An energetic dirty fireball detected in soft X-rays

Einstein Probe (EP) is discovering a new population of fast and unusually soft X-ray transients whose physical origins remain unclear. In this talk, I will present the observations of EP241113a, an extragalactic fast X-ray transient discovered by EP. Classical gamma-ray bursts (GRBs) are produced by ultra-relativistic jets launched during massive-star collapses or compact-object mergers. EP241113a released an energy comparable to that of classical GRBs, but had a markedly lower spectral peak energy and was not detected in gamma-rays. Its X-ray light curve resembles the canonical afterglow light curves of GRBs, suggesting a similar origin from a relativistic jet. Modeling of its early X-ray afterglow indicates that the jet was viewed nearly on-axis and had a Lorentz factor of about 20, much lower than those of typical GRBs. We therefore interpret EP241113a as a prototype of the long-anticipated energetic “dirty fireballs,” in which greater baryon loading prevents the jet from accelerating to the extreme Lorentz factors typical of GRBs. I will also briefly discuss how such a baryon-loaded jet may naturally produce the unusually soft X-ray emission and what this event may tell us about the diversity of stellar explosions.