Outliers from the mainstream: how a massive star can produce a gamma-ray burst

TL;DR

This paper investigates the specific stellar conditions, particularly low metallicity and rapid rotation, necessary for massive stars to produce gamma-ray bursts, supported by X-ray spectral evidence of metal-enriched material.

Contribution

It provides observational evidence that both low metallicity and fast rotation are essential for a massive star to generate a gamma-ray burst, refining the understanding of GRB progenitors.

Findings

Evidence of metal-enriched material ejected before explosion

Only stars with low metallicity and rapid rotation produce GRBs

Supports the link between stellar properties and GRB production

Abstract

It is now recognized that long-duration Gamma-Ray Bursts (GRBs) are linked to the collapse of massive stars, based on the association between (low-redshift) GRBs and (type Ic) core-collapse supernovae (SNe). The census of massive stars and GRBs reveals, however, that not all massive stars do produce a GRB. Only ~1% of core collapse SNe are able to produce a highly relativistic collimated outflow, and hence a GRB. The extra crucial parameter has long been suspected to be metallicity and/or rotation. We find observational evidence strongly supporting that both ingredients are necessary in order to make a GRB out of a core-collapsing star. A detailed study of the absorption pattern in the X-ray spectrum of GRB060218 reveals evidence of material highly enriched in low atomic number metals ejected before the SN/GRB explosion. We find that, within the current scenarios of stellar evolution, only a progenitor star characterized by a fast stellar rotation and sub-solar initial metallicity could produce such a metal enrichment in its close surrounding.