Giant X-ray Bump in GRB 121027A: Evidence for Fall-back Disk Accretion

TL;DR

This paper reports a giant X-ray bump in GRB 121027A, interpreted as fall-back disk accretion from a collapsing massive star, indicating a significant helium envelope surviving prior mass loss.

Contribution

It introduces a fall-back accretion model to explain an unusual X-ray bump in a gamma-ray burst, linking it to stellar collapse and envelope survival.

Findings

X-ray bump re-brightening by over two orders of magnitude

Fall-back radius estimated at about 3.5e10 cm

Progenitor star's helium envelope likely survived mass loss

Abstract

A particularly interesting discovery in observations of GRB 121027A is that of a giant X-ray bump detected by the Swift/X-Ray Telescope. The X-ray afterglow re-brightens sharply at about 1000 s after the trigger by more than two orders of magnitude in less than 200 s. This X-ray bump lasts for more than 10 ks. It is quite different from typical X-ray flares. In this Letter we propose a fall-back accretion model to interpret this X-ray bump within the context of the collapse of a massive star for a long-duration gamma-ray burst. The required fall-back radius of about 3.5e10 cm and mass of about 0.9-2.6 solar masses imply that a significant part of the helium envelope should survive through the mass loss during the last stage of the massive progenitor of GRB 121027A.