Hubble Space Telescope (HST ) observations of the OT associated with GRB 990123 were made between 23:06:54 UT on 8 February 1999 and 03:21:43 UT on 9 February 1999 as part of the Cycle 8 proposal GO-8394 (Beckwith 1999) in order to identify the host galaxy and to study the region around the OT. These observations consisted of six 1300 second exposures taken with the Space Telescope Imaging Spectrograph (STIS) in its Clear Aperture (50CCD) mode. Each exposure was split into two 650 second sub-exposures to allow for the removal of cosmic rays. A six-position spiral dithering pattern, with offsets of pixels (), was used. The CCD gain was set to 1 /ADU, and the read-out noise was 4 /pixel. The data was processed through the standard STIS pipeline and immediately made available to the astronomical community.
Fruchter et al. (1999a) reported a preliminary magnitude of for the OT based on the HST images. Bloom et al. (1999b) used this data to derive magnitudes for the OT and for the purported host galaxy, and noted that there may be a small star-forming region to the north of the OT. Fruchter et al. (1999b) also derived magnitudes for the OT and the host galaxy, and determined a decay rate for the flux from the OT which suggests that the light from the OT may have been beamed for the first days after the burst, and then made a transition to and isotropic expansion.
We combined the six STIS images using the DITHER (v1.2) software (Fruchter & Hook 1999) as implemented in IRAF 2 (v2.11.1)/STSDAS (v2.0.2). A "pixfrac" value of 0.5 was used, and the final output image had a scale of /pixel. Fig. 1 shows the details of the OT and the probable host galaxy. We found the location of the OT to be (J2000 coordinates) based on the World Coordinate System information in the STIS image headers. We have taken the centroid of the bright elliptical object to the north of the OT (Bloom et al. [1999b]'s object A) to be the nucleus of the host galaxy. It is located at . The OT is situated ( kpc) south of the nucleus of the host galaxy ( east of north).
© European Southern Observatory (ESO) 1999
Online publication: March 29, 1999