An energetic dirty fireball detected in soft X-rays

TL;DR

This paper reports the discovery of an extragalactic fast X-ray transient, interpreted as a dirty fireball with a low Lorentz factor, expanding the understanding of energetic stellar explosions beyond traditional gamma-ray bursts.

Contribution

It provides the first observational evidence of a dirty fireball with baryon contamination, expanding the diversity of known relativistic stellar explosions.

Findings

Detected an extragalactic fast X-ray transient with high energy.

Modeling indicates a relativistic jet with Lorentz factor ~20.

Identified the transient as a prototype of a dirty fireball.

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 $\Gamma> 100$, 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 $\Gamma \sim 20$ aligned close to the line-of-sight, signifying the prototype of a dirty fireball.