TITLE: GCN CIRCULAR NUMBER: 6639 SUBJECT: GRB 070721: Swift detection of a possible burst DATE: 07/07/21 10:59:02 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 E. Troja (INAF-IASFPA) report on behalf of the Swift Team: At 10:00:56 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070721 (trigger=285653). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 3.176, -28.546 (J2000) with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). We have very little further information at this time (lightcurve, etc) because there appears to be a problem with the messages. The spacecraft slewed promptly and the XRT found a source at location RA(J2000) = 3.16355 degrees, Dec(J2000) = -28.54928 degrees, which is 40 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 3.25e-10 cgs. At this point we do not have enough data to be able to tell whether this source is fading. We note that there is a bright 2MASS object at this position: 2MASS 00123943-2832566 (2.5 arcseconds from the XRT position). We are waiting for processing of the downlinked Malindi data for more details. At this moment no UVOT data are available. We are waiting for the next Malindi pass to download more data. We note that while writing this Circular, Swift slewed to the next trigger following this one. This is unusual and raises the possibility that this burst is not real. We will not be certain until the Malindi data can be analyzed. 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.) [GCN OPS NOTE(24aug07): Per author's request, the trigger time was corrected from 10:01:08 to 10:00:56. Acknowledgements to F. Marshall.]