electrons accelerated at the forward shock of core-collapse SNRs and their non-thermal radiation, using the RATPaC code that is designed for the time- and spatially dependent treatment of particle acceleration at SNR shocks. We focus on the impact of the spatially inhomogeneous magnetic field through the efficiency of diffusion and synchrotron cooling. It is demonstrated that the structure of the circumstellar magnetic field can leave strong signatures in the spectrum and morphology of the resulting non-thermal emission.

Speaker: Iurii Sushch (North-West University, South Africa)

Sushch_SNR_circumstellar_field_PASTO.pptx

12:05 Gamma-ray and X-ray study of possibly oldest GeV supernova remnant G298.6−0.0

Hadronic γ-ray sources associated with supernova remnants (SNRs) can serve as stopwatches for the escape of cosmic rays (CRs) from SNRs, which gradually develops from highest-energy particles to lowest-energy particles with time. In this work, we analyze the 13.7 yr Fermi-LAT data to investigate the γ-ray feature in/around the SNR G298.6−0.0 region. With ≥16 GeV data, we detect two spatial components: Src-NW at the west of the SNR, and Src-S at the south of the SNR. Then, with ≥8 GeV data, we detect an additional component — Src-NE, which is inside the radio dimension of the SNR. They are all point-like sources. Their GeV spectra are in distinct shapes, suggesting different CR populations and/or different interstellar medium distributions around them. Noteworthily, the component Src-NE inside the SNR dimension demonstrates a spectral break at ≈1.8 GeV, suggesting an old SNR age of >10 kyr. Among the three components, Src-S has the hardest spectrum extending to 30−100 GeV. Considering its separation from the SNR, its hard spectral shape is explainable in terms of the progress of the CR escape. We also look into the X-ray emission from the SNR region, with the Chandra-ACIS data. We detected several point-like keV sources and extended keV emission inside the SNR. We willl discuss the origins of their X-rays as well as the spatial morphology of the diffuse component.

Speaker: Paul (Kin-Hang) Yeung (University of Tokyo (UTokyo))

G298.6-0.0.pdf

12:25 → 13:50 Lunch Break

13:50 → 15:20 Galactic Astrophysical Accelerators

Convener: Martina Cardillo (INAF- IAPS)

13:50 Thematic - Pevatrons (P. Cristofari)

14:25 Thematic talk: High-energy neutrinos from the Galaxy: expectations and experimental results

The detection of a diffuse flux of cosmic neutrinos with energies up to several PeV has opened a new window into the exploration of the extreme non-thermal Universe. Despite several analysis strategies have been implemented, the origin of these neutrinos remains to date unknown. The latter include the investigation of catalogued astrophysical accelerators as well as neutrino auto-correlation studies. Very-high-energy gamma-ray data indicate that our Galaxy is populated by many powerful accelerators, e.g. the several so-called PeVatrons recently detected by LHAASO. Being produced in hadronic interaction processes only, neutrinos would be key to shed light on the nature of the observed radiation. Additionally, the diffuse Galactic neutrino flux originated at cosmic-ray collisions with target gas located along the Galactic Plane represents a guaranteed source of neutrinos, likely contributing to the observed diffuse neutrino flux. In this talk, I’ll review the status of neutrino observations and discuss the most promising results concerning Galactic neutrino candidate sources.

Speaker: Silvia Celli (Sapienza University of Rome & INFN)

Celli_PASTO.pdf

15:00 On the X-ray features associated to some bow-shock pulsar wind nebulae

Elongated X-ray features have been detected in association to some bow-shock pulsar wind nebulae, among which those of PSR B224+65 (the Guitar Nebula), PSR J1101-6101 (the Lighthouse Nebula), PSR B0355+54, PSR J1135-6055, PSR J1509-5850, PSR J2055+2539, PSR J1809-1917 and more recently the outstanding filament associated to PSR J2030+4415.
Distinctive properties of these features are: a very elongated structure, in some cases very straight and narrow; a direction uncorrelated with the pulsar proper motion (incompatible with the hypothesis of a pulsar tail); a very hard non-thermal X-ray spectrum, which does not show any clear softening with distance from the pulsar; the presence in some cases of a counter-feature.
Both semi-analytic and numerical analyses converge to outline a scenario in which the highest-energy electrons may escape from the head of the pulsar bow shock, leak in the ISM, flow along the ambient magnetic flux tubes, emit X-ray synchrotron, and to some extent contribute to amplify the field itself. However, there are some aspects of their nature, structure, and involved micro-physics that are still rather controversial and puzzling.
We will mainly discuss here the way in which the highest energy electrons flow / diffuse in ambient magnetic field (this by focusing on possible effects of a pitch-angle anisotropy), and whether these structures are dynamically passive, or rather real jets (this by introducing a statistical analysis of their curvatures).

