TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1761 SUBJECT: XRF 020903 : HST observations DATE: 02/12/17 18:59:13 GMT FROM: Andrew Levan at U.of Leicester Andrew Levan (U. Leicester/STScI), Andrew Fruchter, Lou Strolger, Ingunn Burud and James Rhoads (STScI) report for the GOSH collaboration. The candidate optical transient for XRF 020903 (Soderberg et al, GCN 1554) was imaged with HST/ACS in F606W on 3rd Dec 2002. 4 dithered exposures were obtained with a total exposure time of 1840s. The candidate appears to be within an irregular galaxy, with four (possibly interacting) components. Astrometry with images obtained with the CTIO 4-m telescope and Mosaic imager on Sept 4th and 9th 2002 (Fruchter et al, GCN 1558) shows that the transient source is located at a position coincident with the peak of the most western of these compact knots (with a positional accuracy of approximately ~0."1). The knot has an F606W AB magnitude of 22.6 and a FWHM of ~0."14, greater than the PSF of ACS. Hence the source is resolved at this resolution, indicating that an AGN does not dominate the optical light from this object. The galaxy, named G2 by Soderberg et al, (GCN 1554) is clearly an interacting system. However, as noted by Soderberg et al the redshift difference (z=0.23 for G2 and z=0.25 for the host of the candidate OT) indicate that the systems are not associated. Using a 1" aperture the magnitude of the entire system in F606W is approximately R=21, roughly consistent with the late time magnitudes determined by Gorosabel et al (GCN 1631). Observations taken at a later date will allow a search for further variability in this source. Images of the host galaxy, and surrounding field can be found at http://www.stsci.edu/~fruchter/GRB/020903