An energetic dirty fireball detected in soft X-rays

Abstract

The collapse of massive stars drives explosions that power relativistic fireballs. If only a small amount of matter is entrained, such clean fireballs can expand with Lorentz factors $\hat{I}“>100$, accounting for gamma-ray bursts (GRBs). It has been hypothesized that energetic explosions with more baryon contamination, dubbed ``dirty fireballs'', may exist in nature, but they have not been observed. Here we report the observation of an extragalactic fast X-ray transient, EP241113a, detected by Einstein Probe. Compared to GRBs, it has a similar isotropic energy of $1.4\times 10^{51}$ erg, but significantly lower spectral peak energy. Theoretical modeling of its early X-ray afterglow suggests a relativistic jet with a low Lorentz factor of $\hat{I}“\sim 20$ aligned close to the line-of-sight, signifying the prototype of a dirty fireball.

61 pages, 14 figures. Manuscript under consideration at Science. In accordance with the journal's policy, this version is the original version submitted by the authors and does not reflect any revisions made in response to peer review