Far-infrared observations of an unbiased sample of gamma-ray burst host
  galaxies
  Gamma-ray bursts (GRBs) are the most energetic phenomena in the Universe;
believed to result from the collapse and subsequent explosion of massive stars.
Even though it has profound consequences for our understanding of their nature
and selection biases, little is known about the dust properties of the galaxies
hosting GRBs. We present analysis of the far-infrared properties of an unbiased
sample of 20 \textit{BeppoSAX} and \textit{Swift} GRB host galaxies (at an
average redshift of $z\,=\,3.1$) located in the {\it Herschel} Astrophysical
Terahertz Large Area Survey, the {\it Herschel} Virgo Cluster Survey, the {\it
Herschel} Fornax Cluster Survey, the {\it Herschel} Stripe 82 Survey and the
{\it Herschel} Multi-tiered Extragalactic Survey, totalling $880$ deg$^2$, or
$\sim 3$\% of the sky in total. Our sample selection is serendipitous, based
only on whether the X-ray position of a GRB lies within a large-scale {\it
Herschel} survey -- therefore our sample can be considered completely unbiased.
Using deep data at wavelengths of 100\,--\,500$\,\mu$m, we tentatively detected
1 out of 20 GRB hosts located in these fields. We constrain their dust masses
and star formation rates (SFRs), and discuss these in the context of recent
measurements of submillimetre galaxies and ultraluminous infrared galaxies. The
average far-infrared flux of our sample gives an upper limit on SFR of
$<114\,{\rm M}\odot\,\mbox{yr}^{-1}$. The detection rate of GRB hosts is
consistent with that predicted assuming that GRBs trace the cosmic SFR density
in an unbiased way, i.e. that the fraction of GRB hosts with
$\mbox{SFR}>500\,{\rm M}\odot\,\mbox{yr}^{-1}$ is consistent with the
contribution of such luminous galaxies to the cosmic star formation density.
