GRB 130925A: an ultra-long Gamma Ray Burst with a dust-echo afterglow, and implications for the origin of the ultra-long GRBs

摘要

GRB 130925A was an unusual GRB, consisting of 3 distinct episodes ofhigh-energy emission spanning $\sim$20 ks, making it a member of the proposedcategory of `ultra-long' bursts. It was also unusual in that its late-timeX-ray emission observed by Swift was very soft, and showed a stronghard-to-soft spectral evolution with time. This evolution, rarely seen in GRBafterglows, can be well modelled as the dust-scattered echo of the promptemission, with stringent limits on the contribution from the normal afterglow(i.e. external shock) emission. We consider and reject the possibility that GRB130925A was some form of tidal disruption event, and instead show that if thecircumburst density around GRB 130925A is low, the long duration of the burstand faint external shock emission are naturally explained. Indeed, we suggestthat the ultra-long GRBs as a class can be explained as those with lowcircumburst densities, such that the deceleration time (at which point thematerial ejected from the nascent black hole is decelerated by the circumburstmedium) is $\sim$20 ks, as opposed to a few hundred seconds for the normal longGRBs. The increased deceleration radius means that more of the ejected shellscan interact before reaching the external shock, naturally explaining both theincreased duration of GRB 130925A, the duration of its prompt pulses, and thefainter-than-normal afterglow.