Short GRBs may not have Emission Below 100 keV

Old list of GRBs detected by Soviet-French instrument APEX onboard "Phobos-2" mission in 1988-89 has mainly long events. This sample conflicted with bimodal distribution of BATSE GRBs, which manifests the existence of two groups of long and short GRBs. Recent statistical re-analysis of spectral and temporal properties of 546 APEX triggering will be presented, which has allowed to identify a group of 28 events (with 16 new events), which are short APEX gamma-ray bursts. APEX had large CsI scintillator detector for the energy range 50 keV - 10 MeV. The method of Bayesian probabilities was used for selection of the best APEX candidates for short GRBs, which was based on statistics of emission time and hardness ratio parameters for classification of candidate events. 16 new short GRBs were found by this method, which had Bayesian probability >95% to belong to this group. The new list of 28 short and 46 long APEX GRBs will be presented. It was found that new sample of 74 APEX GRBs manifests the same bimodal distribution of emission time, as emission time distribution for 1779 BATSE GRBs. Direct comparison of 28 short APEX GRBs will be performed with the total list of triggers of another Soviet-French instrument LILAS on the same spacecraft. LILAS had two small detectors of cleaved NaI for the energy range above 6 keV. There are practically no short GRBs in the list of LILAS events, which is contributed by long GRBs only. LILAS was allowed to trigger for 25 of 28 short APEX bursts, but the actual triggering took place for only 3 of them. The flux of photons was approximated by the power low down from the APEX energy range into the LILAS energy range, and different efficiency of two instruments has also been taken into account. It was found that at least for 17 short APEX burst LILAS should be able to trigger, provided the power low approximation were valid. According to data from BATSE and PHEBUS, short GRBs have harder energy spectra than long GRBs. The absence of short APEX GRBs in the list of LILAS events shows that larger spectral hardness of short bursts is not resulted from stepper power low of the energy spectra. Instead, one may conclude from the comparison of APEX and LILAS lists, that short GRBs may not have emission below 100 keV at all.