Hot gas around SN 1998bw: Inferring the progenitor from its environment
  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, as well as 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 ($A_V\sim0.1\ \mathrm{mag}$) progenitor of zero-age
main-sequence mass between 25 $M_{\odot}$ and 40 $M_{\odot}$ and oxygen
abundance 12+log(O/H)~8.2 ($Z\sim0.3\ {Z}_\odot$), which is slightly lower than
the one of an integrated measurement of the galaxy (12+log(O/H)~8.3) and a
prominent nearby HII region (12+log(O/H)~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 [OIII] and/or
[NII] 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.
