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

¹ Institute of Physics and Astronomy, University of Aarhus, 8000 Århus C., Denmark e-mail: bt@ifa.au.dk
² Department of Physics, University of Notre Dame, Notre Dame, IN 46556-5670, USA e-mail: sholland@nd.edu
³ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany e-mail: jfynbo@eso.org; pmoller@eso.org
⁴ Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark e-mail: jens@astro.ku.dk; holger@astro.ku.dk
⁵ Danish Space Research Institute, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark e-mail: jgu@dsri.dk
⁶ Division of Astronomy, University of Oulu, PO Box 3000, 90014 Oulu, Finland e-mail: manderse@sun3.oulu.fi
⁷ Department of Physics, Technion, Haifa 32000, Israel e-mail: arnon.dar@cern.ch
⁸ Science Institute, Dunhagi 3, University of Iceland, 107 Reykjavik, Iceland e-mail: gulli@raunvis.hi.is
⁹ European Southern Observatory, Casilla 19001, Santiago 19, Chile e-mail: ajaunsen@eso.org
¹⁰ Department of Astronomy, Yale University, New Haven, CT 06520-8181, USA e-mail: priya@astro.yale.edu
¹¹ Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Hertfordshire, AL10 9AB, UK e-mail: nrt@star.herts.ac.uk

Corresponding author: S. Holland, sholland@nd.edu

Received: 11 December 2000
Accepted: 27 February 2001

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 , , 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 , assuming no extinction in the host. We find that the galaxy is best fit by a Sersic profile with and a half-light radius of ( proper kpc). This corresponds to an exponential disk with a scale radius of ( 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 . 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.