TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1452
SUBJECT: GRB 020124, HST observations of the fading afterglow
DATE:    02/07/12 17:08:20 GMT
FROM:    Edo Berger at Caltech  <ejb@astro.caltech.edu>

J. S. Bloom, E. Berger, and S. R. Kulkarni (Caltech), on behalf of the
larger HST Collaboration, report:

"After thorough analysis of the three epochs of HST STIS/Clear imaging
observations of GRB 020124, we now find that the faint source reported in
Bloom et al. (GCN #1389) has faded between epoch 1 (11 Feb 2002 UT) and
epoch 3 (25 Feb 2002 UT).  This source ("S1"), is astrometrically
consistent with the rapidly fading optical transient reported from Palomar
200-inch imaging (Bloom, GCN #1225).  We therefore conclude that S1 is the
afterglow, rather than the host. The source was R=28.6+/-0.2 mag on Feb
11.09 UT, and fainter than R~29.2 mag on Feb 25.71 UT. See the associated
figure at:

  http://www.astro.caltech.edu/~jsb/GRB/grb020124/grb020124-hst.gif

This figure shows the first epoch (9972 sec) and the combined images from
18 Feb and 25 Feb (14836 sec) with the same flux scaling.

When combined with ground-based data, these observations reveal that the
rate of decay of the afterglow has steepened from a value of -1.6 to <-1.9
approximately 15 days after the burst.

Furthermore, we do not detect a persistent source (i.e. a host galaxy)
within 1.75 arcsec of the OT position down to R~29.5 mag.  Thus, a
positionally-coincident host galaxy of GRB 020124 is the faintest host to
date."

This message may be cited.