Timetable

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Detailed program

Speaker: Dr. Ore Gottlieb

Title: From Birth to Blast: Tracing Progenitors to Central Engines

Abstract: It has long been known that GRB jets can be launched from either black holes or neutron stars. While some indirect observations have been used to infer the nature of the central engine, a direct connection between the central engine and GRB emission has yet to be established, preventing us from determining the central engine type from GRB observations. In the first part of this talk, I will focus on compact binary mergers and present a novel unifying theoretical framework that links binary neutron star and black hole-neutron star merger populations with the fundamental physics governing GRBs from compact binaries to infer the central engine type. In the second part, I will link collapsar progenitors with the central engines of collapsar jets and introduce a novel idea on how collapsar black holes acquire the strong magnetic fields necessary for powering long GRB jets.

Speaker: Dr. James Leung

Title: Nature or Nurture: revealing the jet properties and environments of gamma-ray bursts through radio observations

Abstract: Radio observations are powerful probes for understanding the afterglow of gamma-ray bursts (GRBs), the nature of the jets that produce them, and the environments that nurture them. Together, they reveal important insights about their progenitor systems: massive stellar collapses, binary coalescences, and possibly more. In this talk, I will present results from two recently concluded observing campaigns. (1) 400-day radio campaign following the most distant ultra-long GRB detected to date, GRB 220627A at z=3.084. The light curve reveals a secondary component in excess of predictions from the standard afterglow model. I will walk through a few physical scenarios considered to explain the excess. The GRB 220627A was also the first GRB to be followed with Very Long Baseline Interferometry (VLBI) to test a possible lensing hypothesis by directly imaging the multiple images. (2) 2000-day radio campaign of GRB 171205A/SN 2017iuk, including multi-epoch VLBI observations up until very late-times. GRB171205A is a very nearby low-luminosity GRB, with previous studies pointing to an off-axis geometry featuring a thermal cocoon component. I present results from our high-cadence, multi-scale, radio observations spanning 5 years, revealing key insights into the nature of this system, including the unprecedented details on the jet structure/geometry and the circumstellar environment. Despite the wealth of information that can be obtained from a well-sampled radio campaign, <10 GRBs in the 25+ years since the discovery of the first afterglow have had comprehensive radio follow-up, like for GRB 171205A. I will conclude the talk by discussing initial results from our PanRadio GRB program to comprehensively follow all Swift-localised GRBs in the Southern sky with the Australia Telescope Compact Array (ATCA) radio telescope from <1 day to years post-burst.

Speaker: Dr. Antonios Nathanail

Title: Short GRBs from Binary Neutron Star Mergers: Jet Breakouts, Ejecta Effects, and Disk Efficiency

Abstract: This talk explores the 3D magnetohydrodynamic simulations of a jet breaking out from the ejecta, confirming a hollow core structure and wide opening angle. These simulations are then used to model the afterglow emission observed in GRB170817A, revealing the observer angle and constraining some of the properties of the kilonova emission. Furthermore, we investigate the impact of inhomogeneous ejecta on the jet's properties. We find that while the jet structure near its axis remains largely unaffected, the high-latitude regions exhibit significant sensitivity to the ejecta's distribution. This has important implications for interpreting afterglow signals, particularly for off-axis observations. Finally, we analyze data from GW170817 to estimate the disk mass surrounding the BNS remnant and infer the accretion-to-jet efficiency. Our results suggest that a substantial portion of short GRBs may require unrealistically massive disks if the efficiency distribution observed in GRB170817A is universal. 

