TITLE:   GCN CIRCULAR
NUMBER:  752
SUBJECT: GRB 990712: Late time HST/STIS Observations
DATE:    00/07/23 21:05:09 GMT
FROM:    Andrew S. Fruchter at STScI  <fruchter@stsci.edu>

A. Fruchter (STScI), P. Vreeswijk (UvA),  R. Hook (ST-ECF) and E. Pian
(OA Trieste) report for a larger HST GRB collaboration:

We have observed the field of GRB 990712 with the HST STIS CCD on April
24, 2000, 287 days after outburst.  Observations with total exposure
times of 3720 s were taken in both the (50CCD) clear aperture and (LP)
long pass filter.   We have compared these images with those taken of
this GRB and its host galaxy with the same instrumentation 48 days
after outburst (Fruchter et al. 2000).

We find that the optical transient (OT) associated with the GRB is
located on the bright red knot towards the southeastern end of this
elongated galaxy.  Thus the OT is not associated with the faint blue
knot toward the northwest end of the galaxy as was claimed in the
literature on the basis of the first HST observation alone (Hjorth et
al. 2000).

After subtracting the images from the two epochs we find an OT
magnitude on August 29, 1999 of R=24.35 +/- 0.15, where the error is
dominated by the uncertainty due to the wide STIS filters.   If the OT
were to have fallen between the two epochs as a simple power-law of
t^{-1} (Sahu et al. 2000) this would have oversubtracted the OT by
about 0.15 mags.  However, if a supernova provided a significant
contribution at the first epoch, there would be little
oversubtraction.

The color of the OT at day 48 is quite red, with a ratio of counts
between LP and 50CCD of 0.73 +/- 0.1.  If the OT were indeed dominated by a
supernova of color similar to that of SN1998bw, we would have expected
a ratio about 1 sigma bluer, and an OT of the colors reported by Sahu
et al. (2000) would be about 1 sigma bluer still.  In any event, as
noted in our previous GCN, a supernova would have to be at least one
magnitude fainter than SN1998bw to agree with these observations.

The host galaxy is approximately 1.5" in length, and has an ellipticity
of ~0.5.  The R magnitude of the host is found to be 21.95 +/- 0.15 in
good agreement with Sahu et al. (2000).   The color of the galaxy is
seen to vary across the stellar disk, with perhaps some evidence for a
dust lane near the galaxy's center.

The images are displayed at:  http://www.stsci.edu/~fruchter/GRB/990712 .

Fruchter, A., et al. 2000, GCN 565 
Hjorth, J., et al. 2000, ApJ, 534, L147
Sahu, K., et al. 2000, ApJ, in press (astro-ph/0003378)