1998A&A...337..356Z

Optical observations of the GRB 970508 optical remnant were continued with the 6-m telescope of SAO RAS in standard the BVR_cI_c bands in Oct.-Dec. 1997 and in Jan. 1998. The results of photometry of the GRB 970508 remnant and of three nearby galaxies are presented. The BVR_cI_c light curves of the GRB 970508 remnant can be described by a power law plus a constant F(t)=F₀(t-t₀)^α+C. In determination of parameters of the faint host galaxy we used the results of our BVR_cI_c photometry of May-August, 1997, the data of recent observations with Keck-II and WHT telescopes and also the data of photometry of R_c 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 I_c band: the difference on the last date of the I_c observations reaches ∼1.3mags. The best χ²-fits for F_o, α, C parameters of the data in each of 4 bands point to the presence of a constant faint source with I_c=24.13±0.28, R_c=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 BVR_cI_c spectrum together with the absolute magnitude of the constant component M_Brest=-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 ∼20kpc from GRB.