Speaker: Dr. Karelle Siellez

Title: Uncovering Neutron Stars Mergers through a new class of disguised Long Gamma Ray Bursts

Abstract: With the discovery of 170817, from the gamma ray burst [GRB] to its gravitational waves’ [GW] detections, through the Kilonova [KN] emission, and after 7 years of search, we are now, more than ever, eager to detect another event displaying those different multi-messengers emissions, including ideally the missing neutrinos. With the start of LIGO/Virgo/KAGRA, the GW detectors, in May 2023, coupled with the launch of SVOM, a franco-chinese GRB dedicated satellite in June 2024, and the use of the large-scale survey using the optical observatory Rubin in 2024, we are entering in the big data era. We need to understand which trigger to follow to be able to look for more Kilonovae leading us to solve the Hubble tension, and the physics of Neutrons Stars Mergers. Combining all informations on GRBs, I will show how long and short GRBs, especially those with Extended Emission, could lead us to more potential kilonovae using the right detectors and will introduce a correction for the intrinsic luminosity of short GRBs, as well as the rate of Neutron Stars Mergers, that could lead us to the next multi-messenger event.   

Speaker: Dr. Sayan Kundu

Title: Dynamics and afterglow emission properties of structured Gamma-ray burst jet

Abstract: Gamma-ray burst (GRB) afterglows are produced by collimated outflows of ultra-relativistic plasma. Recent work has shown an increasing role for jet structure in the modelling of these events, where the energy of the jet is not evenly distributed with angle, already from an early stage of jet evolution before lateral spreading. This in turn impacts the observed temporal slope of afterglow emission across the broadband. It also affects the spectra directly, in particular if the synchrotron cooling of shock-accelerated electrons is accounted for locally within the flow, revealing the hot zones of structured jets. To fully numerically model such jets and their emission characteristics across distance scales requires special relativistic hydrodynamics (RHD) methods. I will present the results from multi-dimensional simulations of structured GRB jets performed with our moving-mesh code, GAMMA. The emission from the jets is computed via a local cooling approach, where each computational cell possesses a distinct synchrotron cooling cutoff, and the collective contribution from these cells determines the overall emission profile. I will show how the interplay between jet structure, cooling, and jet orientation can be used to explore the diversity of afterglow observations, including off-axis observations and jets with delayed or absent jet breaks.

Speaker: Dr. Vikas Chand

Title: Realistic Predictions and Survey Strategies for Orphan GRB Afterglows Using a Semi-Analytic Model Calibrated by Numerical Simulations

Abstract: A gamma-ray burst (GRB) afterglow is considered an orphan when it is detected without a targeted search triggered by the prompt GRB emission. This can occur when the GRB jet points away from us or if the prompt emission along our line of sight is dim (e.g. a ``dirty fireball''). We present a semi-analytic model for the afterglow lightcurves based on and calibrated with numerical simulations. Such an approach better captures the peak time and flux for off-axis observers, as well as the shape of the lightcurves near the peak at different frequencies, which are the most relevant properties for transient surveys or targeted searches triggered by gravitational-wave detections. We use this model to calculate the rates of both single-epoch and multiple-epoch detections of orphan afterglows for different surveys as a function of their wavelength, sensitivity, and cadence. Additionally, we discuss an optimization scheme for observational strategies and potential methods for distinguishing orphan afterglows from other transients.

Speaker: Yonatan Lerner

Title: Fragmentation in collapsar disks

Abstract: We present a comprehensive study of fragmentation in collapsar disks, using a revised version of the Chen & Beloborodov (2007) model that determines the structure of steady state disks in a general relativistic and neutrino cooled framework. We conduct a parameter survey of gravitational instability alongside an exploration of the cooling time parameter β, which together determine if fragmentation will occur. We estimate the initial mass and density of fragments, finding that they occupy a unique region in the space of self-gravitating astrophysical objects, with masses M_frag ∼ (10^−3 − 10^−1)M⊙ and densities ρ_frag ∼ (10^6 − 10^11) g cm−3. We then calculate their migration and mass growth (via Bondi-Hoyle accretion) as they inspiral through the collapsar disk, where our calculation suggests mass growth by a factor between 2-6000. We speculate that these bound fragments may give rise to various astrophysical phenomena, including the variability observed in gamma ray bursts, the generation of non-vacuum gravitational waves accompanied by electromagnetic counterparts, and even the ejection of unique compact objects near or below the minimum cold neutron star mass.

