Heterogeneity in Short Gamma-Ray Bursts

Dr. Jay P. Norris
Boise State University
Department of Physics

GRBs, briefly the most powerful explosions since the Big Bang, come in at least two types, as distinguished by their stellar-system progenitors.  Long bursts (duration ~> 2 s) -- widely believed to arise from core collapse of rapidly rotating massive, metal-poor stars -- are found preferentially internal to small irregular galaxies at early epochs, in regions with enhanced star formation. Where sub-arcsecond localizations are available for short bursts, they do not indicate such a preference.  Instead, nuclear offsets -- sometimes placing the source outside the galaxy -- seem to favor no particular galaxy type, nor epoch, but do favor a timescale consistent with coalescence of compact-object binary systems (~ 1 Gyr).  Whether a dichotomy in offsets is definitive -- clouded by the often less accurate short-burst localizations -- is unclear, but it does suggest that short bursts may arise from two kinds of compact-object systems.

Short bursts themselves do appear dichotomous:  three quarters are truly short, with durations <~ 2 s and robust evidence of absence in many cases of any gamma-ray component after the burst.  The other quarter are accompanied by a very low-level extended emission (EE) component, with duration ~ 100 s.  The two groups have markedly different prompt emission and X-ray afterglow timescales, and total energies:  median durations, pulse structure widths, and peak intervals for EE bursts are factors of ~ 2–3 longer than for non-EE bursts. The median flux of X-ray afterglows at initial detection time for EE bursts is > 20 times brighter than for non-EE bursts, and the median X-ray afterglow duration for EE bursts is ~ 30 times longer than for non-EE bursts.

The tendency for EE bursts toward longer prompt-emission timescales and higher initial X-ray afterglow fluxes implies larger energy injections powering the afterglows.  The longer-lasting X-ray afterglows of EE bursts suggest that a significant fraction explode into more dense environments than non-EE bursts, and/or that the sometimes-dominant EE component efficiently powers the X-ray afterglow.  All things considered:  Different progenitor systems are favored for EE and non-EE short bursts.