HST/STIS Images of GRB 990712

A color image of the host of GRB 990712 taken on August 29, 1999, or 48 days after outburst using the STIS 50CCD (clear) and LP (long-pass) exposures. The arrow points to the location of the GRB, now revealed by the subtraction of later images. The field of view is about 1."6. North is up, East to the left. .

On this page the HST GRB Collaboration presents the Hubble Space Telescope images of GRB 990712. The Gamma-Ray Burst of July 12, 1999 was first located by the Beppo-Sax satellite. The optical transient was discovered in images taken 4 hours later by Kailash Sahu and collaborators at the SAAO 1m telescope at Sutherland, South Africa. A redshift of z=0.43 was determined by Vreeswijk et al. using the VLT. A complete list of GCN circulars for this burst can be found at the UCO Lick REACT web pages.

We observed the field of GRB 990712 with the HST STIS CCD on August 29, 1999, 48 days after outburst and on April 24, 2000, 287 days after outburst. On both occasions observations with total exposures times of 3720s were taken in both the (50CCD) clear aperture and (LP) long pass filters. At the position of the previously located OT (GCN reports starting with GCN 387), we find a disk galaxy with two apparent knots along its major axis.

On the left in the above image is the August 1999 STIS 50ccd image of GRB 990712, on the right April 2000 50ccd image. The field of view in each case is 1."6. The images have been drizzled onto an output grid with pixels one-half the size of the original STIS pixels, or about 0."025 on a side. The bright knot on to the south-east can be seen to have varied.

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

On the left is the STIS LP image of GRB 990712 from August 1999, and on the right is the subtraction of the April 2000 LP image from the August 1999 image. Only the optical transient remains.

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.

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)