ISO detection of a 60 μm source near GRB 970508

The Infrared Space Observatory observed the field of the γ-ray burst GRB 970508 with the CAM and PHT instruments on May 21 and 24, 1997 and with PHT in three filters in November 1997. A source at 60 μm (flux in May of 66 ± 10 mJy) was detected near the position of the host galaxy of this γ-ray burst. The source was detected again in November 1997, at a marginally lower flux (43 ± 13 mJy). A Galactic cirrus origin and a stellar origin for the emission can be ruled out on the basis of the infrared colours. The marginal evidence for variability in the 60 μm flux between May and November is not sufficient to warrant interpretation of the source as transient fireball emission. However, the infrared colours are physically reasonable if attributed to conventional dust emission from a single blackbody source. The probability of detecting a 60 μm source by chance in a PHT beam down to a detection limit of 50 mJy is ∼ 5 × 10-3. If the source is at the redshift of the host galaxy of the γ-ray burst the fluxes and upper limits at wavelengths from 12 μm to 170 μm indicate it is an ultraluminous infrared galaxy (Lir ∼ 2 × 1012 L⊙). The star formation rate is estimated to be several hundred solar masses per year, depending significantly on model-dependent parameters. If this source is associated with the host galaxy of GRB 970508, progenitor models which associate GRBs with star-forming regions are favoured.