Hot gas around SN 1998bw: inferring the progenitor from its environment

Abstract

Spatially resolved spectroscopy of the environments of explosive transients carries detailed information about the physical properties of the stellar population that gave rise to the explosion, and thus the progenitor itself. Here, we present new observations of ESO184-G82, the galaxy hosting the archetype of the gamma-ray burst/supernova connection, GRB 980425/SN 1998bw, obtained with the integral field spectrograph MUSE mounted at the Very Large Telescope. These observations have yielded detailed maps of emission-line strength for various nebular lines along with physical parameters such as dust extinction, stellar age, and oxygen abundance on spatial scales of 160 pc. The immediate environment of GRB 980425 is young (5-8 Myr) and consistent with a mildly extinguished (A(V) similar to 0.1 mag) progenitor of zero-age main-sequence mass between 25 M-circle dot and 40 M-circle dot and an oxygen abundance 12 + log(O = H) similar to 8.2 (Z similar to 0.3 Z(circle dot)), which is slightly lower than that of an integrated measurement of the whole galaxy (12 + log(O = H) similar to 8.3) and a prominent nearby H II region (12 + log(O = H) similar to 8.4). This region is significantly younger than the explosion site, and we argue that a scenario in which the GRB progenitor formed in this environment and was subsequently ejected appears very unlikely. We show that empirical strong-line methods based on [O III] and/or [N II] are inadequate to produce accurate maps of oxygen abundance at the level of detail of our MUSE observation as these methods strongly depend on the ionization state of the gas. The metallicity gradient in ESO184-G82 is 0.06 dex kpc(-1), indicating that the typical offsets of at most few kpc for cosmological GRBs on average have a small impact on oxygen abundance measurements at higher redshift.