HST/STIS Images of GRB 990123

The HST images of GRB 990123. HST has observed this field in the 50CCD (open) filter on the 8 February 1999, 23 March 1999, and the 7 February 2000, or 16, 59 and 380 days after outburst. These are shown in the animated gif above. Each square is 3."2 on a side. East is to the left; North is up. A wider field-of-view is found at the bottom of this page. Three orbits of HST time were used to obtain the first image; only two orbits each were available for the second and third observations -- and thus the slightly higher background noise in these images. The animated gif will stop running after 100 cycles. To restart, reload the page. By putting your cursor on the center of the optical transient (OT) you can easily locate the position of the GRB in the late-time host image.

On this page we present the Hubble Space Telescope images of GRB 990123. The Gamma-Ray Burst of January 23, 1999 was first located by the Beppo-Sax satellite. Most remarkably, an optical transient (OT) associated with the GRB was detected only ~25 seconds after the start of the burst by the ROTSE camera at Los Alamos. The optical emission peaked about 45 seconds after the start of the burst, at a visual magnitude of about 9. Optical spectra taken at the W.M. Keck Observatory and the Nordic Optical Telescope revealed absorption lines corresponding to a redshift of z=1.6. A complete list of notices sent out to the Gamma Ray Burst community on this object can be found in the GCN Archive.

HST has observed GRB 990123 on four occasions. In the first three cases, the observations were performed using the STIS CCD camera in with no filter (clear/50CCD mode). In the last case (a year and a half after outburst) the observation was with the LP (long pass) filter. The first observation, on 8 February 1999, was performed by the Director of HST, Steven Beckwith, as a service to the astronomical community. The data were made available immediately to the public, and were reduced by various groups. The image made by the HST GRB Collaboration HST GRB Collaboration, using this data, is shown on the left above. The OT, the point source at the center of the image, is seen to be superposed on an irregular galaxy, which could perhaps be an interacting system. A wider field-of-view image is shown at the bottom of this page. By the time that the first HST image was taken the OT had faded to V=25.4 +/- 0.1, or by a factor of about 4 million since its peak. Our paper on the results of these observations, including both the behavior of the OT and the nature of the host galaxy have been published in the Astrophysical Journal. If you or your Institution does not have a subscription to the Ap. J., you can obtain the paper from the LANL preprint server.

The second and third sets of observations, on 23 March 1999, and 7 February 2000 were obtained by the HST GRB Collaboration as part of our Cycle 8 Target of Opportunity program. We find that the OT has faded to V=27.7 +/- 0.15 by the time of the 23 March 99 observation. The OT is not visible above the galactic background in the 7 February image. The images below show the images from the first two epochs on the left and the difference image between each image and the last epoch on the right. The fourth and most recent observation, on the 15 June 2000, was using the LP (long-pass) filter as part of a public survey of a number of GRB hosts. By this time the OT has faded to insiginificance. However, the LP filter, which only passes light longward of ~5500 Angstroms provides some color information when compared with the open mode (50CCD) image.

On the left in the above image is the February 1999 image of GRB 990123, on the right the difference between that image and the February 2000 image. The field of view in each case is, as in Figure 1, 3."2. 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.

On the left in the above image is the March 1999 image of GRB 990123, on the right the difference between that image and the February 2000 image.

On the left, the February 2000 50CCD image of the host of GRB 990123, and on the right, the June 2000 red (LP) image. As can be seen here and in the animated gif, the GRB lay on the edge of the visible stellar field, and does not appear to be associated with a particularly strong region of star formation as had been previously reported (Hjorth et al. 1999).

A color image of the host of GRB 990123. The LP image is used as red. All of the 50CCD data has been combined (with the OT removed from two of the datasets) to produce the green. A scaled subtraction of the 50CCD and LP images is used for the blue. Again, while the whole galaxy is relatively blue, the OT does not fall on one of its bluest regions. This image reproduces best on the standard monitor settings of a PC. Sun users may find it improved by increasing their monitor's brightness setting to the highest level.

The Decline of the OT

The graphs below shows the decline of the OT with time. The HST values are shown as filled boxes. In order to compare the HST observations with the observations from the ground in R band, we have converted HST magnitude to the R band assuming that the spectral index of the OT was constant after day 3. Note that thus R~=V-0.4. We show two plots, one with three power-laws fitted to different sections of the data, and one with no fits. The first and third power-laws are equal within the errors (-1.7 +/- 0.1 for the later power-law), and this may suggest a connection between the early and late-time emission.

Wide-field Image of GRB 990123 Field

A wide-field color image (23."6 on a side) of the field of GRB 990123. North is up; East is to the left. The host of GRB 990123 can be seen near the center of the field. This GIF shows about one-quarter of the total area of the HST/STIS (50CCD) image. The LP filter covers the same region in the detector X axis, but half as much in the detector Y axis. Again, this image is best viewed on a PC. On a Sun monitor, the reader will most likely wish to raise the brightness to the maximum.

Availability of Data

The reduced, drizzled FITS image of the combined HST data from the February 1999 observation is freely available from this web site. The drizzled image has a pixel size of 0."025, or one-half (in linear scale) that of the orginal image. This FITS image, unlike the GIFs on this page, has not been rotated from the original telescope orientation. A rotated FITS image is also available; however, its corners have been rather ignominiously cut off, to reduce the data volume. Drizzle afficionados may also wish to retrieve the output weight files for the original orientation and rotated images. For more information on these images, feel free to email Andy Fruchter (fruchter@stsci.edu). The pipeline reduced (but not combined) images are publicly available from the HST archives (see GCN 254 for the simplest means of retrieval).

The March 1999 and February 2000 drizzled images will be made available shortly.

If you use the reduced FITS images provided on this page in any publication, we ask that you reference the paper "HST and Palomar Imaging of GRB 990123: Implications for the Nature of Gamma-Ray Bursts and their Hosts" (Fruchter et al., 1999, the Astrophysical Journal, Volume 519, page L519) as the source of the images.