Speaker: Prof. Zeljka Bosnjak

Title: TBD

Abstract: TBD

Speaker: Dr. Gor Oganesyan

Title: TBD

Abstract: TBD

Speaker: Dr. Mike Moss

Title: The Durations and Fluences of High-z GRBs are Underestimated

Abstract: In this study we investigated how Gamma-Ray Burst (GRB) prompt duration and fluence measurements are affected by increasing source redshift. We took bright GRBs observed by the Burst Alert Telescope on board the Neil Gehrels Swift Observatory (Swift/BAT) at redshifts z<1 and simulated what Swift/BAT would have observed if the GRBs were instead at higher redshifts (i.e., z>3), taking into account proper distance corrections and instrument response folding. We then used a Bayesian block algorithm to measure the duration of the simulated GRBs. We found that almost all durations and fluences measured for simulated high-z simulated GRBs are shorter than their observed durations when corrected for time dilation and luminosity distance, respectively.  The underestimations are due to low signal-to-noise burst emission being lost into the background, i.e., the so-called “tip-of-the-iceberg” effect. The amount of signal lost depends on light curve structure and brightness. We then compared the duration and fluence distributions of the GRBs we simulated at high-z to that of Swift/BAT GRBs observed at high redshifts and found that the two samples are consistent. These results imply that the GRB prompt duration and fluences of high-z GRBs observed by Swift/BAT are all underestimations.

Speaker: Dr. Mukesh Vyas

Title: Theory of photon scattering in shearing plasma: A novel mechanism to produce power-law spectra in GRBs

Abstract: We investigate a photon analogue of Fermi acceleration where a photon scatters with shearing layers of relativistic plasma and produces power-law-shaped spectra at high energies. It is an alternative to existing explanations of power law spectra such as synchrotron process or inverse Comptonization. Among several potential applications of this phenomenon, we explain the high energy spectra of Gamma-ray bursts (GRBs) prompt phase where we recover the range of their observed high energy photon indices by this mechanism.

Speaker: Dr. Rahul Gupta

Title: A detailed time-resolved and energy-resolved spectro-polarimetric study of bright GRBs observed using Fermi and AstroSat

Abstract: The radiation mechanism of the prompt emission is still an open issue and can be resolved using a systematic and uniform time-resolved spectro-polarimetric study. In this work, we performed a comprehensive investigation of the spectral, temporal, and polarimetric characteristics of five bright GRBs observed using the AstroSat CZTI, Fermi GBM, and Swift-BAT to provide insight into the prompt emission radiation mechanisms. These GRBs are detected by CZTI during its first year of operation, and the average polarisation characteristics of these GRBs are already published in Chattopadhyay et al. 2022. In the present work, we investigated the time-resolved (in 100-600 keV) and energy-resolved polarization measurements of these GRBs with an improved polarimetric technique, such as increasing the effective area and bandwidth (by using data from low-gain pixels), using an improved event selection logic to reduce noise in the double events and extend the spectral bandwidth. In addition, we also carried out detailed time-resolved spectral analyses of these GRBs using empirical and physical synchrotron models. By these improved time-resolved and energy-resolved spectro-polarimetric studies, we could pin down the elusive prompt emission mechanism of these GRBs. Our spectro-polarimetric analysis reveals that GRB 160623A, GRB 160703A, and GRB 160821A have a Poynting flux-dominated jet. On the other hand, GRB 160325A and GRB 160802A have a baryonic-dominated jet with mild magnetization. Furthermore, we observe a rapid change in polarization angle by ~ 90 degrees within the main pulse of very bright GRB 160821A, consistent with our previous results. Additionally, I will also discuss the spectro-polarimetric analysis of the second brightest burst, GRB 230307A, using data from Fermi and AstroSat. In this analysis, we discovered evidence of a transition from a Baryonic to a Poynting flux-dominated jet composition within the burst’s duration. Our study suggests that the jet composition of GRBs may exhibit a wide range of magnetization, which can be revealed by utilizing spectro-polarimetric investigations of the bright GRBs.

