Optical observations of GRB afterglows : GRB 970508 and GRB 980326 revisited ⋆ , ⋆ ⋆

Since January 1997, we have monitored 15 GRB fields, detecting 6 optical/IR afterglows. We have revisited GRB 970508 and GRB 980326. For GRB 970508, we derive a power-law decay exponent α =-1.19 (R-band). The luminosity of the host galaxy L relative to the characteristic luminosity L ⋆ is in the range 0.06–0.15, i.e. a dwarf galaxy. For GRB 980326, we derive a power-law decay exponent α =-1.7, taking into account the new upper limit for the host as R ≥ 27.3 provided by Bloom and Kulkarni (1998). This implies one of the fastest GRB optical decays ever measured. The fact that only about 50 % of optical transients have been found within the γ/X-ray error boxes, suggest that either considerable intrinsic absorption is present or that some optical transients display a very fast decline. We also propose that the ′′ secondary maximum ′′ detected on 17 Apr 1998 could be explained in the context of the ′′ SN-like ′′ light curves 2-3 weeks after the GRB, as recently suggested by Woosley (1999).