GCN Circular 17728

C. Pagani (U Leicester), V. D'Elia (ASDC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and T. N. Ukwatta (LANL) report on behalf of the
Swift Team:

At 06:28:04 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150423A (trigger=638808). Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 221.600, +12.288 which is 
   RA(J2000) = 14h 46m 24s
   Dec(J2000) = +12d 17' 18"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). The BAT light curve showed a single peak 
with a duration of about 0.5 sec. The peak count rate was ~6400 
counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 06:29:14.8 UT, 70.1 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 221.5787, 12.2833 which
is equivalent to:
   RA(J2000)  = 14h 46m 18.90s
   Dec(J2000) = +12d 16' 60.0"
with an uncertainty of 2.4 arcseconds (radius, 90% containment). This
location is 76 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.77
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 73 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 29% 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 expected extinction corresponding to E(B-V) of
0.03. 

Burst Advocate for this burst is C. Pagani (cp232 AT star.le.ac.uk). 
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.)