Speaker: Prof. Shigehiro Nagataki

Title: A Review of Simulations of Prompt Emission and Related Particle Acceleration 

Abstract: There is a long history of research on the mechanisms of prompt emission, and various theoretical studies and observations have been conducted. In this talk, I would like to present the current status of the theories of prompt emission, especially the current status of numerical simulations and prospects. Since particle acceleration is often considered to occur in the prompt and afterglow phases, I would like to briefly introduce the current status of numerical simulations of particle acceleration there.

Speaker: Dr. Arthur Charlet

Title: Internal hydrodynamical shocks as a mechanism for GRB prompt emission

Abstract: In the wake of recent analytical work highlighting the role as synchrotron emission sources of both the forward and reverse shock emerging from the collision of two ultra relativistic shells, we perform a numerical study of such a system and its observed flux. We precise the hydrodynamical structure, properties, and resulting emission timings by extending the approach to spherical geometry. We confirm the reproduction of GRB spectral features explained by the simpler analytical approach such as the sub-dominant low-energy spectral component and the doubly-broken power-law spectrum without any fine-tuning of the physical conditions, and lay the foundation to explore emission from internal shocks of more complex outflows.

Speaker: Dr. Sheikh Minhajur Rahaman

Title: Prompt phase of Gamma-ray bursts --Implications from internal shocks

Abstract: Internal shocks are one of the leading dissipation mechanisms for powering the prompt phase of Gamma-ray bursts (GRBs). The basic paradigm is that a central engine produces a variable outflow wherein the faster trailing material collides with the slower leading material at a distance from the engine. Each collision produces a pair of shock fronts -- a reverse (forward) shock front propagating in the faster (slower) material. In general, the physical conditions in both the shocked regions are very different. In my talk, I will demonstrate that with few basic central engine parameters, a self-consistent hydrodynamic solution of both shocked regions provides a predictive framework for understanding the spectra and the light pulses of prompt-GRBs. Besides, internal shocks in structured jets can provide a unifying theme for classical GRBs (CGRBs), X-ray riches (XRRs), and  X-ray flashes (XRFs). 

Speaker: Dr. Filip Alamaa Samuelsson

Title: TBD

Abstract: Photospheric radiation is a natural consequence of most GRB jet acceleration models and its potential role in the prompt emission has been studied extensively. To account for the observations, subphotospheric dissipation should occur before the photons decouple from the plasma. Due to the high radiation pressure, shocks below the photosphere are not collisionless but so called radiation-mediated shocks (RMSs), and the distinction is crucial for the resulting spectrum. In this talk, I present the first-ever fit of a prompt GRB spectrum with an RMS model. I also show that RMS spectra are in many ways similar to the observations, as they consist of a broad, soft power law across the sub-MeV-band with an additional break in X-rays. Therefore, photospheric radiation with properly modeled dissipation is a promising candidate for the prompt emission in GRBs. Finally, I present preliminary results regarding the predicted time-resolved signal from the photosphere. One such prediction is a correlation between the high and low-energy spectral indices at early times.

Speaker: Prof. Frederic Daigne

Title: The GRB prompt emission: recent results, open issues and perspectives

Abstract: The GRB prompt emission allows probing the properties of GRB relativistic ejecta. Many issues remain open regarding its physical origin. What is the nature and efficiency of the internal dissipation processes in GRB ejecta? What are the respective contributions emitted below and above the photosphere? What are the dominant radiative processes in the optically thin regime? What is the nature of the emission observed outside the standard soft gamma-ray range (visible, high or very-high energy gamma-rays)? What are the respective contributions of internal dissipation processes and of the ejecta deceleration (external forward, reverse shocks)?  I will discuss several of these issues, focusing on some recent observational and theoretical results, and look at the short-term prospects offered by some new instruments. 

