3 Observations of GRB 970508 with CAM

GRB 970508 was observed on May 21 and May 24 1997 (Fig. 1) with CAM's LW10 filter ($8-15~\mu$m, reference wavelength 12 $\mu$m, assuming a 1/$\lambda$ spectrum). Observations made with the PHOT instrument will be presented elsewhere ([Hanlon et al. 1999]).

  

Figure 1: The central $45''\times45''$ region of the CAM LW10 mosaic image taken on May 24 overlaid with the HST STIS image from June 2. The images have been rotated to have North up and East left. RA and Dec scaling and LW10 pixel scaling are shown on the axes. The white "divets" are due to a flatfielding error in the STIS standard processing pipeline. The contour levels are: 2, 3 and 4 $\mu$Jy/arcsec2

The most significant ($\sim3\sigma$ above background) source which appears in this, the deepest region of the May 24 mosaic, is coincident with the OT associated with GRB970508. The astrometric pointing accuracy of ISO ($\sim5''$) has been improved by matching several sources in the combined May 21 and May 24 full-field mosaics with optical sources. The flux of the source is $\sim 60~\mu$Jy. The overall photometric error, including relative and absolute calibration uncertainties, is approximately 50%. From Fig. 1 it can be seen that the source is not coincident with either of the faint nearby galaxies G1 and G2 ([Pian et al. 1998]).

The source was not detected in the May 21 observation. The flux on May 21 can be inferred by extrapolating the observed flux on May 24, assuming a decay proportional to t^-1.1 days, as was seen in the R band (Galama et al. 1998a; Castro-Tirado et al. 1998). The 12 $\mu$m flux on May 21, extrapolated back from May 24 is $79\pm35~\mu$Jy, consistent with the measured $3\sigma$ upper limit of $60~\mu$Jy. We therefore cannot rule out the possibility that this is a constant source, rather than transient emission. However, the emission detected at 12 $\mu$m on May 24 is consistent with the extrapolated optical to radio spectrum which has been modelled as synchrotron radiation from a population of relativistic electrons, a significant fraction of which is cooled rapidly ([Galama et al. 1998b]; [Sari et al. 1998]).