1998A&A...334..117S

We present the results of photometric observations of the variable optical source associated to the remnant of the gamma-ray burst GRB 970508 performed at the Special Astrophysical Observatory of the Russian Academy of Science (SAO RAS) from May to August 1997. The observations were carried out with the standard (Johnson-Kron-Cousins) photometric BVR_cI_c system using the 1-meter and 6-meter telescopes. The brightness of the optical source increased from R_c=21.19±0.25 (May 9.75 UT) to R_c = 19.70±0.03 (May 10.77 UT), whereupon it was decaying in all the four BVR_cI_c bands during about one month after the burst. Between the 3rd and the 6th night after the GRB 970508 event, the flux decrease of the optical source follows an exponential law in all the four bands. In that period the broad-band spectrum of the object does not change and can be approximately described by the power law F_ν∝ν^–1.1. The subsequent observations in BVR_cI_c bands have shown a reduction of the source brightness decrease rate. The source flux decay after the maximum in the R_c band is well described by a single power-law F ∝t^{–1.171(±0.012)} for the whole set of observations during 86 days. In the other bands the decay seems to slow down from the 31th day onwards. In particular, in the I_c band the source magnitude is about 23.1 from the 36th day after the GRB to the end of the observations (August 4, 1997).