GCN Circular 715

Stephen Holland, Johan Fynbo, Bjarne Thomsen (University of Aarhus),
Michael Andersen (University of Oulu),
Gunnlaugur Bjornsson (University of Iceland),
Jens Hjorth (University of Copenhagen),
Andreas Jaunsen (University of Oslo),
Priya Natarajan (Universities of Cambridge, & Yale), and
Nial Tanvir (University of Hertfordshire)

	We have obtained 8224 seconds of STIS images with the F28X50LP
(long pass) aperture of the host galaxy of GRB 990123.  This data was
taken as part of the Survey of the Host Galaxies of Gamma-Ray Bursts
(Holland et al. GCN 698
Not found
 
 
) approximately 509 days after the burst.
Combined images are now available at
"http://www.ifa.au.dk/~hst/grb_hosts/data/index.html".

	Using the light curve fits of Holland et al. (2000, submitted
to A&A) we predict that the optical afterglow will have R = 31.6 on 15
June 2000, and thus will not be visible in the STIS images.
Therefore, we used aperture photometry to determine the AB magnitudes,
in the long pass filter, of the three knots found by Holland & Hjorth
(1999, A&A, 344, L67).  We find the following colours for the three
extended knots where CL is the AB magnitude in the STIS clear
aperture, LP is the AB magnitude in the STIS long pass aperture, and
beta is the corresponding spectral index, f = k*nu^beta.

Knot   CL     LP         CL-LP       beta
  1   28.3   28.2    +0.1 +/- 0.4    -0.4
  2   28.1   27.5    +0.6 +/- 0.4    -2.6
  3   28.0   27.5    +0.5 +/- 0.4    -2.2

       The GRB occurred on the southeast edge of Knot 1.  This knot is
approximately one sigma bluer than the other two knots, and two sigma
bluer than the overall colour of the galaxy (V-R = 0.43 +/- 0.18;
Castro-Tirado et al., 1999, Science 283, 2069).  This suggests that
Knot 1 might be undergoing stronger star formation than the rest of
the galaxy.