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GCN Circular 712

Subject
GRB 990123: Late-time HST/STIS 50CCD observations of the host
Date
2000-06-17T02:51:46Z (24 years ago)
From
Andrew S. Fruchter at STScI <fruchter@stsci.edu>
A. Fruchter (STScI), S. Thorsett (UCSB), R. Wijers (SUNY) report for the
larger HST GRB collaboration:

Public HST observations in the red (LP) STIS filter should shortly be
available of the host galaxy of GRB 990123.  We therefore report here
on the results of late-time imaging of the host by our group using the
wide-band open (50CCD) filter.  The images described here can be found
at http://www.stsci.edu/~fruchter/GRB , and will be submitted to a
journal shortly.

The field of GRB 990123 was reobserved by HST with the STIS CCD in open
filter (50CCD) mode during two orbits on 7 February 2000, or 380 days
after the GRB.  The optical transient was no longer visible.   However
we have been able to use this image to further refine our earlier
photometry on the OT (Fruchter et al. 1999a,b).    We now find 
a V magnitude for the OT of 25.4 +/- 0.1 on 8 February 1999 and
V = 27.7 +/- 0.15  on 23 March 1999, implying that the late time
late-time light curve falls as t^{-1.7 +/- 0.1}.  This represents a
break of about 0.65 from the power-law decay a couple of days after
outburst, and thus is roughly consistent with the beaming model of
Meszaros and Rees (1999).  However, this also agrees within the errors
with the power-law of the early time light curve reported by ROTSE, and
this may suggest a connection between the early and late-time
emission.

The GRB is found to reside near the edge of the visible stellar field
of the host, and is not superposed on a strong region of star-formation.

References:

Akerlof, C. et al, 1999, Nature, 398 400.
Fruchter, A. et al., 1999, Ap. J. (Letters), 519, L13.
Fruchter, A. et al., 1999, GCN 354.
Meszaros, P. and Rees, M., 1998, MNRAS, 306L, 39.
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