A new population of ultra-long duration gamma-ray bursts

TL;DR

This paper reports the discovery and analysis of a new class of ultra-long gamma-ray bursts, characterized by durations of thousands of seconds, unusual lightcurves, and host galaxies, suggesting a different progenitor mechanism.

Contribution

It identifies a new population of ultra-long GRBs, providing redshift measurements and detailed multiwavelength observations, and proposes their origin from larger stellar progenitors or tidal disruption events.

Findings

GRBs have redshifts of 0.677 to 1.773.

Host galaxies are faint, compact, star-forming dwarf galaxies.

These bursts exhibit unusual, highly variable X-ray and optical lightcurves.

Abstract

We present comprehensive multiwavelength observations of three gamma-ray bursts (GRBs) with durations of several thousand seconds. We demonstrate that these events are extragalactic transients; in particular we resolve the long-standing conundrum of the distance of GRB 101225A (the "Christmas-day burst"), finding it to have a redshift z=0.847, and showing that two apparently similar events (GRB 111209A and GRB 121027A) lie at z=0.677 and z=1.773 respectively. The systems show extremely unusual X-ray and optical lightcurves, very different from classical GRBs, with long lasting highly variable X-ray emission and optical light curves that exhibit little correlation with the behaviour seen in the X-ray. Their host galaxies are faint, compact, and highly star forming dwarf galaxies, typical of "blue compact galaxies". We propose that these bursts are the prototypes of a hitherto largely unrecognized population of ultra-long GRBs, that while observationally difficult to detect may be astrophysically relatively common. The long durations may naturally be explained by the engine driven explosions of stars of much larger radii than normally considered for GRB progenitors which are thought to have compact Wolf-Rayet progenitor stars. However, we cannot unambiguously identify supernova signatures within their light curves or spectra. We also consider the alternative possibility that they arise from the tidal disruption of stars by supermassive black holes.