GCN Circular 1135

GRB 010921: HST Detection of the Afterglow and Host Galaxy

J. S. Bloom, S. R. Kulkarni, F. A. Harrison, T. J. Galama, P. A. Price,
S. G. Djorgovski, D. W. Fox, E. Berger, and D. A. Frail report on behalf
the larger Caltech-NRAO-CARA GRB Collaboration:

"We observed the field of GRB 010921 (Ricker et al., GCN #1096
Not found
 
 
; Hurley et
al., GCN #1097
Not found
 
 
; Price et al., GCN #1107
Not found
 
 
) on 2001 October 26-27 UT using
the WFPC F450W, F555W, F606W, F814W, F850LP filters on the Hubble Space
Telescope for a total of 1200 s integration in each bandpass.  The
observations were conducted as part of the HST-GO Proposal #8867 (S.
Kulkarni, P.I.).

An astrometric comparison of the HST images with the Price et al.
discovery image from Palomar reveals that the transient position is
consistent (to better than 1 sigma) with a red compact source O.41 arcsec
to the East of the center of a galaxy. The Palomar localization is
inconsistent (at the 3-sigma level) with the center of the galaxy (note:
no correction has yet been made to the Palomar transient position in the
Palomar image for the contribution of the galaxy to the centroid position;
correcting for this effect would tend to push the Palomar transient
position further from the galaxy center).  Given that the red color of the
compact source is consistent with the Price et al. spectral slope and that
the localization is consistent, we identify this source as the afterglow
of GRB 010921 and the galaxy (z=0.45; Djorgovski et al. #1108) as the
host."

An image of the host and the optical transient may be found at:

   http://www.astro.caltech.edu/~jsb/GRB/grb010921-host-reg.ps

The 1 and 3-sigma uncertainty contours of the OT position measured from
the Palomar discovery images is shown.

Links to false-color images of the WFPC field and close-up images of the
OT-host system may be found at:

   http://www.astro.caltech.edu/~jsb/grb010921

Analysis of the broadband afterglow spectra is underway and more HST
observations are scheduled.

This message may be cited.