GCN Circular 21792

S. D. Barthelmy (GSFC), J. A. Kennea (PSU) and
K. L. Page (U Leicester) report on behalf of the Swift Team:

At 08:59:48 UT, the Swift Burst Alert Telescope (BAT) triggered on MAXI J1535-571
(trigger=770502).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 233.806, -57.244, which is 
   RA(J2000)  =  15h 35m 13s
   Dec(J2000) = -57d 14' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows several weak peaks
with a total duration of about 40 sec.  The peak count rate
was ~600 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 09:02:22.6 UT, 154.3 seconds after
the BAT trigger. XRT found a bright, X-ray source located
at RA, Dec 233.8300, -57.2303 which is equivalent to:
   RA(J2000)  = +15h 35m 19.20s
   Dec(J2000) = -57d 13' 49.1"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This position
is 4.3 arc-seconds from the previously reported (ATEL #10700) XRT position of
MAXI J1535-571. 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 3.16e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of  69 seconds with the White filter
starting 163 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.