Speaker: Rino Bandiera (Istituto Nazionale di Astrofisica (INAF))

Rino Bandiera - PASTO 2022.pdf

15:20 → 15:50 Coffee Break

15:50 → 17:20 Galactic Astrophysical Accelerators

Convener: Lucio Angelo Antonelli (Istituto Nazionale di Astrofisica (INAF))

15:50 The formation of gamma-ray halos around supernova remnants through particle escape

Supernova remnants (SNRs) are known to accelerate particles to relativistic energies, from the detection of nonthermal emission. The particularities of the acceleration mechanism are still debated. Here, we discuss how particle escape modifies the observable spectra as well as morphological features that might be revealed by the observational progress from radio to gamma-ray energies.
We use our time-dependent acceleration code RATPaC to study the formation of extended gamma-ray halos around supernova remnants and the morphological implications that arise when the high-energetic particles start to escape from the remnant.
We find a strong difference in the morphology of the gamma-ray emission from supernova remnants at later stages, dependent on the emission process. At early times, both the inverse-Compton and the Pion-decay morphology are shell-like. However, as soon as the maximum-energy of the freshly accelerated particles starts to fall, the inverse-Compton morphology starts to become center-filled, whereas the Pion-decay morphology keeps its shell-like structure. Both emission-spectra show a spectral softening caused by the escape of the highest-energetic particles. Escaping high-energy electrons start to form an emission halo around the remnant at this time. There are good prospects for detecting this spectrally hard emission with the future Cerenkov Telescope Array, as there are for detecting variations in the gamma-ray spectral index across the interior of the remnant. Due to the projection effects there is no significant variation of the spectral index expected with current-generation of gamma-ray observatories.

Speaker: Robert Brose (Dublin Institute for Advanced Studies)

Brose_SNRHalo.pptx

16:10 First X-ray polarimetry measurements of supernova remnants with IXPE

Young supernova remnants (SNRs) such as Cas A, Tycho, and SN1006 are relativistic particle accelerators and likely the sources of most of Galactic cosmic rays.
The X-ray synchrotron emission from their shock fronts has been expected to be polarized for some time.
The measurement of X-ray polarization degree and direction provides unique constraints on the turbulence level of the magnetic field, which plays a crucial role in theories of diffusive shock acceleration with efficient magnetic field amplification in supernova remnants, and on the morphology of the magnetic field where particle are accelerated.
The NASA/ASI Imaging X-ray Polarimetry Explorer (IXPE), that launched in December 2021, is the first mission entirely dedicated to X-ray polarimetry.
Its imaging-capable detectors allow us to perform spatially resolved X-ray polarimetry of extended sources such as supernova remnants.
Here I present the first results obtained from the IXPE observation of the SNRs Cas A, Tycho, and SN1006.

Speaker: Riccardo Ferrazzoli (Istituto Nazionale di Astrofisica (INAF))

SNR_PASTO_2022_09_RF.pdf

16:30 How radio observations can support ASTRI Mini-Array Galactic science

Both radio and gamma astronomy are about to hugely profit from new and upcoming instruments. SKA and CTA precursors and pathfinder are going to give us an unprecedented view of our Galaxy in the two most extreme parts of the electromagnetic spectrum. Supernova remnants (SNRs) are emblematic sources where the radio emission mirrors the energy distribution of the accelerated particles.
In this talk, I present some results of Galactic surveys with the SKA precursors. I will bring examples of how the analysis of these radio data is giving hints on the emission mechanisms taking place in SNRs (spectral breaks, spinning dust, spatial spectral variations) and how these results may be exploited with follow-up gamma studies. The aim is to show how radio and gamma can work synergistically and stimulate new discussions and collaborations.
I finally focus on the case study of Kes73, a known radio and TeV SNR. We show our current and past radio studies on this object and the possibility to have this source as a target for the ASTRI Mini-Array, which consists of nine dual-mirror Cherenkov telescopes currently being built at the Observatorio del Teide (Spain).

