TITLE: GCN GRB OBSERVATION REPORT NUMBER: 31 SUBJECT: GRB970508 SAO RAS optical observations DATE: 98/03/26 22:08:00 GMT FROM: V. V. Sokolov at SAO RAS S. V. Zharikov, V. V. Sokolov, SAO RAS and Yu. V. Baryshev, St.Petersburg University report on behalf of the SAO RAS optical follow-up team: "Optical observations of the GRB 970508 optical remnant were continued with the 6-m telescope of SAO RAS in the standard B, V, Rc, Ic bands in October-December 1997 and in January 1998. We found: Rc (UT Oct.09.94, 1997) = 24.30+/-0.20 (integ., 2100sec.) Rc (UT Nov.10.04, 1997) = 24.70+/-0.15 (integ., 2400sec.) V (UT Nov.10.04, 1997) = 25.10+/-0.17 (integ., 2700sec.) Rc (UT Nov.25.97, 1997) = 24.70+/-0.14 (integ., 2500sec.) Ic (UT Nov.25.99, 1997) = 23.90+/-0.14 (integ., 4800sec.) V (UT Nov.26.06, 1997) = 25.50+/-0.35 (integ., 2100sec.) B (UT Dec.01.91, 1997) = 25.75+/-0.30 (integ., 2400sec.) Rc (UT Jan.24.87, 1998) = 24.96+/-0.17 (integ., 2580sec.) V (UT Jan.24.87, 1998) = 25.44+/-0.25 (integ., 2400sec.) B band images have been obtained by A. I. Kopylov. In determination of parameters of the faint host galaxy we used the results of our BVRcIc photometry of May-August, 1997 (see at http://xxx.lanl.gov/astro-ph/9802341), the data of recent observations with Keck-II (Bloom et al., 1998, GCN Circular #30) and WHT telescopes (Castro-Tirado et al., IAU Circular No. 6848), and also the data of Rc and B bands photometry obtained by other authors based on our secondary standards: Pedersen H. et al., 1997, astro-ph/9710322; Garsia M.R., et al., 1997, astro-ph/9710346; Galama T.J. et al., 1998, astro-ph/9802160. The level-off from the initial power-law decline seen in the first months after the burst is observed in all bands. The effect is the most strong in the Ic band: the difference at the last date of Ic observations achieves ~1.3 mags. The best X^2-fits of the data in each of 4 bands point to the presence of a faint constant source with Ic = 24.13 +/- 0.28, Rc = 25.55 +/- 0.19, V = 25.80 +/- 0.14, B = 26.68 +/- 0.14. The fitted magnitudes of the constant component and small angular size of underlying host galaxy (Pian et al. 1998, ApJ 492, L103) correspond to a dwarf elliptical galaxy with an absolute magnitude of M_B = -16.76 +/- 0.14 mags for z = 0.835 (Metzger et al. 1997, Nature 387, 879). Corresponding figures for all 4 photemetrical bands can be seen in http://www.sao.ru/~zhar/home/GRB/970508.html This message is quotable in publications."