GCN Circular 354

TITLE: Late-time HST Observations of GRB 990123
Subject: HST/STIS Observations of GRB 990123

A. Fruchter (STScI), S. Thorsett (Princeton), and E. Pian (ITESRE)
report for the HST GRB collaboration:

The field of GRB 990123 was reobserved by HST with the STIS CCD in open
filter (50CCD) mode during two orbits on 23 March 1999, or 59.5 days
after the GRB.  The total exposure time was 5040s.  The optical
transient was visible, but had declined by 2.1 +/- 0.2 mags from that
observed on 8 February 1999 by HST (Fruchter et al. 1999, Kulkarni et
al 1999), to V = 27.55 +/- 0.2 mags.

Both of the HST observations of the OT of GRB 990123 fall well below
the t^{-1.09} power-law behavior of the OT during the first three days
(Fruchter et al. 1999).  The 8 February observation lies more than 2
mags below the continuation of such a power-law.   Additionally, a
power-law fit of the flux density of the OT finds between the two
HST observations finds t^{-1.52 +/- 0.15}; this slope is nearly 3 sigma 
steeper than that found through day 3, but is also noticeably shallower than
that predicted from a break in the power-law due to either our now
observing the edge of a collimated outflow (Meszaros and Rees 1999) or
the sideways expansion of such an outflow (Kulkarni et al. 1999, Rhoads
1999).

The HST images from both epochs will be made available in gif format 
at http://www.stsci.edu/~fruchter/GRB/990123.  The reduced HST data 
from the first epoch are also available in fits format at this
site.   The reduced data of the second observation will be made
available later, in accordance with the HST GRB Collaboration
policy of making reduced data publicly available upon publication 
in a refereed journal.

References:

Fruchter, A. et al., 1999, Ap. J. (Letters), 519, L13.
Kulkarni, S. et al., 1999, Nature, 398, 389.
Meszaros, P. and Rees, M., 1999, MNRAS (submitted), astro-ph/9902367.
Rhoads, J., 1999, Ap. J. (submitted), astro-ph/990399.