The host galaxy and optical light curve of the gamma-ray burst GRB 980703

Abstract

We present deep HST/STIS and ground-based photometry of the host galaxy of the gamma-ray burst GRB 980703 taken 17, 551, 710, and 716 days after the burst. We find that the host is a blue, slightly over-luminous galaxy with V_gal = 23.00 +/- 0.10, (V-R)_gal = 0.43 +/- 0.13, and a centre that is ~0.2 mag bluer than the outer regions of the galaxy. The galaxy has a star-formation rate of 8-13 M_sun yr^-1, assuming no extinction in the host. We find that the galaxy is best fit by a Sersic R^1/n profile with n ~ 1.0 and a half-light radius of 0farcs13 (= 0.72 h₁₀₀^-1 proper kpc). This corresponds to an exponential disk with a scale radius of 0farcs22 (= 1.21 h₁₀₀^-1 proper kpc). Subtracting a fit with elliptical isophotes leaves large residuals, which suggests that the host galaxy has a somewhat irregular morphology, but we are unable to connect the location of GRB 980703 with any special features in the host. The host galaxy appears to be a typical example of a compact star forming galaxy similar to those found in the Hubble Deep Field North. The R-band light curve of the optical afterglow associated with this gamma-ray burst is consistent with a single power-law decay having a slope of alpha = -1.37 +/- 0.14. Due to the bright underlying host galaxy the late time properties of the light-curve are very poorly constrained. The decay of the optical light curve is consistent with a contribution from an underlying type Ic supernova like SN1998bw, or a dust echo, but such contributions cannot be securely established. Based on observations made with the NASA/ESA Hubble Space Telescope (HST), obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Based on observations made with the Nordic Optical Telescope (NOT), operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden. Based on observations made at the 2.2-m telescope of the German-Spanish Calar Alto Observatory.