A section of the HST STIS image of GRB 991216. The image was taken on April 17, 2000, or 123 days after outburst. The section above is 3."2 across. North is up, East is to the left. The estimated position of the GRB (from earlier observations of the optical transient) is shown by the circle. Two nearby galaxies, N and S are marked. We believe N to be the host. This image was made by summing the 50CCD and LP filter images. Again North is up and East to the left.
On this page the HST GRB Collaboration presents the Hubble Space Telescope images of the host of GRB 991216.
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 an 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 (Vreewijk et al., GCN 496), 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 Ar=1.6 mags predicted by the Schlegel et al. 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) and Vreeswijk 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.
A section of the HST STIS image of GRB 991216. This is a larger view (6."4 on a side) of the image shown at the top of this page. Two additonal galaxies can be seen, at least one of which may be contributing to the absorption line systems seen in the VLT spectrum of the optical transient.