GIANT X-RAY BUMP IN GRB 121027A: EVIDENCE FOR FALL-BACK DISK ACCRETION

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 similar to 10(3) 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(4) s. 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 similar to 3.5x10(10) cm and mass of similar to 0.9-2.6M(circle dot) 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.