Speaker: Ariel Perera

Title: Searching for Undetected Gamma Ray Bursts in Fermi GBM Data

Abstract: The recent discovery of simultaneous gravitational wave (GW) emission and short gamma-ray burst from a binary neutron star (BNS) merger has opened a new window to study such compact binary systems. The joint detection rate from this event is estimated at approximately 0.1 to 1.4 per year, with no other confirmed sources identified yet. In my research, I develop a GRB detection pipeline with rigorous statistical analysis to scan the Fermi GBM data, with the aim of increasing the search sensitivity. This will result in a larger number of known GRBs, and hence increased probability of another joint GRB-GW detection. This expectation is supported by a comparison of the detection probability of the pipeline and the significance of GBM triggering algorithm; our pipeline shows between 2 to 15 times higher signal-to-noise ratio, depending on source direction and intrinsic spectrum. Another feature of this pipeline is a statistical estimation of the location of the burst and discrimination between cosmological, terrestrial, and stellar origins. This allows for a robust classification of the signal, lowering the contamination of the sample. Our analysis includes operating the pipeline on ”time-slided” copies of the data, which allows exact significance assessment and pastro computation, akin to the state-of-the-art GW data analysis pipelines. This, along with a sky-map for the direction to the GRB, will facilitate a rigorous joint GW-GRB search on O3 and O4 LVK data.

Speaker: Dimitrios Skiathas

Title: Electromagnetic outflows of inspiraling binary neutron stars

Abstract: Broadband electromagnetic emissions originate from the magnetospheres of neutron stars. In the case of binary systems involving those objects, emissions are fueled by the dynamic interaction of their magnetospheres during the inspiraling motion of the neutron stars. These emissions, produced prior to the merger and the sGRB, could serve as precursor signals to those events. Such signals provide crucial information about the nature of the merging neutron stars, assisting in follow-up observations and their localization. Additionally, emissions redirected after interacting with interstellar gases and observed as echoes could potentially contribute to afterglow rebrightenings with a time delay. Understanding the patterns and characteristics of these pre-merger emissions requires a comprehensive exploration of the magnetospheres’ global electromagnetic field structure. Our study employs numerical simulations to thoroughly explore the multidimensional parameter space that involves the relative strengths and directions of the dipole moments, their direction relative to the orbital axis, and the star spin periods.  We calculate the Poynting flux and the gamma-ray emission patterns of the system in different cases, finding diverse, non-uniform, and non-symmetric distributions. Additionally, we investigate how the Poynting fluxes depend on the varying orbital angular frequency over time, revealing the diverse rates at which the magnetospheres lose energy. Finally, we discuss the potential implications of asymmetric outflows on the orbits and magnetic alignments of the neutron stars due to the corresponding kicks and torques. We also explore the expected echoes during the inspiral evolution, highlighting their potential contributions to our understanding of these complex systems.

Speaker: Prof. Ramandeep Gill 

Title: TBD

Abstract: TBD

Speaker: Dr. Gal Birenbaum

Title: Afterglow Linear Polarization Signatures from Shallow GRB Jets: Implications for GRB 221009A

Abstract: Gamma-ray bursts (GRBs) are powered by ultra-relativistic jets. The launching sites of these jets are surrounded by dense confining media, which the jets must cross before they can accelerate and release the high energy emission. Interaction with this medium leads to the formation of a mildly relativistic sheath around the jet and forms angular structures in the jet's asymptotic Lorentz factor and energy per solid angle, which modifies the afterglow emission. The processes that the jet undergoes prior to breakout from the confining medium are encoded within its energy angular profile and can be probed through its emission. We build a semi-analytical tool to analyze the light curve and polarization signatures of jets observed from a wide range of viewing angles, and focus on ones with slowly declining energy profiles known as shallow jets. We provide an analytical expression for the peak polarization degree as a function of the energy profile power-law index, magnetic field configuration and viewing angle, and show that it occurs near the light curve break time for all viewers. When applying our tool to GRB 221009A, suspected to involve a shallow jet, we find that the suggested jet structures for this event agree with the upper limits placed on its afterglow polarization in the optical and X-ray bands

