We examined the properties of a sample of BATSE Gamma–Ray Bursts ( GRBs ) comprising events which have indications of association with a supernova ( SN ) , some on the basis of indications of re–brightening in the optical afterglow light curve , but in most cases based only on the ‘ loose ’ temporal and directional coincidence inferred from the cross correlation of catalogs .
Despite of the large uncertainties in the latter selection method , the temporal and spectral analysis reveal three interesting statistical results when the sample is compared with that of all the BATSE GRBs : the GRBs tentatively associated with SNe are found to predominantly ( in \sim 80 % of the cases ) have single-peaked light curves , a softer spectrum ( i.e .
low energy power law index \alpha \sim –1.5 ) and tend not to follow the Lag-Luminosity and Isotropic Energy–Peak Energy correlations .
These three independent statistical properties point toward the existence of a significant number of under-luminous , GRB 980425-like events constituting – at least from an observational point of view – a tail or a separate class with respect to the whole of the BATSE GRB events .
The unusually high percentage of SN Ibc among those identified by the catalog cross–correlation ( factor \sim 4 higher than expected from SN catalog statistics ) reinforces the non-randomness of ( some of ) the selected events .