X-ray Through Radio Observations and Modeling of the Soft X-ray Flash GRB 250419A

Genevieve Schroeder; Anna Y. Q. Ho; Gavin P. Lamb; Tanmoy Laskar; Daniel A. Perley; Ore Gottlieb; Tomas Ahumada; Igor Andreoni; Aleksandra Bochenek; Michael Bremer; Michael Camilo; Jonathan Carney; S. Bradley Cenko; Alessandra Corsi; Michael W. Coughlin; James Freeburn; Erica Hammerstein; Rahul Jayaraman; Mansi M. Kasliwal; Noel Klingler; Brendan O'Connor; Cassie Sevilla; Gokul P. Srinivasaragavan; Robert Stein; Jada L. Vail; Sylvain Veilleux; Lin Yan

arXiv · 2610.00508

X-ray Through Radio Observations and Modeling of the Soft X-ray Flash GRB 250419A

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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 $E_p<4\,$keV) 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 $\lesssim2\,$d. We find that the event is well explained by a high-Lorentz factor jet with low kinetic energy ($E\sim10^{49}$-$10^{50}\,$erg), 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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Genevieve Schroeder, Anna Y. Q. Ho, Gavin P. Lamb, Tanmoy Laskar, Daniel A. Perley, Ore Gottlieb, Tomas Ahumada, Igor Andreoni, Aleksandra Bochenek, Michael Bremer, Michael Camilo, Jonathan Carney, S. Bradley Cenko, Alessandra Corsi, Michael W. Coughlin, James Freeburn, Erica Hammerstein, Rahul Jayaraman, Mansi M. Kasliwal, Noel Klingler, Brendan O'Connor, Cassie Sevilla, Gokul P. Srinivasaragavan, Robert Stein, Jada L. Vail, Sylvain Veilleux, Lin Yan. 2026-09-30. X-ray Through Radio Observations and Modeling of the Soft X-ray Flash GRB 250419A. https://arxiv.org/abs/2610.00508

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