Onset of Particle Acceleration during the Prompt Phase in Gamma-Ray Bursts as Revealed by Synchrotron Emission in GRB 160821A

Ryde, F.; Iyyani, Shabnam; Ahlgren, Björn; Pe’er, Asaf; Sharma, V.; Axelsson, M.

doi:10.3847/2041-8213/ac73fe

Cited by 7 publications

(2 citation statements)

References 74 publications

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“…For this burst, further independent indications corroborate that the emission is indeed synchrotron. First, the flare has a large polarization degree (Sharma et al 2019;Gill et al 2020; although see Lazzati et al 2004), and second, the direct fits with an actual synchrotron model yield good agreement to the data and reveal an accelerated electron distribution consistent with the theory of particle acceleration in weakly magnetized flows (Ryde et al 2022). Additional indirect arguments in favor of synchrotron origin for this burst are that the emission occurs late and has a long duration (Li et al 2021).…”

Section: Double Smoothly Broken Power Lawmentioning

confidence: 56%

Observational Characteristics of Radiation-mediated Shocks in Photospheric Gamma-Ray Burst Emission

Samuelsson,

Ryde

2023

ApJ

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Emission from the photosphere in gamma-ray burst jets can be substantially affected by subphotospheric energy dissipation, which is typically caused by radiation-mediated shocks. We study the observational characteristics of such emission, in particular the spectral signatures. Relevant shock initial conditions are estimated using a simple internal collision framework, which then serve as inputs for a radiation-mediated shock model that generates synthetic photospheric spectra. Within this framework, we find that if the free fireball acceleration starts at r 0 ∼ 1010 cm, in agreement with hydrodynamical simulations, then the typical spectrum consists of a broad, soft power-law segment with a cutoff at high energies and a hardening in X-rays. The synthetic spectra are generally well fitted with a standard cutoff power-law (CPL) function, as the hardening in X-rays is commonly outside the observable energy range of current detectors. The CPL-fits yield values for the low-energy index, α, and the peak energy, E peak, that are centered around ∼ −0.8 and ∼220 keV, respectively, similar to typical observed values. We also identify a nonnegligible parameter region for what we call optically shallow shocks: shocks that do not accumulate enough scatterings to reach a steady-state spectrum before decoupling and thereby produce more complex spectra. These occur for optical depths τ ≲ 55 u u − 2 , where u u = γ u β u is the dimensionless specific momentum of the upstream as measured in the shock rest frame.

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Section: Double Smoothly Broken Power Lawmentioning

confidence: 56%

Observational Characteristics of Radiation-mediated Shocks in Photospheric Gamma-Ray Burst Emission

Samuelsson,

Ryde

2023

ApJ

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“…However, the physical processes driving particle acceleration in the sGRB prompt phase and the related gamma-ray emission are unsettled. Specifically, sGRB observations are difficult to reconcile with optically thin synchrotron models (Mészáros et al 2001;Ajello et al 2019;Li 2020;Ryde et al 2022; however, see also Burgess et al 2019a). An alternative is provided by photospheric models of (modified) thermal spectra released as the outflow becomes optically thin (see, e.g., Ramirez-Ruiz 2005;Beloborodov & Mészáros 2017;Pe'er & Ryde 2017).…”

Section: Introductionmentioning

confidence: 99%

Subphotospheric Emission from Short Gamma-Ray Bursts: Protons Mold the Multimessenger Signals

Rudolph,

Tamborra,

Gottlieb

2024

ApJL

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The origin of the observed Band-like photon spectrum in short gamma-ray bursts (sGRBs) is a long-standing mystery. We carry out the first general relativistic magnetohydrodynamic simulation of an sGRB jet with initial magnetization σ 0 = 150 in dynamical ejecta from a binary merger. From this simulation, we identify regions along the jet of efficient energy dissipation due to magnetic reconnection and collisionless subshocks. Taking into account electron and proton acceleration processes, we solve for the first time the coupled transport equations for photons, electrons, protons, neutrinos, and intermediate particle species up to close to the photosphere (i.e., up to 1 × 1012 cm), accounting for all relevant radiative and cooling processes. We find that the subphotospheric multimessenger signals carry strong signatures of the hadronic interactions and their resulting particle cascades. Importantly, the spectral energy distribution of photons is significantly distorted with respect to the Wien one, commonly assumed below the photosphere. Our findings suggest that the bulk of the nonthermal photon spectrum observed in sGRBs can stem from hadronic processes occurring below the photosphere and previously neglected, with an accompanying energy flux of neutrinos peaking in the GeV energy range.

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Insights into the physics of gamma-ray bursts from the high-energy extension of their prompt emission spectra

Ravasio,

Ghirlanda,

Ghisellini

2024

A&A

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The study of the high-energy part (MeV-GeV) of the spectrum of gamma-ray bursts (GRBs) can play a crucial role in investigating the physics of prompt emission, but it is often hampered by low statistics and the paucity of GeV observations. In this work, we analyze the prompt emission spectra of the 22 brightest GRBs which have been simultaneously observed by Fermi/GBM and Fermi/LAT, spanning six orders of magnitude in energy. The high--energy photon spectra can be modeled with a power-law $N(E) beta $ possibly featuring an exponential cutoff. We find that, with the inclusion of the LAT data, the spectral index beta is softer than what is typically inferred from the analysis of Fermi/GBM data alone. Under the assumption that the emission is synchrotron, we derived a median value of the index $p 2.79$ of the power-law energy distribution of accelerated particles ($N( -p $). In nine out of 22 GRB spectra, we find a significant presence of an exponential cutoff at high energy, ranging between 14 and 298 MeV. By interpreting the observed cutoff as a sign of pair-production opacity, we estimate the jet bulk Lorentz factor Gamma , finding values in the range 130–330. These values are consistent with those inferred from the afterglow light curve onset time. Finally, by combining the information from the high-energy prompt emission spectrum with the afterglow light curve, we exploited a promising method to derive the distance $R$ from the central engine where the prompt emission occurs. The distances ($R > 10^ $ cm) inferred for the only two GRBs in our sample that are suitable for the application of this method, which have only lower limits on their cutoff energies, suggest large emitting regions, although they are still compatible with the standard model. Larger samples of GRBs with measured cutoff energies and afterglow deceleration time will allow for more informative values to be derived. These results highlight the importance of including high-energy data, when available, in the study of prompt spectra and their role in addressing the current challenges of the GRB standard model.

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Onset of Particle Acceleration during the Prompt Phase in Gamma-Ray Bursts as Revealed by Synchrotron Emission in GRB 160821A

Cited by 7 publications

References 74 publications

Observational Characteristics of Radiation-mediated Shocks in Photospheric Gamma-Ray Burst Emission

Observational Characteristics of Radiation-mediated Shocks in Photospheric Gamma-Ray Burst Emission

Subphotospheric Emission from Short Gamma-Ray Bursts: Protons Mold the Multimessenger Signals

Insights into the physics of gamma-ray bursts from the high-energy extension of their prompt emission spectra

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