BV RcIc light curves of GRB970508 optical remnant and colors of underlying host galaxy

Optical observations of the GRB970508 optical remnant were continued with the 6-m telescope of SAO RAS in standard the BV RcIc bands in Oct.-Dec. 1997 and in Jan. 1998. The results of photometry of the GRB970508 remnant and of three nearby galaxies are presented. The BV RcIc light curves of the GRB970508 remnant can be described by a power law plus a constant F (t) = F0(t− t0) +C. In determination of parameters of the faint host galaxy we used the results of our BV RcIc photometry of May-August, 1997, the data of recent observations with Keck-II and WHT telescopes and also the data of photometry of Rc and B bands obtained by other authors based on our secondary standards. The level-off from the initial power law decline seen in the first months after the burst was observed in all bands. The effect is the strongest in the Ic band: the difference on the last date of the Ic observations reaches ∼1.3 mags. The best χ2-fits for Fo, α, C parameters of the data in each of 4 bands point to the presence of a constant faint source with Ic = 24.13 ± 0.28, Rc = 25.55 ± 0.19, V = 25.80 ± 0.14, B = 26.68 ± 0.14. The average α is −1.23 ± 0.04. The optical remnant has a power law spectrum with a spectral slope equal to −1.10 ± 0.08 and does not change after the optical curve maximum. The BV RcIc spectrum together with the absolute magnitude of the constant component MBrest = −17.5 ± 0.3, and the linear size of the underlying host galaxy d ∼ 3 kpc conform to a host galaxy, such as a starburst dwarf, a red starburst dwarf, a irregular dwarf, a HII galaxy, or a blue compact dwarf galaxy. All these types of dwarf galaxies show evidence of starburst or post starburst activity. The galaxy G2 has a spectrum similar to that of the host GRB galaxy and lies at the projected distance of ∼ 20 kpc from GRB.