Year: 2026
Type: preprint
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.
Source: arXiv (Cornell University)
Authors: Schroeder, Genevieve, Ho, Anna Y. Q., Lamb, Gavin P., Laskar, Tanmoy, Perley, Daniel A., Gottlieb, Ore, Ahumada, Tomas, Andreoni, Igor, Bochenek, Aleksandra, Bremer, Michael, Camilo, Michael, Carney, Jonathan, Cenko, S. Bradley, Corsi, Alessandra, Coughlin, Michael W., Freeburn, James, Hammerstein, Erica, Jayaraman, Rahul, Kasliwal, Mansi M., Klingler, Noel, O'Connor, Brendan, Sevilla, Cassie, Srinivasaragavan, Gokul P., Stein, Robert, Vail, Jada L., Veilleux, Sylvain, Yan, Lin
Institutions: University of Toronto, Cornell University, Liverpool John Moores University, University of Utah, Cerro Tololo Inter-American Observatory, University of North Carolina at Chapel Hill, Institut de Radioastronomie Millimétrique, Goddard Space Flight Center, Joint Space Science Institute, University of Maryland, College Park, Johns Hopkins University, University of Minnesota, University of California, Berkeley, California Institute of Technology, Northwestern University, Massachusetts Institute of Technology
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Science Foundation, Alfred P. Sloan Foundation, Research Corporation for Science Advancement, Government of Canada, University of Leicester, Liverpool John Moores University, Max-Planck-Gesellschaft, Centre National de la Recherche Scientifique, Science and Technology Facilities Council
Award: G336187100
OpenAlex ID: W7216221813