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1. Generation of Large-scale Magnetic Fields Upstream of Gamma-Ray Burst Afterglow Shocks

   https://doi.org/10.3847/1538-4357/add404  

   Golant, Ryan; Vanthieghem, Arno; Grošelj, Daniel; Sironi, Lorenzo (June 2025, The Astrophysical Journal)

   Abstract The origins of the magnetic fields that power gamma-ray burst (GRB) afterglow emission are not fully understood. One possible channel for generating these fields involves the pre-conditioning of the circumburst medium: in the early afterglow phase, prompt photons streaming ahead of the GRB external shock can pair produce, seeding the upstream with drifting electron–positron pairs and triggering electromagnetic microinstabilities. To study this process, we employ 2D periodic particle-in-cell simulations in which a cold electron–proton plasma is gradually enriched with warm electron–positron pairs injected at mildly relativistic speeds. We find that continuous pair injection drives the growth of large-scale magnetic fields via filamentation-like instabilities; the temporal evolution of the field is self-similar and depends on a single parameter, ${\left(\alpha /\left(t_{\mathrm{f}}{\omega }_{\mathrm{pi}}\right)\right)}^{1/2}t{\omega }_{\mathrm{pi}}$ , whereαis the ratio of final pair beam density to background plasma density,t_fis the duration of pair injection, andω_piis the plasma frequency of background protons. Extrapolating our results to parameter regimes realistic for long GRBs, we find that upstream pair enrichment generates weak magnetic fields on scales much larger than the proton skin depth; for bright bursts, the extrapolated coherence scale at a shock radius ofR ∼ 10¹⁷cm is $\left({\lambda }_y\right)\sim 100c/{\omega }_{\mathrm{pi}}$ , and the corresponding magnetization isσ ∼ 10⁻⁸for typical circumburst parameters. These results may help explain the persistence of magnetic fields at large distances behind GRB shocks. 

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2. Prospects for Gamma-Ray Burst Afterglow Discovery with the Eric and Wendy Schmidt Observatory System

   https://doi.org/10.3847/1538-4357/ae8e6b  

   Freeburn, James; Dobie, Dougal; Anumarlapudi, Akash; Andreoni, Igor; O’Connor, Brendan; Burns, Eric; Carney, Jonathan; Corbett, Hank; Coughlin, Michael W; Goldstein, Adam; et al (August 2026, The Astrophysical Journal)

   Abstract Two time domain surveys, recently funded as part of the Eric and Wendy Schmidt Observatory System: the Argus Array, in the optical, and the Deep Synoptic Array (DSA), in the radio, will transform gamma-ray burst (GRB) science via the serendipitous discovery of hundreds of GRB afterglows per year. In this work, we simulate DSA and Argus observations of GRB afterglows. We find that, of the long-duration GRBs (LGRBs) detected by the Fermi Gamma-ray Burst Monitor, (26 ± 3)% will yield afterglow detections with Argus and (44 ± 3)% with DSA, corresponding to rates of 51 ± 5 and 86 ± 8 per year, respectively. We also compute rates for both upcoming and proposed GRB monitors: the forthcoming StarBurst Multimessenger Pioneer, with 84 ± 6 detections per year in Argus and 158 ± 11 detections per year in DSA, and the Moon Burst Energetics All-sky Monitor (MoonBEAM) concept, with 90 ± 9 per year in Argus and 160 ± 14 per year in DSA. The observatory system will also detect 118 ± 10 optical and 199 ± 17 radio afterglows per year, independent of GRB triggers, exceeding the current annual rate with global follow-up. Afterglow counterparts to short-duration GRBs, originating from neutron star mergers, will be detected at 5%–10% of the LGRB afterglow rate, which is promising for multimessenger detections of gravitational wave sources and constraining the neutron star merger rate. The Argus Array, with its second–minute cadence, will detect afterglows before they peak ∼18% of the time, which will dramatically increase the sample of observed reverse shock and prompt optical emission. 

