GCN Circular 17611

A. Amaral-Rogers (U Leicester), S. D. Barthelmy (GSFC),
F. E. Marshall (NASA/GSFC), C. J. Mountford (U Leicester),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 02:49:14 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150323A (trigger=635887).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 128.191, +45.434 which is 
   RA(J2000) = 08h 32m 46s
   Dec(J2000) = +45d 26' 03"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a small peak at the
time of the trigger, followed by a much larger peak at T+130 s,
for a total duration of at least 170 sec.  The peak count rate
was ~5600 counts/sec (15-350 keV), at ~130 sec after the trigger. 

The XRT began observing the field at 02:51:41.2 UT, 146.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 128.17951,
45.46355 which is equivalent to:
   RA(J2000)  = 08h 32m 43.08s
   Dec(J2000) = +45d 27' 48.8"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 110 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 3.05
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 0.1 s image was 3.25e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 156 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 expected extinction corresponding to E(B-V) of
0.03. 

Burst Advocate for this burst is A. Amaral-Rogers (aar14 AT 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.)