GCN Circular 8021

S. Immler (CRESST/GSFC/UMD), W. H. Baumgartner (GSFC/UMBC),
N. Gehrels (NASA/GSFC), S. D. Hunsberger (PSU), W.B Landsman (GSFC),
C. B. Markwardt (CRESST/GSFC/UMD), K. L. Page (U Leicester),
D. M. Palmer (LANL), A. M. Parsons (GSFC), D. Perez (U Leicester),
T. Sakamoto (NASA/UMBC) and T. N. Ukwatta (GSFC/GWU) report on behalf
of the Swift Team:

At 05:57:39 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080727 (trigger=318094).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 208.389, -18.549 which is 
   RA(J2000) = 13h 53m 33s
   Dec(J2000) = -18d 32' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
structure with a duration of about 5 sec.  The peak count rate
was ~700 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 05:59:28.5 UT, 109.2 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 208.39099,
-18.54505 which is equivalent to:
   RA(J2000)  = 13h 53m 33.84s
   Dec(J2000) = -18d 32' 42.2"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 15 arcseconds from the BAT onboard position, within the BAT
error circle. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
6.75e+20 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm)  filter starting 114 seconds after the BAT trigger. No
afterglow candidate has  been found in the initial data products. The
2.7'x2.7' sub-image covers 25% of  the BAT error circle and 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 
18.5 mag. The list of sources is typically complete to about 18 mag. 
No  correction has been made for the expected extinction corresponding 
to E(B-V) of 0.07. 

Burst Advocate for this burst is S. Immler (immler AT milkyway.gsfc.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.)