GCN Circular 6639

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.]