TITLE: GCN CIRCULAR NUMBER: 15231 SUBJECT: GRB 130903A: Further Swift XRT observations. DATE: 13/09/17 18:40:19 GMT FROM: Dirk Grupe at PSU/Swift-XRT Dirk Grupe (Swift MOC/PSU) report on behalf of the Swift XRT team: We report on two additional Swift XRT observations of the INTEGRAL-discovered burst GRB 130903A (Mereghetti et al., GCN circ. 15169). As reported in Grupe & Holland (GCN circ. 15173), we found an X-ray source at a level of 5e-3 counts/s within the INTEGRAL error circle. Swift obtained a second observation on September 06 for 5.0 ks. The source reported in Grupe & Holland was still present but at a significantly lower level at (1.1+0.5-0.4)e-3 counts/s suggesting a rather slow decay slope during the 'normal' X-ray afterglow phase (e.g. see Grupe et al. 2013, arXiv:1305:3236). However, a decay by a factor of 5 is still consistent with a background AGN (e.g. Grupe et al. 2010, AJ 187, 64). Therefore we performed a third observation which was executed on September 16 for a total of 4.6 ks. In this new observation the source seems to be gone. The 3-sigma upper limit at the position of the previously detected X-ray source is at a level of 1.25e-3 counts/s. We can conclude that the X-ray source reported in Grupe & Holland (GCN circ. 15173) is most-likely the X-ray afterglow of GRB 130903A. This circular is an official product of the Swift XRT team.