A fast X-ray transient from a weak relativistic jet associated with a type Ic-BL supernova

TL;DR

This paper reports the discovery of a fast X-ray transient linked to a type Ic supernova, revealing a new class of Wolf-Rayet star explosions with weak, successful relativistic jets distinct from classical gamma-ray bursts.

Contribution

It introduces the first observation of a weak relativistic jet associated with a type Ic supernova, expanding understanding of stellar explosions and jet diversity.

Findings

Detected a bright, soft X-ray transient with unique spectral properties.

Follow-up confirmed a weak relativistic jet interacting with surrounding material.

Event suggests a new explosion class from Wolf-Rayet stars with less powerful engines.

Abstract

Massive stars end their lives as core-collapse supernovae, amongst which some extremes are broad-lined type Ic supernovae from Wolf-Rayet stars associated with long-duration gamma-ray bursts (LGRBs) having powerful relativistic jets. Their less-extreme brethren make unsuccessful jets that are choked inside the stars, appearing as X-ray flashes or low-luminosity GRBs. On the other hand, there exists a population of extragalactic fast X-ray transients (EFXTs) with timescales ranging from seconds to thousands of seconds, whose origins remain obscure. Here, we report the discovery of the bright X-ray transient EP240414a detected by the Einstein Probe (EP), which is associated with the type Ic supernova SN 2024gsa at a redshift of 0.401. The X-ray emission evolution is characterised by a very soft energy spectrum peaking at $< 1.3$ keV, which makes it different from known LGRBs, X-ray flashes, or low-luminosity GRBs. Follow-up observations at optical and radio bands revealed the existence of a weak relativistic jet that interacts with an extended shell surrounding the progenitor star. Located on the outskirts of a massive galaxy, this event reveals a new population of explosions of Wolf-Rayet stars characterised by a less powerful engine that drives a successful but weak jet, possibly owing to a progenitor star with a smaller core angular momentum than in traditional LGRB progenitors.