GCN Circular 18050

J. L. Racusin (NASA/GSFC), A. P. Beardmore (U Leicester),
J. R. Cummings (NASA/UMBC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 05:46:35 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150724A (trigger=650141).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 97.540, -19.154 which is 
   RA(J2000) = 06h 30m 09s
   Dec(J2000) = -19d 09' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed multiple peaks
structure with a duration of about 40 sec.  The peak count rate
was ~1412 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 05:48:18.6 UT, 102.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 97.56060,
-19.16395 which is equivalent to:
   RA(J2000)  = 06h 30m 14.54s
   Dec(J2000) = -19d 09' 50.2"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 78 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.94
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.27e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
108 seconds after the BAT trigger. No credible afterglow candidate has been
found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the
XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. The list of sources is typically complete to about 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.27. 

Burst Advocate for this burst is J. L. Racusin (judith.racusin AT nasa.gov). 
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.)