llar continuum. The [BPASS]{} models consider the instantaneous-burst and constant star formation rate cases. In both cases, we modify the templates using the @2000ApJ...533..682C dust extinction law. This was derived for local infrared-luminous galaxies with active star formation, and would appear appropriate in this case, given the bright infrared fluxes measured.

The photometry shows a challenging combination of a very flat optical spectrum (which implies little extinction and potentially even a non-stellar continuum) and evidence for strong dust emission in the infrared (see figure \[fig:bpass\]). The [BPASS]{} population synthesis models include a treatment of stellar evolution pathways through binary evolution. For young stellar populations, this treatment results in a relatively blue UV-optical continuum at a given age (and hence larger energy budget for heating dust) compared to models which neglect such pathways. To model the re-emission of thermal photons at longer wavelengths, we adopt the energy-balance prescription of @2008MNRAS.388.1595D, and re-emit the energy lost from the UV-optical as a combination of black body and PAH emission components. For the latter we scale the composite mid-infrared spectrum determined for star forming galaxies by @2007ApJ...656..770S. The W3 and W4 bands were excluded from the fitting procedure, in order to assess whether the observed UV-optical continuum were able to correctly predict the mid-infrared flux, or whether an additional, heavily dust-extincted component was required.

The best fitting single SED model to the host photometry is not one dominated by nebular emission or continuous star formation, but rather a post-starburst template, observed 500Myr after the initial starburst, as shown in figure \[fig:bpass\]. The derived stellar mass is log$_{10}$(M$_\ast$/M$_\odot$)=9.58$^{+0.12}_{-0.16}$, and a relatively low extinction of A$_V$=0.16$\pm$0.02 is required for the dominant stellar component. This model reproduces the GRB host galaxy’s ultraviolet and infr