New soft gamma-ray bursts in the BATSE records and spectral properties of X-ray rich bursts

A population of X-ray dominated gamma-ray bursts (GRBs) observed by Ginga, BeppoSAX and the High Energy Transient Explorer (HETE-2) should be represented in the Burst and Transient Source Experiment (BATSE) data as presumably soft bursts. We have performed a search for soft GRBs in the BATSE records in the 25-100 keV energy band. The softness of a burst spectrum could explain why it has been missed by the on-board procedure and by the previous searches for untriggered GRBs tuned to the 50-300 keV range. We have found a surprisingly small number (~ 20yr^-1 with fluxes down to 0.1 photon cm^-2s^-1) of soft GRBs where the count rate is dominated by the 25-50 keV energy channel. This fact, as well as the analysis of HETE-2 and common BeppoSAX/BATSE GRBs, indicates that the majority of GRBs with a low E_peak have a relatively hard tail with a high-energy power-law photon index β > -3. An exponential cutoff in GRB spectra below 10-15 keV may be a distinguishing feature of non-GRB events.