TITLE: GCN GRB OBSERVATION REPORT NUMBER: 751 SUBJECT: GRB 991216, HST/STIS observations DATE: 00/07/17 09:18:02 GMT FROM: Paul Vreeswijk at U of Amsterdam 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