X-ray Through Radio Observations and Modeling of the Soft X-ray Flash GRB 250419A
Abstract
Soft X-ray flashes (XRFs) are predicted from a variety of hypothesized phenomena, including "dirty fireballs" (relativistic jets with significant baryon loading), and gamma-ray bursts viewed marginally off-axis. For a given burst, detailed multiwavelength observations are necessary for distinguishing between possible progenitor scenarios, but the number of XRFs with such data remains small. Here we present X-ray, optical, and radio (mm to cm) data of GRB 250419A, an XRF (with peak energy Ep<4keV) discovered by the Space-based multi-band astronomical Variable Objects Monitor. We observe a clear achromatic break in the X-ray and optical bands, and likely two spectral components in the radio emission. The afterglow is similar to the long-duration gamma-ray burst population, but slightly underluminous, and the optical afterglow has an unusually shallow decay phase at ≲2d. We find that the event is well explained by a high-Lorentz factor jet with low kinetic energy (E∼1049-1050erg), with energy injection that could arise from radial stratification and/or the jet being viewed marginally off-axis. We conclude that at least some X-ray flashes are an extension of classical gamma-ray bursts down to lower energies.
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BibTeX
@misc{schroeder2026xray,
title = {{X-ray Through Radio Observations and Modeling of the Soft X-ray Flash GRB 250419A}},
author = {Genevieve Schroeder and Anna Y.Q Ho and Gavin P Lamb and Tanmoy Laskar and Daniel A Perley and Ore Gottlieb and Tomas Ahumada and Igor Andreoni and Aleksandra Bochenek and Michael Bremer and Michael Camilo and Jonathan Carney and S. Bradley Cenko and Alessandra Corsi and Michael W Coughlin and James Freeburn and Erica Hammerstein and Rahul Jayaraman and Mansi M Kasliwal and Noel Klingler and Brendan O'Connor and Cassie Sevilla and Gokul P Srinivasaragavan and Robert Stein and Jada L Vail and Sylvain Veilleux and Lin Yan},
howpublished = {HAL},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.00508},
eprint = {2610.00508},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05842235v1},
}
RIS
TY - UNPB
TI - X-ray Through Radio Observations and Modeling of the Soft X-ray Flash GRB 250419A
AU - Genevieve Schroeder
AU - Anna Y.Q Ho
AU - Gavin P Lamb
AU - Tanmoy Laskar
AU - Daniel A Perley
AU - Ore Gottlieb
AU - Tomas Ahumada
AU - Igor Andreoni
AU - Aleksandra Bochenek
AU - Michael Bremer
AU - Michael Camilo
AU - Jonathan Carney
AU - S. Bradley Cenko
AU - Alessandra Corsi
AU - Michael W Coughlin
AU - James Freeburn
AU - Erica Hammerstein
AU - Rahul Jayaraman
AU - Mansi M Kasliwal
AU - Noel Klingler
AU - Brendan O'Connor
AU - Cassie Sevilla
AU - Gokul P Srinivasaragavan
AU - Robert Stein
AU - Jada L Vail
AU - Sylvain Veilleux
AU - Lin Yan
PY - 2026
DA - 2026/10/01
PB - HAL
DO - 10.48550/arxiv.2610.00508
UR - https://hal.science/hal-05842235v1
AB - Soft X-ray flashes (XRFs) are predicted from a variety of hypothesized phenomena, including "dirty fireballs" (relativistic jets with significant baryon loading), and gamma-ray bursts viewed marginally off-axis. For a given burst, detailed multiwavelength observations are necessary for distinguishing between possible progenitor scenarios, but the number of XRFs with such data remains small. Here we present X-ray, optical, and radio (mm to cm) data of GRB 250419A, an XRF (with peak energy E_p<4keV) discovered by the Space-based multi-band astronomical Variable Objects Monitor. We observe a clear achromatic break in the X-ray and optical bands, and likely two spectral components in the radio emission. The afterglow is similar to the long-duration gamma-ray burst population, but slightly underluminous, and the optical afterglow has an unusually shallow decay phase at ≲2d. We find that the event is well explained by a high-Lorentz factor jet with low kinetic energy (E∼10^49-10^50erg), with energy injection that could arise from radial stratification and/or the jet being viewed marginally off-axis. We conclude that at least some X-ray flashes are an extension of classical gamma-ray bursts down to lower energies.
ER -