Energy-dependent Spectral Lags of short GRBs detected by Fermi-GBM

The Gamma-ray Burst Monitor (GBM) on board the Fermi satellite has detected over 600 GRBs from its launch in June 2008 up to the end of 2010. Approximately 20% of these are short bursts based on the traditional classification of T90 durations less than 2 seconds. Many long-duration GRBs exhibit a spectral lag which is seen as high-energy γ-ray emission arriving earlier than photons in a low-energy γ-ray band. While long-duration GRBs may have lags ranging from zero to several tens of seconds, short GRBs exhibit negligible or zero lags between energy channels within the 25-1000 keV range of the BATSE Large Area Detectors. The spectral lag has therefore been proposed as a tool to assist in differentiating between long and short duration GRBs. Here we present the spectral lags of the sample of short GRBs detected by GBM in its first 2 years of operation, utilising data from the 12 NaI detectors and 2 BGO detectors on board. The wide energy coverage of GBM (8 keV-40 MeV) allows the dependence of the spectral lag on energies up to the MeV range to be investigated. There is some evidence to suggest that short GRBs may have measurable lags at such high energies which may indicate a requirement for an additional spectral component in these bursts.