Speaker: Adriano Ingallinera (Istituto Nazionale di Astrofisica (INAF))

Ingallinera_PASTO.pdf

16:50 Modeling the Gamma-ray Emission from the Symbiotic Nova RS Ophiuchi

Novae, among the brightest transients in the night sky, are luminous eruptions occurring in a binary system in which a white dwarf accretes mass from a stellar companion. As an accreted layer accumulates conditions are reached for a thermonuclear runaway. The resulting energy release causes the accreted envelope to expand, leading to its ejection. During this process shock waves are produced that provide extreme conditions required to accelerate particles to high energies. Novae are valuable laboratories for studying time-dependent shock particle acceleration. Evidence of the presence of high-energy particles in these systems are the detections of radio synchrotron emission, of gamma rays in the range $\sim$ 100 MeV to $\sim$ 10 GeV, and of very-high-energy radiation ($>$ 100 GeV) with the very recent detection at these energies of the recurrent symbiotic nova RS Ophiuchi. I present here a spatial-temporal model for the non-thermal emission of RS Ophiuchi. The model solves the transport of relativistic particles, electrons and protons, injected inside the system, computing the produced radiation. Additionally, the model estimates the particle escape into the interstellar medium.

Speaker: María Victoria del Valle

RSOph-delValle22.pdf

17:10 Particle Acceleration at Reflected Shocks in Supernovae Remnants:

Supernovae remnants (SNRs) are widely believed to be one of the prime sources of Galactic cosmic rays. They are known to be efficient particle accelerators which is indirectly confirmed by detection of non-thermal emission across the whole electromagnetic spectrum from radio to very-high-energy gamma-rays. Protons and electrons can be accelerated to very high energies of at least several tens of TeV both at the forward and at the reverse shock of the remnant. About 80% of all SNRs originate in core-collapse events and are expected to expand into a complex environment of the stellar wind bubble blown up by their progenitor stars, where forward shock might interact with various density inhomogeneities. Such interaction would cause the formation of reflected shocks propagating inside the remnant which can potentially be strong enough to also accelerate particles. Current investigations of particle acceleration in SNRs are usually limited to forward and reverse shocks ignoring the complexity of the hydrodynamic picture. Although for most SNRs the observed shell-like morphology generally agrees with an idea that high energy particles originate predominantly from the forward shock (for some remnants the significant contribution from the reverse shock was also confirmed), precise spatially resolved measurements do not always agree with a simplified picture giving rise to alternative ideas such as interaction with dense cloudlets . This work is focused on the investigation of particle acceleration at the reflected shocks formed through the interaction of the forward shock with density inhomogeneities and its potential impact on the overall observational properties.

Speaker: Jacobus Frederik Le Roux (North-West University)

19:30 → 19:45 Meeting point in Frascati and bus trip

19:45 → 20:00 Bus trip

20:00 → 23:00 Social dinner

23:00 → 23:15 Bus trip to Frascati

Wednesday 7 September

08:20 → 08:30 Meeting point in Frascati and bus trip

08:30 → 09:00 Bus trip

09:00 → 10:35 Extragalactic Astrophysical accelerators

Convener: Antonio Stamerra (Istituto Nazionale di Astrofisica (INAF))

09:00 Review talk: Fermi acceleration in relativistic outflows

The physics of acceleration of charged particles in relativistic astrophysical sources represents a central question in modern high-energy astrophysics and multi-messenger astronomy. This talk will discuss modern developments in our understanding of Fermi-type processes in relativistic plasmas: acceleration at shock waves, acceleration in magnetized turbulence, and finally during the interaction between a relativistic shock wave and a magnetized turbulent plasma.

