GCN Circular 22580

A. Y. Lien (GSFC/UMBC), A. P. Beardmore (U Leicester),
J.D. Gropp (PSU) and K. L. Page (U Leicester) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 09:44:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180402A (trigger=821103).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 251.905, -14.963 which is 
   RA(J2000) = 16h 47m 37s
   Dec(J2000) = -14d 57' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a short peak
with a duration of about 0.3 sec.  The peak count rate
was ~9000 counts/sec (15-350 keV), at ~0 sec after the trigger. 
There are some additional variation in the light curve, however, 
this variation is also consistent with the presence of the 
currently-active black hole transient MAXI J1820+070 in the FOV. 
Further analysis of the full downlinked data will be required
to determine the cause of the variation. 

The XRT began observing the field at 09:46:19.2 UT, 79.9 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 251.9335,
-14.9683 which is equivalent to:
   RA(J2000)  = 16h 47m 44.05s
   Dec(J2000) = -14d 58' 05.8"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 100 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 consistent with the Galactic value of 2.94
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
139 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 none of the
XRT error circle. 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 expected extinction
corresponding to E(B-V) of 0.77. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien AT 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.)