An unusual stellar death on Christmas Day

TL;DR

This paper reports on the peculiar long-duration gamma-ray burst 101225A, proposing a helium star-neutron star merger as its progenitor, with detailed observations of its thermal and supernova-like emissions, and discusses alternative explanations.

Contribution

It introduces a new interpretation of GRB 101225A as a helium star-neutron star merger with unique thermal emission features, expanding understanding of stellar death mechanisms.

Findings

GRB 101225A had an exceptionally long gamma-ray emission.

Optical emission evolved as an expanding, cooling blackbody.

A faint supernova component was observed emerging after 10 days.

Abstract

Long Gamma-Ray Bursts (GRBs) are the most dramatic examples of massive stellar deaths, usually as- sociated with supernovae (Woosley et al. 2006). They release ultra-relativistic jets producing non-thermal emission through synchrotron radiation as they interact with the surrounding medium (Zhang et al. 2004). Here we report observations of the peculiar GRB 101225A (the "Christmas burst"). Its gamma-ray emission was exceptionally long and followed by a bright X-ray transient with a hot thermal component and an unusual optical counterpart. During the first 10 days, the optical emission evolved as an expanding, cooling blackbody after which an additional component, consistent with a faint supernova, emerged. We determine its distance to 1.6 Gpc by fitting the spectral-energy distribution and light curve of the optical emission with a GRB-supernova template. Deep optical observations may have revealed a faint, unresolved host galaxy. Our proposed progenitor is a helium star-neutron star merger that underwent a common envelope phase expelling its hydrogen envelope. The resulting explosion created a GRB-like jet which gets thermalized by interacting with the dense, previously ejected material and thus creating the observed black-body, until finally the emission from the supernova dominated. An alternative explanation is a minor body falling onto a neutron star in the Galaxy (Campana et al. 2011).