Speaker: Martin Lemoine (Institut d'Astrophysique de Paris (CNRS, France))

Lemoine_PASTO22.pdf

09:55 Search for candidate neutrino emitters with similar features as TXS 0506+056

The detection of high energy astrophysical neutrinos is an important step towards finding the long-sought sources of cosmic rays. However, the long-exposure neutrino sky map by IceCube has not shown significant excesses so far and the sources of such energetic neutrinos remain unknown. Among the potential extragalactic neutrino sources, blazars are interesting candidates, as suggested by the detection of the flaring blazar TXS 0506+056 in coincidence with a high-energy neutrino, IceCube-170922A, in 2017. This is the first – and up to now the only – evidence for a neutrino point source. In this contribution, we present a sample of candidate neutrino-emitting blazars taken from the most recent 4LAC-DR2 catalog (based on 10 years of Fermi-LAT data) and selected by constraining a number of key properties to be similar as those of TXS 0506+056. Important properties of the broad line region, narrow line region and disk of the candidates will be discussed, with particular attention to the (in)efficience of the accretion flow. In addition, theoretical interpretation of the spectral energy distribution of the candidates through lepto-hadronic models will be shown, providing information on the neutrino flux from these sources and the detectability prospects at TeV energies.

Speaker: Ilaria Viale (University and INFN Padova)

PASTO2022_IViale.pdf

10:15 Simulating the recollimation shocks, instabilities and particle acceleration in relativistic jets.

AGN jets are the most powerful persistent emitters in the Universe, but the mechanisms through which they dissipate part of their energy flux and convey it to relativistic particles are still elusive. Despite advances on the numerical and theoretical side, the identification of the processes at work is made difficult by the huge range of spatial and temporal scales involved and by the strong interplay between kinetic-scale processes and large-scale dynamics, with the important role of instabilities. Numerical simulations are therefore the natural tool for exploring the vast range of jet phenomenology.
In this framework, 3D MHD simulations of relativistic jets surprisingly reveal that the (intensively studied in 2D) recollimation caused by pressure unbalance with the external medium triggers a rapidly growing instability that leads to the development of strong turbulence, eventually resulting in the complete disruption of the flow (Komissarov et al. 2019). Existing simulations are inadequate to fully characterize the instability and the level of turbulence injected in the plasma, therefore in order to understand the impact of this newly discovered instability, we are pursuing a vigorous program of relativistic MHD simulations. Preparatory 2D and complete 3D simulations are carried out with the state-of-the-art PLUTO code, and the treatment of particle acceleration and transport is included via a hybrid approach. In particular the setup we are considering is designed to be applied to the most extreme and still enigmatic blazars, the EHBL. Their observational properties are different from the bulk of the blazar population, challenging the standard emission scenarios, and the multiple-shock model proposed by Zech &Lemoine, justified by 2D simulations, is instead questioned by the results of these 3D simulations. In addition, the presence of strong turbulence downstream of the recollimation shock is an interesting feature for building a new model for the VHE-emission of the EHBL.

Speaker: Agnese Costa (Istituto Nazionale di Astrofisica (INAF))

Simulations of relativistic jets and recollimation in extreme blazars.pdf

10:35 → 11:15 Coffee Break

11:15 → 12:50 Extragalactic Astrophysical accelerators

Convener: Dr Luca Foffano (Istituto Nazionale di Astrofisica (INAF))

11:15 Thematic talk: Diffusive Shock Acceleration in Galactic Winds: Multimessenger prospects

Starburst Galaxies (SBGs) and Active Galactic Nuclei (AGNi) can launch and sustain powerful outflows of very high velocity and large opening angle.
Such winds develop a bubble structure characterized by an inner wind shock and an outer forward shock.
During the time the forward shock expands in the surrounding medium, the inner wind shock quickly decelerates while remaining strong, thereby creating ideal conditions for stationary particle acceleration.
We model the diffusive shock acceleration process at the wind shock of such winds and we explore the multimessenger implications in terms of high energy photons, neutrinos and escaping cosmic rays.

Speaker: Enrico Peretti (Niels Bohr Institute)

Peretti_PASTO.pdf

11:50 Extreme TeV Blazar: a stochastic acceleration model

Active Galactic Nuclei are the most powerful persistent sources in the Universe. Among them, 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 blazars. These sources exhibit a peak of radiation at TeV energies and a hard intrinsic spectrum at sub-TeV range. In most cases their exceptional TeV emission appear to be steady over years.

Explaining the features of the extreme TeV blazars is still an open challenge, in fact the most used phenomenological models, based on shock acceleration alone, are not able to totally reproduce their SED and the parameters required by the fit are close to the theoretical limits.

