TITLE: GCN GRB OBSERVATION REPORT NUMBER: 757 SUBJECT: GRB 990510: Possible detection of a host galaxy DATE: 00/07/30 04:57:38 GMT FROM: Andrew S. Fruchter at STScI A. Fruchter (STScI), R. Hook (ST-ECF), E. Pian (ITESRE-CNR, Bologna) report for a larger HST GRB collaboration: The field of GRB 990510 was imaged by HST for a third time on 29 April 2000 using the STIS/CCD camera in Open Filter (50CCD) mode, as part of our program to study this GRB (see GCN 386 for earlier results. A total exposure time of 5840 s, divided between 8 dithered observations was obtained. Based on our previous observations, we would expect the GRB to be well below the threshold of detectability -- which is for this image a V=29 point source. As predicted the OT was not detected. However, our June 1999 image had suggested the possible presence of an underlying host galaxy (see GCN 386). We therefore convolved the image with a gaussian of FWHM ~ 0."3, which is roughly comparable to the typical size of galaxies in other deep fields near the detection limit of the image for an extended source (V~28). The convolved image reveals a >3 sigma source lying under the position of the OT, with its peak lying ~0."075 East of the position of the OT. This we believe to most likely be the host of the GRB. Although the formal significance of the object is above 3 sigma, it lies in a region of significant scattered light from bright nearby stars and it is difficult to quantify the possible errors in the sky subtraction. We therefore regard the detection of this host as tentative. Our best estimate for the host's magnitude, assuming its correct identification, is V=28 +/-0.3. Deeper integrations, particularly ones taken at different roll angles, would be able to unambiguously determine the reality of this detection. The images of the field of GRB 990510 giving a clear idea of the significance of the detection can be found at: http://www.stsci.edu/~fruchter/GRB/990510