Letter to the Editor

Massive star-formation rates of γ-ray burst host galaxies: An unobscured view in X-rays

¹ Niels Bohr Institute, Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark e-mail: darach@astro.ku.dk
² USRA (Universities Space Research Association), NSSTC, SD-50, 320 Sparkman Drive, Huntsville, AL 35805, USA
³ NASA Marshall Space Flight Center, NSSTC, SD-50, 320 Sparkman Drive, Huntsville, AL 35805, USA

Received: 12 July 2004
Accepted: 16 August 2004

Abstract

The hard X-ray (2–10 keV) luminosity of a star-forming galaxy tracks its population of high mass X-ray binaries and is essentially unobscured. It is therefore a practically unbiased measure of star-formation in the host galaxies of γ-ray bursts (GRBs). Using recent and archival observations of GRBs with the XMM-Newton and Chandra X-ray observatories, limits are placed on the underlying X-ray emission from GRB hosts. Useful limits on the current massive star-formation rates (SFRs), unaffected by obscuration, are obtained for the hosts of three low redshift GRBs: GRB 980425, GRB 030329 and GRB 031203. These limits show that though the specific SFRs may be high (as in dwarf starburst galaxies), none have massive obscured star-formation at the levels implied by the sub-mm detection of some GRB hosts. It is also shown that in cases where the faint luminosities of the late time afterglow or supernova emission are of interest, the contribution of the host galaxy to the X-ray flux may be significant.