In our model we suppose that the non-thermal particle are firstly accelerated by a jet recollimation shock caused by the difference between the jet and the external pressure, which then induces turbulence in the rest of jet, presuming a low plasma magnetization. Non-thermal particles are therefore reaccelerated by the turbulence, which harden the particle spectra and accordingly the radiative emission.

Since we are treating stochastic acceleration, in order to study the time evolution and the steady state of the system, we must describe the phase-space distribution of the non-thermal particles and of the turbulence through diffusion equations. Supposing isotropy and homogeneity, their interaction and spectra have been studied solving a system of two non-linear and entangled Fokker Planck equations, while the radiative emission has been calculated through the Synchrotron Self Compton model. The emission from our model has been compared with prototype extreme TeV blazar 1ES 0229+200.

Speaker: Alberto Sciaccaluga (Istituto Nazionale di Astrofisica (INAF))

Slide_Talk.pdf

12:10 Constraining blazar emission models with the study of waiting time between gamma-ray flares of FSRQs

I report here the results of a study of waiting times between Gamma-ray flares of FSRQ, defined as the time intervals between consecutive activity peaks.
I will show that this study constrains the physical mechanism responsible for gamma-ray emission.
We obtained that waiting times between flares can be described with a Poissonian process, consisting of a set of overlapping bursts of flares, with an average burst duration of ∼0.6 year and average rate of ∼1.3 y−1. For short waiting times (below 1 d host-frame) we found a statistically relevant second population, the fast component, consisting of a few tens of cases, most of them revealed for CTA 102. Interestingly, the period of conspicuous detection of the fast component of waiting times for CTA 102 coincides with the reported crossing time of the superluminal K1 feature with the C1 stationary feature in radio.
To reconcile the recollimation shock scenario with the bursting activity, we have to assume that plasma should have a typical length of ~2 pc (in the stream reference frame) when it reaches the recollimation shock. Otherwise, the distribution of waiting times can be interpreted as originating from relativistic plasma moving along the jet for a deprojected length of ∼30−50 pc (assuming a bulk Lorentz factor = 10) that sporadically causes gamma-ray flares, hopefully triggered by magnetic field structures along its path.
In the magnetic reconnection scenario, reconnection events or plasma injection to the reconnection sites should be intermittent. Individual plasmoids can be resolved in a few favourable cases only, and could be responsible for the fast component.

Speaker: Dr Luigi Pacciani (IAPS-INAF)

waiting_times_lp_v5.pdf

12:30 BL Lacertae: Unravelling the change in spectral upturn with flux variation

BL Lacertae is an intermediate BL Lac object. It entered a flaring state in gamma energy range in August 2020. It was also found entering a flaring X-ray state. In this work, we examine the change in spectra of BL Lacertae with the variation in flux in X-ray energy range of 0.2-10.0 keV. For this, we took observations from the EPIC-PN instrument onboard XMM-Newton satellite. We did the spectral modeling for the different flaring and quiescent states of BL Lacertae and examined the variation of upturn from synchrotron to inverse compton emission with the variation in flux. A steep power law model represents the soft X-ray emission due to synchrotron process and another power law model was used to describe the hard energy emission explained by inverse compton emission. We notice the change in the energy of the upturn that was observed in low X-ray energies in small time ranges as the flux of the source changes.

Speaker: Angel Priyana Noel

Angel_BLLac_PASTO2022.pdf

12:50 → 14:00 Lunch Break

14:00 → 15:10 Extragalactic Astrophysical accelerators

Convener: Antonio Stamerra (Istituto Nazionale di Astrofisica (INAF))

14:00 Thematic talk: Particle Acceleration and radiative cooling in blazar jets

Relativistic jets are a common manifestation of accreting black holes. Blazars are jets from supermassive black holes moving close to our line of sight. A common hypothesis for jet formation is that they are launched by powerful magnetic fields that thread the black hole. Here, I discuss the trip of the jet from the black hole to the much larger scales where it radiates. I argue that the jet emission is result of MHD instabilities that result in dissipation in the jet through the process of magnetic reconnection. I will review our latest understanding of the physics of magnetic reconnection and show that it could naturally produce the emitting plasmoids commonly invoked when modeling the blazar flares. Our 3D first-principle simulations of magnetic reconnection show that the dominant mechanism for (fast) particle acceleration involves particles that escape the current sheet and are accelerated by the large-scale electric field in the reconnection upstream. If the reconnection layers extend to a large fraction of the jet cross section, this mechanism can accelerate hadrons to ultra-high energies.