Speaker: Prof. Haocheng Zhang

Title: Probing the prompt emission physics with MeV polarization variability

Abstract: GRB prompt emission physics remain unknown despite decades of efforts. Although it has been suggested that the prompt MeV polarization distribution of a large population of GRBs may tell apart a few simplified prompt radiation models, the radiation mechanism, particle acceleration, and jet physical evolution of individual GRBs cannot be determined in this way. Recent progress in numerical simulations suggest promising observable signatures that can help to probe the prompt emission physics via MeV polarization variability. Future MeV polarimeters such as COSI and POLAR-2 have the capability to resolve the temporal evolution of polarization for a couple of GRBs during their mission lifespan. This talk will share with you some preliminary studies of combined plasma simulations and radiation transfer that deliver MeV polarization variability predictions.

Speaker: Prof. Jianchao Sun

Title: GRB Polarimetry with POLAR-2

Abstract: Gamma-Ray Bursts (GRBs) are the most powerful cosmic explosions and thus probe some of the most extreme conditions known in nature, which are well beyond our reach on Earth. Although impressive progress has been achieved in our understanding of GRB physics over the last few decades, much of their inner workings are still poorly understood. In particular, many fundamental questions still remain unanswered, such as the outflow geometry and composition, the γ-ray emission mechanism, as well as the GRB jet launching, acceleration, collimation and energy dissipation. The key to answering many of these important open questions may lie in polarization measurements of the GRB prompt γ-ray emission, which are unique probes of the elusive γ-ray emission mechanism, as well as the magnetic field structure and outflow geometry. The fulfillment of this great promise still awaits, mainly because of the technical difficulties of measuring polarization at hard X-rays to soft γ-rays. However, this field is maturing and coming of age, with steady improvements in the instrumental sensitivity and reliability, and several planned space missions that hold great promise. One of the most promising among all the current or past experiments, is the GRB polarimeter POLAR onboard the China’s space lab Tiangong-2 launched in 2016. Given the exciting details and new insight of the GRB prompt emission polarization measurements by POLAR, its successor POLAR-2 experiment, with enhanced scientific potentials, was proposed and has been accepted for flight onboard the China Space Station. Here we will firstly present a summary of the POLAR mission and its GRB polarization measurement results, then give an overview on prospects for GRB polarimetry with the POLAR-2 experiment.

Speaker: Prof. Michela Negro

Title: High energy polarization from the Brightest of All Time

Abstract: GRB 221009A is an exceptionally bright gamma-ray burst (GRB) that reached Earth on 2022 October 9th after traveling through the dust of the Milky Way. The Imaging X-ray Polarimetry Explorer (IXPE) pointed at GRB 221009A on October 11th and measured, for the first time, the 2-8 keV X-ray polarization of both a GRB afterglow and rings of dust-scattered photons which are echoes of the GRB prompt emission. This observation provided constraints on the jet opening angle of the GRB and other properties of the emitting region, opening up a new observable to infer the physics of GRBs. In this contribution, I present the results of the IXPE observation of GRB 221009A and discuss future prospects for high-energy polarization in GRBs.

Speaker: Eliza Neights

Title: Studying GRBs Using COSI’s Polarization Measurements

Abstract: Our understanding of the physics of gamma-ray bursts (GRBs) and in particular, their early explosions producing the prompt emission, is still limited. New detection capabilities, such as polarization measurements, may provide additional information on the emission mechanisms, jet geometry, and magnetic field structure of GRBs. The Compton Spectrometer and Imager (COSI) is a wide field-of-view gamma-ray space telescope set to launch in 2027. COSI’s polarization capabilities, excellent energy resolution, and sub-degree localizations may reveal insight into GRB prompt emission. In this presentation, I will give a brief overview of the COSI mission and discuss the GRB science its polarization measurements will enable.

