TITLE: GCN GRB OBSERVATION REPORT NUMBER: 386 SUBJECT: Late-time HST/STIS Observations of GRB 990510 DATE: 99/07/12 23:05:53 GMT FROM: Andrew S. Fruchter at STScI A. Fruchter, H. Ferguson, J. Pepper, R. Gibbons, K. Sahu (STScI) and E. Pian (ITESRE-CNR, Bologna) report for the HST GRB collaboration: The field of GRB 990510 was imaged by HST on 8.1 June 1999 (UT) and 17.9 June 1999 (UT) during one and two orbits, respectively, with the STIS/CCD camera in Open Filter (50CCD) mode. The OT of GRB 990510 was detected in both cases. Assuming the color of the OT has not changed from the first few days when it could be modeled as f(nu) = nu^{-0.6} passing through a Galactic extinction of E(B-V)=0.22 (Stanek et al. 1999, Harrison et al. 1999), we find magnitudes of V = 27.0 +/- 0.2 on 8.1 June and 27.8 +/- 0.3 on 17.9 June. The STIS CCD in open filter mode is sensitive to wavelengths between 300 and 900 nm, and therefore both the conversion from counts to flux density, and the estimated PSF (which is diffraction limited) depend strongly on the assumed spectrum of the observed emission. This is of particular concern in the 17.9 June observation, where we observe some sign of extended emission about the PSF. Even though the extended emission appears to be asymmetric, subtracting a red PSF (rather than the relatively blue PSF implied by the early color) largely removes this emission. In any event, the GRB does not have a host galaxy brighter than V~28. The fits of Stanek et al. and Harrison et al. to the early time light curve predict, at both epochs, an OT fainter than observed by at least several tenths of a magnitude. However, the excess counts above predicted are between a factor of three (first epoch) to seven (second epoch) less than would exist were a supernova Type Ic of the luminosity of SN1998bw at the probable redshift of the GRB (z=1.6, Vreeswijk et al. 1999) superposed on the decaying power-law light curve. Sections of the images in gif format can be retrieved from http://www.stsci.edu/~fruchter/GRB/990510 . References: Harrison, F.A. et al. 1999, astro-ph/9905306. Stanek, K., Garnavich, P.M., Kaluzny, J., Pych, W. and Thompson, I. 1999, Ap. J. (Letter,submitted), astro-ph/990534. Vreeswijk, P. et al. 1999, GCN 324.