Speaker: Dimitrios Giannios (Purdue University)

Giannios_Rome.pptx

14:35 Thematic talk: Relativistic accelerators: Gamma-Ray Bursts

Gamma-Ray Bursts (GRBs) are the strongest explosions in the Universe, and are powered by ultra-relativistic jets. They produce very bright emission both within the relativistic outflow (prompt gamma-ray emission, X-ray flares, reverse shock emission - optical flash and radio flare) and from the relativistic shock that they drive into the external medium (the long-lived broad-band afterglow emission). This emission provides strong evidence for non-thermal particle acceleration in these regions and teaches us about its properties along with the physical conditions and processes at work. This talk will discuss our current understanding along with existing open questions and puzzles that can motivate related theoretical work.

Speaker: Jonathan Granot (Open University of Israel)

PASTO_7September2022_Granot.pdf

15:10 → 15:40 Coffee Break

15:40 → 16:20 Extragalactic Astrophysical accelerators

Convener: Lara Nava (Istituto Nazionale di Astrofisica (INAF))

15:40 Onset of particle acceleration during the prompt phase in gamma-ray bursts

The physical processes of the gamma-ray emission and particle acceleration during the prompt phase in gamma ray bursts (GRBs) are still unsettled. In order to perform an unambiguous physical modelling of observations, a clear identification of the emission mechanism is needed.
An instance of a clear identification is the synchrotron emission during the very strong flare in GRB160821A, that occurs during the prompt phase at 135 s. In this talk, we show that the distribution of the radiating electrons in the flare is initially very narrow, but later develops a power-law tail of accelerated electrons. We thus identify for the first time the onset of particle acceleration in a GRB jet. The flare is consistent with a late energy release from the central engine causing an external-shock as it encounters a preexisting ring nebula of a progenitor Wolf-Rayet star. Relativistic forward and reverse shocks develop, leading to two distinct emission zones with similar properties.
The particle acceleration only occurs in the forward shock, moving into the dense nebula matter. Here, the magnetisation also decreases below the critical value, which allows for Fermi acceleration to operate. Using this fact, we find a bulk Lorentz factor of $420 < \Gamma < 770$, and an emission radius of $R \sim 10^{18}$ cm, indicating a tenuous gas of the immediate circumburst surrounding. The observation of the onset of particle acceleration thus gives new and independent constraints on the properties of the flow as well as on theories of particle acceleration in collisionless astrophysical shocks.

Speaker: Felix Ryde (KTH)

FelixRydePASTO22.pdf

16:00 Prompt GRB Polarization: Temporal Evolution and Energy Dependence

The dominant radiation mechanism that produces the prompt emission in gamma-ray bursts (GRBs) remains a major open question. Spectral information alone has proven insufficient in elucidating its nature. Time-resolved linear polarization has the potential to distinguish between popular emission mechanisms, e.g. synchrotron radiation from electrons with a power-law energy distribution or inverse Compton scattering of soft seed thermal photons, which can yield the typical GRB spectrum but produce different levels of polarization. Furthermore, it can be used to learn about the outflow’s composition (i.e. whether it is kinetic-energy-dominated or Poynting-flux-dominated) and angular structure. For synchrotron emission, it is a powerful probe of the magnetic field geometry. In this talk, I will discuss synchrotron emission from a thin ultrarelativistic outflow and use a phenomenological pulse model to construct the energy-dependent temporal evolution of polarization for both coasting and accelerating flows. I will present results for a top-hat jet with sharp and smooth edges with observers having different viewing angles. I will then use the single pulse model to construct the polarization evolution for multiple overlapping pulses that arise due to episodic internal dissipation in the outflow. In the end, I will discuss how energy dependent polarization can be used to distinguish between different particle acceleration/heating scenarios in a magnetized outflow.

Speaker: Ramandeep Gill

16:20 → 16:35 Closing remarks

LucaFoffano_PASTO2022_conclusion_slides.pdf