TITLE: GCN CIRCULAR NUMBER: 6640 SUBJECT: GRB 070721B: Swift detection of a burst DATE: 07/07/21 11:16:00 GMT FROM: Scott Barthelmy at NASA/GSFC H. Ziaeepour (UCL-MSSL), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), P. A. Evans (U Leicester), K. L. Page (U Leicester) and P. Schady (MSSL-UCL) report on behalf of the Swift Team: At 10:33:48 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070721B (trigger=285654). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 33.131, -2.184 (J2000) with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Like the previous trigger, there continues be a problem with the messages, so we are not able to say anything about the lightcurve at this time. The XRT found a bright, uncatalogued source at RA(J2000) = 33.13506 DEC(J2000) = -2.19602 which is 45.7 arcseconds from the BAT position. We do not have an error estimate, but the typical uncertainty for on-board centroids is 6 arcseconds. The source flux was about 1.296e-09 cgs. While we don't have the usual GCN notices for this trigger, the realtime telemetry from the spacecraft indicates that the source is fading. At the moment no UVOT data are available. We are waiting for the next Malindi pass to download more data. Burst Advocate for this burst is H. Ziaeepour (hz AT mssl.ucl.ac.uk). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.)