GCN Circular 17686

D. Malesani (DARK/NBI), D. M. Palmer (LANL) and T. N. Ukwatta (LANL)
report on behalf of the Swift Team:

At 19:36:25 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a burst from SAX J1808.4-3658 (trigger=637765).  Swift slewed
immediately to the burst.  The BAT on-board calculated location is 
RA, Dec 272.073, -36.974 which is 
   RA(J2000) = 18h 08m 18s
   Dec(J2000) = -36d 58' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a duration of about 20 sec.  The peak count rate
was ~2400 counts/sec (15-350 keV), at ~6 sec after the trigger. 

The XRT began observing the field at 19:37:11.2 UT, 46.1 seconds after
the BAT trigger. XRT found a bright X-ray source located at RA, Dec
272.1135, -36.9774 which is equivalent to:
   RA(J2000)  = +18h 08m 27.24s
   Dec(J2000) = -36d 58' 38.6"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
location is 117 arcseconds from the BAT onboard position, within the
BAT error circle. This position is 6.7 arcseconds from a known X-ray
source: SAX J1808.4-3658. This source is in the Swift XRT 1SXPS
catalogue with a mean 0.3-10 keV count-rate of 0.3550 +/- 0.0038
ct/sec; see http://www.swift.ac.uk/1SXPS/1SXPSJ180827.6-365843 for
details of these previous observations. No event data are yet available
to determine the column density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 6.62e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 54 seconds after the BAT trigger. The source is clearly visible
in the UVOT image, but the crowded field prevents automatic estimation
of its brightness at this time. 

This source is currently in outburst (Sanna et al., ATel 7364). 
Its BAT lightcurve is given at
http://swift.gsfc.nasa.gov/results/transients/weak/SAXJ1808.4-3658/