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3. A Radio Flare in the Long-lived Afterglow of the Distant Short GRB 210726A: Energy Injection or a Reverse Shock from Shell Collisions?

   https://doi.org/10.3847/1538-4357/ad49ab  

   Schroeder, Genevieve; Rhodes, Lauren; Laskar, Tanmoy; Nugent, Anya; Rouco_Escorial, Alicia; Rastinejad, Jillian C; Fong, Wen-fai; van_der_Horst, Alexander J; Veres, Péter; Alexander, Kate D; et al (July 2024, The Astrophysical Journal)

   Abstract We present the discovery of the radio afterglow of the short gamma-ray burst (GRB) 210726A, localized to a galaxy at a photometric redshift ofz∼ 2.4. While radio observations commenced ≲1 day after the burst, no radio emission was detected until ∼11 days. The radio afterglow subsequently brightened by a factor of ∼3 in the span of a week, followed by a rapid decay (a “radio flare”). We find that a forward shock afterglow model cannot self-consistently describe the multiwavelength X-ray and radio data, and underpredicts the flux of the radio flare by a factor of ≈5. We find that the addition of substantial energy injection, which increases the isotropic kinetic energy of the burst by a factor of ≈4, or a reverse shock from a shell collision are viable solutions to match the broadband behavior. Atz∼ 2.4, GRB 210726A is among the highest-redshift short GRBs discovered to date, as well as the most luminous in radio and X-rays. Combining and comparing all previous radio afterglow observations of short GRBs, we find that the majority of published radio searches conclude by ≲10 days after the burst, potentially missing these late-rising, luminous radio afterglows. 

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4. First Detection of Faraday Rotation in a Gamma-Ray Burst Afterglow: Low Polarization and High Rotation Measure in GRB 260310A Reveal the Jet's Magnetic Structure and Environment

   https://doi.org/10.3847/2041-8213/ae80bc  

   Christy, Collin T; Laskar, Tanmoy; Alexander, Kate D; Franz, Noah; Granot, Jonathan; Chornock, Ryan; Margutti, Raffaella; Gill, Ramandeep; Pearson, Jeniveve; Berger, Edo; et al (July 2026, The Astrophysical Journal Letters)

   Abstract We report the detection of linear polarization in the radio afterglow of GRB 260310A, representing the first centimeter-wavelength polarization detection of a gamma-ray burst (GRB) afterglow and the first measurement of Faraday rotation in a GRB environment. We detect linearly polarized emission across 11–25 GHz, with a polarization fraction decreasing monotonically from (3.18 ± 0.18)% at 25 GHz to (0.69 ± 0.22)% at 11 GHz. We model the multiwavelength data as emission from a refreshed forward shock (FS) that dominates in the optical and X-rays and a reverse shock (RS) in a structured, relativistic jet that dominates at radio wavelengths. The observed depolarization toward the lower radio frequencies is consistent with suppression by RS synchrotron self-absorption, while the low observed polarization at high frequencies relative to the theoretical maximum suggests a patchy magnetic field in the jet with a coherence scale,θ_B ≈ 10⁻²rad. We identify a frequency-dependent rotation of the polarization angle consistent with Faraday rotation, with a rotation measure (RM) of RM = −(8300 ± 90) rad m⁻²at the GRB redshift. The magnitude of the RM is consistent with propagation through a dense, magnetized environment, such as a progenitor HIIregion. These findings demonstrate that GRB afterglows exhibit measurable linear polarization at centimeter wavelengths, and that their polarimetric properties probe both intrinsic jet magnetization and the surrounding medium. Future multifrequency polarimetric monitoring over timescales of days to weeks will enable detailed studies of the evolution of magnetic field structure and provide new constraints on the role of magnetic fields in GRB afterglows. 

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5. Two-component jet model for multiwavelength afterglow emission of the extremely energetic burst GRB 221009A

   https://doi.org/10.1093/mnrasl/slad038  

   Sato, Yuri; Murase, Kohta; Ohira, Yutaka; Yamazaki, Ryo (March 2023, Monthly Notices of the Royal Astronomical Society: Letters)

   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 work, 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. We also discuss how model degeneracies can be disentangled with observations. 

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