GCN Circular 13831

V. D'Elia (ASDC), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC),
N. Gehrels (NASA/GSFC), S. T. Holland (STScI), J. A. Kennea (PSU),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB/PSU),
M. H. Siegel (PSU), E. Sonbas (NASA/GSFC/Adiyaman Univ.) and
E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift Team:

At 18:23:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located trigger 535026.  Swift slewed immediately to the location.
The BAT on-board calculated location is
RA, Dec 276.033, -5.667 which is
   RA(J2000) = 18h 24m 08s
   Dec(J2000) = -05d 40' 00"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty).  The BAT light curve showed a weak multi-peaked
structure with a duration of about 25 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 18:25:03.7 UT, 121.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
276.0325, -5.6654 which is equivalent to:
   RA(J2000)  = 18h 24m 07.80s
   Dec(J2000) = -05d 39' 55.5"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 6.0 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 in excess of the Galactic value (4.53 x
10^21 cm^-2, Kalberla et al. 2005), with an excess column of 5.5
(+4.26/-3.49) x 10^21 cm^-2 (90% confidence).

The initial flux in the 2.5 s image was 5.49e-10 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
287 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 large, but uncertain extinction expected.

The classification of this trigger is uncertain, but it's quite
likely that it is a galactic transient.  The duration (~20 sec) and
low energy emission (<25 keV) are consistent with it being a
thermonuclear X-ray burst from a neutron star.  The location is 3.5
degrees from the galactic plane.

Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.it).
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.)