Speaker: Prof. Davide Miceli

Title: TBD

Abstract: TBD

Speaker: Yuri Sato

Title: Two-component jet model for multiwavelength afterglow emission of the very-high-energy gamma-ray burst GRB 221009A

Abstract: Recently gamma-ray bursts (GRBs) have been detected at very-high-energy (VHE) gamma-rays by imaging atmospheric Cherenkov telescopes, and a two-component jet model has often been invoked to explain multiwavelength data. In this talk, multiwavelength afterglow emission from an extremely bright GRB, GRB 221009A, is examined. The isotropic-equivalent gamma-ray energy of this event is among the largest, which suggests that similarly to previous VHE GRBs, the jet opening angle is so small that the collimation-corrected gamma-ray energy is nominal. Afterglow emission from such a narrow jet decays too rapidly, especially if the jet propagates into uniform circumburst material. In the two-component jet model, another wide jet component with a smaller Lorentz factor dominates late-time afterglow emission, and we show that multiwavelength data of GRB 221009A can be explained by narrow and wide jets with opening angles similar to those employed for other VHE GRBs. 

Speaker: Dr. Monica Barnard

Title: Probing the external Compton mechanism for the multiwavelength afterglow of the VHE-GRBs

Abstract: Over the past few years the field of gamma-ray burst (GRB) physics has been revolutionised after very-high energy (VHE; >100) emission was detected from at least seven GRBs by ground-based telescopes. These observations have challenged our understanding of the particle acceleration, emission mechanisms and blastwave evolution of GRB afterglow emission in this domain. We probe the VHE afterglows of GRB 180720B and GRB 190829A as external Compton (EC) radiation, considering the blastwave evolution in both the wind and interstellar medium scenarios. We also include both synchrotron and synchrotron self-Compton (SSC) emission, since the latter is primarily used to explain this VHE emission. For emission >100 GeV,  gamma-ray attenuation by absorption of photons through their interaction with the extragalactic background light needs to be taken into account, and we corrected the EC and SSC models accordingly. We predicted multiwavelength light curves and spectra of the above mentioned GRBs, assuming the Cosmic Microwave Background as the external radiation field. In our work we found that for a given set of fixed model parameters the EC gives a satisfactory fit for GRB 180720B, whereas SSC dominates the fit for GRB 190829A. For both GRBs a wind environment is preferred over constant density inter-stellar medium. By including the data of these extreme energies, we will apply our EC model to this VHE population and thereby obtain better limits on the GRB afterglow model parameters.

Speaker: Dr. Ankur Ghosh

Title: Multi-wavelength modelling of VHE-GRB afterglows using NAIMA

Abstract: In recent years, the detection of very high-energy gamma ray emission (VHE, $\geq$ 100 GeV) from a subset of GRBs has opened new avenues for GRB research. Understanding the temporal and spectral evolution of VHE GRB afterglows requires proper modeling and high cadence datasets across the electromagnetic spectrum (radio to VHEs). The standard afterglow emission from VHE GRBs encompasses a blend of synchrotron radiation from both forward and reverse shocks, synchrotron self-Compton (SSC) emission, and external Compton radiation. Until now SSC, assuming both the stellar wind and constant density circumburst scenarios, has been primarily used to interpret the VHE emission from GRBs. Hadronic emission mechanisms such as photo-pion and photo-pair processes, and proton synchrotron have also been considered in some cases. The publicly available NAIMA radiation modeling code includes several radiative models and the Markov chain Monte Carlo (MCMC) optimization tools to fit the multi-wavelength data and obtain the best-fit parameters. In our study, we extensively use the NAIMA code to model the energy spectra and light curve. Panchromatic data of seven VHE detected GRBs till date was acquired from the literature. Through fitting the observational data with the model, we constrain model parameters associated with the burst and surrounding environment. Our findings provide valuable insights into the the physical mechanisms driving the VHE emission in GRB afterglows. This study contributes to a deeper understanding of the future VHE emission in GRBs.

Speaker: Prof. Martin Lemoine

Title: Particle acceleration in GRBs

Abstract: This presentation will review our current understanding -- and discuss some recent topics of interest --  on particle acceleration in relativistic outflows, with applications to GRBs. More specifically, I will discuss in a first part the microphysics of the afterglow and in a second, the physics of Fermi-type particle acceleration (shocks, turbulence and shear) in the context of the prompt emission.

