1999A&A...344...43S

Recent observations of GRB970508 with the SAO-RAS 6-m telescope in July-August 1998 show that the optical transient (OT) has varied very little since November 1997. Here we report on the behaviour of the BVR_cI_c light-curves for up to 470 days from the date of the GRB occurrence. After ≃200 days any power-law decay has ceased and the OT contamination of the host galaxy flux is already less than the observational errors. We derive the broad band spectrum of the host galaxy, without OT, and compare it with the average continuum spectra of galaxies of different Hubble types. The spectral distribution of normal galaxies with types earlier than Sbc is confidently excluded. For H₀=60km/s/Mpc and three Friedmann models with matter density and cosmological constant parameters (Ω_m, Ω_Λ)=(1,0); (0,0); (0,1) the derived host galaxy absolute magnitudes (M_Brest_) are -18.0±0.3, -18.5±0.3 and -19.0±0.3 respectively. The absolute B magnitude ≃-18.5 corresponds to the luminosity L_B≃0.1L^*_gen where L^*_gen is the knee of the local general Schechter luminosity function. However, the luminosity of the GRB host is roughly at the knee of the local luminosity function L^*_late for late type galaxies Sd-Sm-Irr (Binggeli et al. 1988). Comparison of the BVR_cI_c spectral energy distribution of the GRB host with local starburst galaxies leads to best fits for the Scd starburst HII galaxy NGC 7793 and the blue compact galaxy Mrk 1267. The position of the host galaxy in the M_B vs. logD₂₅ diagram for local late-type LEDA (Lyon-Meudon Extragalactic database) galaxies allows us to attribute the GRB 970508 host galaxy to the blue compact galaxies.