TITLE:   GCN CIRCULAR
NUMBER:  14607
SUBJECT: GRB 130508A: Swift detection of a probable burst
DATE:    13/05/08 17:44:12 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

S. T. Holland (STScI), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), V. D'Elia (ASDC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), D. Malesani (DARK/NBI),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU),
E. Troja (NASA/GSFC/UMCP) and B.-B. Zhang (PSU) report on behalf of
the Swift Team:

At 17:08:53 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a probable GRB 130508A (trigger=555413).  Swift slewed immediately to the 
source location. 
The BAT on-board calculated location is 
RA, Dec 305.312, +34.949 which is 
   RA(J2000)  =  20h 21m 15s
   Dec(J2000) = +34d 56' 58"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The real-time lightcurve shows a weak peak
with a duration of about 40 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~18 sec after the trigger. 

The XRT began observing the field at 17:11:03.9 UT, 130.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued
X-ray source with an enhanced position: RA, Dec 305.3217, 34.9583 which
is equivalent to:
   RA(J2000)  = 20h 21m 17.20s
   Dec(J2000) = +34d 57' 29.9"
with an uncertainty of 2.5 arcseconds (radius, 90% containment). This
location is 44 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.  We
cannot determine whether the source is fading at the present time. 

The on-board calculated light curve shows evidence for a strong flare in 
Windowed Timing mode data at about 145-150 s after the BAT trigger. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (7.72 x
10^21 cm^-2, Kalberla et al. 2005), with an excess column of 1
(+1.60/-1.01) x 10^22 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 135 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 19.6 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 mag. No
correction has been made for the large, but uncertain extinction expected. 

Even though this is a weak detection and it is on the Galactic Plane
(lon,lat= 74,-1), we determine that this is probably a GRB, but can not rule out
some other type of astrophysical source.  We will need the full data set 
to make the determination. 

Burst Advocate for this burst is S. T. Holland (sholland AT stsci.edu). 
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.)