Speaker: Dr. Biswajit Banerjee

Title: The origin of the second spectral component of GRB 221009A

Abstract: Long-duration gamma-ray bursts (GRBs) are produced by ultra-relativistic jets that emerge immediately following the collapse of massive stars. Initially, we observe a highly variable MeV radiation lasting a few minutes, resulting from the internal dissipation within the jet. This is succeeded by an afterglow radiation lasting for several days, originating from the non-thermal electrons in the medium accelerated by the blast wave. The mechanisms of synchrotron and synchrotron self-Compton radiation are commonly used to explain the observed afterglow emissions observed in radio frequencies to TeV energies.  Yet so far, for GRBs detected in the HE gamma rays, distinctly identifying the TeV spectral component has been challenging due to the lack of sensitivity in MeV-GeV observations. In this talk,  I will  present the unique very-high-energy (VHE) spectral component from GRB 221009A observed during the initial 30 minutes. For the first time, the modelling of the data consisting of LAT in the initial time bins and AGILE in the late time for the GeV regime along with the TeV data from LHAASO provides stringent constraints on the magnetic field and the energy levels of the electrons accelerated in the relativistic shock. We also show that the intense prompt MeV radiation impacts the early TeV afterglow radiation through the external Compton cooling of electrons which also explains the absence of the variability in the early observation from LHAASO coinciding with the promt emission period observed with GBM.  

Speaker: Riki Matsui

Title: High-energy neutrinos from late-time jets of gamma-ray bursts seeded with cocoon photons

Abstract: In gamma-ray bursts (GRBs), ∼ 100 - 1000 s after the prompt emission, afterglow observations have consistently shown X-ray excesses detected in the form of flares (XFs; in long GRBs) or extended emission (EEs; in short GRBs). These observations are interpreted as emissions from jets launched by late central engine activity. However, the characteristics of these late-time jets, particularly the dissipation radius, Lorentz factor remain unknown despite their importance. In order to understand the properties of the late-time jets with future multi-messenger observations, we estimate the detectability of neutrinos associated with late-time emissions for a wide range of the dissipation radius and Lorentz factor. We take into account external seed photons from the cocoon around the jets, which can enhance the neutrino production through photohadronic interaction in the jet dissipation region. Our results are still consistent with the upper limit obtained by IceCube. Our calculations indicate a promising prospect for neutrino detection with IceCube-Gen2 through the stacking of ∼ 1000 − 2000 events, for a wide range of the dissipation radius and Lorentz factor. Furthermore, even in the case of non-detection, we show that meaningful constraints on the characteristics of the late-time jets can be obtained. This talk will include the discussion of an optimal energy threshold to reduce the noise of neutrinos. 

Speaker: Prof. Tsvi Piran

Title: Implications of the TeV emission from GRb 221009A

Abstract: TBD

Speaker: Dr. Clement Pellouin

Title: The VHE emission of structured GRB jets: GW 170817 and short merger-time BNS progenitors.

Abstract: Several GRB afterglows have now been observed at Very High energy (VHE, > 1 TeV). In addition, GW 170817 has allowed to probe the lateral structure of the associated relativistic jet. In this talk, I will present a complete numerical model of the afterglow of a laterally-structured relativistic ejecta from radio to VHE. This includes a self-consistent calculation of the synchrotron radiation, with its maximum frequency, and of Synchrotron Self-Compton (SSC) scatterings taking into account the Klein-Nishina regime. The attenuation due to pair production is also included. This model is computationally-efficient, allowing for multi-wavelength data fitting. While the SSC flux at the peak of GW 170817 was much dimmer than the H.E.S.S. upper limit, I show that smaller viewing angles or larger external densities would make similar off-axis events detectable in the future at VHE. Interestingly, large external densities are expected in the case of fast mergers, but the existence of a formation channel for such Binary Neutron Stars (BNS) is still uncertain. I will discuss how an evolutionary track including the accretion-induced collapse of a white dwarf into a neutron star could produce fast-merging BNSs, using results obtained with the population synthesis code COSMIC.