TITLE: GCN CIRCULAR NUMBER: 6676 SUBJECT: GRB070721A: Swift/UVOT Detection of a Weak Afterglow DATE: 07/07/27 21:14:26 GMT FROM: Frank Marshall at GSFC F. E. Marshall (NASA/GSFC), P. Schady (MSSL-UCL) and H. Ziaeepour (MSSL-UCL) report on behalf of the Swift/UVOT team: A comparison of the initial Swift/UVOT observations with those taken several days after the BAT trigger (Ziaeepour et al. GCN Circ. 6639) reveals a weak, fading source inside the refined XRT error circle (Evans et al. GCN Circ. 6644). The source is only detected in the initial 100-s exposure with the white filter starting 88 sec. after the trigger and the initial 400-s exposure with the V filter starting 194 sec. after the trigger. The 2.4-sigma detection in white corresponds to 21.4 mag., and the 3.4-sigma detection in V corresponds to 20.2 mag. The best-fit position is RA, Dec = 3.16343,-28.55020, which is equivalent to: RA (J2000): 00 12 39.22 Dec (J2000): -28 33 00.7 with an uncertainty of 2.0" (radius, 90% confidence). The source is slightly blended with 18th mag star noted by Schady and Ziaeepour (GCN Circ. 6648), which is 4.7" away.