GCN Circular 23989

S. D. Barthelmy (GSFC), J.D. Gropp (PSU), A. Y. Lien (GSFC/UMBC),
D. M. Palmer (LANL) and B. Sbarufatti (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 20:30:47 UT, the Swift Burst Alert Telescope (BAT) triggered and
located LS I +61 303 (trigger=894557).  Swift slewed immediately to 
the source. 
The BAT on-board calculated location is 
RA, Dec 40.153, +61.246 which is 
   RA(J2000) = 02h 40m 37s
   Dec(J2000) = +61d 14' 45"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed multiple short
spikes with a total duration of about 20 sec.  The peak count rate
was ~2400 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 20:32:12.5 UT, 85.3 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
with an enhanced position: RA, Dec 40.1330, 61.2290 which is equivalent
to:
   RA(J2000)  = 02h 40m 31.91s
   Dec(J2000) = +61d 13' 44.2"
with an uncertainty of 2.0 arcseconds (radius, 90% containment). This
location is 70 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. This
position is 2.0 arcseconds from LS I +61 303. This source is in the 
Swift XRT 1SXPS catalogue with a mean 0.3-10 keV count-rate of 
0.19600 +/- 0.00093 ct/sec; see 
http://www.swift.ac.uk/1SXPS/1SXPSJ024031.5%2B611344 
for details of these previous observations. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 9.01
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT data cannot properly be analyzed at this time. A full analysis
will be possible once the full dataset comes to the ground. 

The light curve from the BAT transient monitor is available at
https://swift.gsfc.nasa.gov/results/transients/weak/LSIp61303/