HST ACS Images of the Afterglow of GRB 021004

The optical transient and host galaxy of GRB 021004. As seen by HST/ACS on Oct 22nd (top left) and Nov 26th (top right) respectively. The position of the OT on its host galaxy is marked. This position is consistent with the peak of the light in the later image and hence we cannot rule out significant contamination from the OT at this time.

The field of GRB 021004 has been observed using the ACS/WFC on HST in four epochs: the 7,11 and 22 Oct and 26 Nov 2002.

A difference image of the two later epochs, which were dithered, reveals a declining point source whose position is consistent within the astrometric errors (~0."01) with the peak of the light in the later image. The source is marginally resolved in that image (with a FWHM of 0".11, compared to a point source FWHM of ~0."08) indicating a host galaxy directly underlying the OT. This differs from the astrometry of Mirabal et al (astro-ph/0303616) who find the OT position well offset from the host. As the source lies on the peak of the light, and the object is only marginally resolved, it is possible that the OT continues to contribute significantly to the observed brightness even at the later epoch. Hence the magnitude found for the source in November, F606W(AB)=24.3 +/- 0.1, is an upper limit on the true brightness of the host galaxy.

We have scheduled late time observations of GRB 021004 which will enable an accurate determination of both the host galaxy magnitude and the late time decay of the afterglow.

This result was reported as GCN 2238 .

A wide field taken with ACS/WFC in F606W. The field is approximately 45" across.

The same field of view in a subtracted image of the Oct22nd data minus the Nov 26th data, the lack of residuals (other than the optical transient) indicates that the astrometry bewteen the two images is accurate.