TITLE:   GCN CIRCULAR
NUMBER:  751
SUBJECT: GRB 991216, HST/STIS observations
DATE:    00/07/17 09:18:02 GMT
FROM:    Paul Vreeswijk at U of Amsterdam  <pmv@astro.uva.nl>

P.M. Vreeswijk, A. Fruchter, H. Ferguson and C. Kouveliotou report for
a larger HST GRB Collaboration:

The afterglow of GRB 991216 (c.f. Kippen et al. 1999 ; Uglesich et
al. 1999) was observed using HST/STIS at approximately UT 2000 April
17.6 through the clear (50CCD) and long pass (LP) filters, each for a
total of 4790s.  The pipeline reduced images were drizzled onto output
images with pixels one-half native scale, or approximately 0."025 on a
side.
	
We have projected the OT position from an early VLT image taken 1.5
days after the burst, to the frame of the HST drizzled images. Four
bright nearby reference stars were used, and the estimated error in
the resulting position is 0."1, corresponding to 4 pixels.

The position coincides with the visible extent of a faint galaxy,
presumably the host of GRB 991216. The galaxy appears irregular, with
a diameter of about 0."3. Another, probably separate, faint galaxy is
located 0."4 to the southwest of the afterglow position.  These two
objects may explain the presence of two MgI absorption line systems in
the VLT spectrum of 991216 (Vreeswijk et al. 1999), while one of the
other galaxies visible at a separation of about 2" could potentially
be responsible for the third absorption line system.

Using an aperture of diameter 0."4, we measure R=26.9 +/- 0.2 for the
probable host of GRB 991216. The galaxy to the southwest has R = 26.1
+/- 0.2 inside an aperture of diameter 0."6.  The large errors reflect
the difficulty of matching the colors of these objects -- all objects
in the field are quite red, perhaps indicating that the foreground
extinction is even higher than the A(R)=1.6 mags predicted by the
Schlegel et al. (1998) model.  Additionally, it is probable that the
small apertures used underestimate the total magnitudes of these
galaxies by at least a couple of tenths.

The transient afterglow may still be present in these observations,
but the low signal to noise does not allow unambiguous identification
of the bright patch at the edge of the galaxy as a point source.  We
estimate that any remaining OT is no brighter than R=27.6.  Assuming
the single power law decay index, alpha = -1.36, of Garnavich et
al. (2000), the afterglows expected magnitude at the time of our
observations is R ~ 27 (not corrected for Galactic extinction). Our
observations therefore suggest a break in the light curve, as already
inferred by Halpern et al. (2000). A supernova of type SN1998bw at a
redshift of z=1.02 would have R>30 at the epoch of our observations.

Images of the host and surrounding region can be found at:
http://www.stsci.edu/~fruchter/GRB/991216

References:

Garnavich et al. 2000, ApJ, in press, preprint astro-ph/0003429
Halpern et al. 2000, ApJ, in press, preprint astro-ph/0006206
Kippen et al. 1999, GCN Circ. No. 463
Schlegel et al. 1998, ApJ, 500, 525
Uglesich et al. 1999, GCN Circ. No. 472
Vreeswijk et al. 1999, GCN Circ. No. 496