GCN Circular 17888

R. L. C. Starling (U Leicester), N. Gehrels (NASA/GSFC),
J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC) and
K. L. Page (U Leicester) report on behalf of the Swift Team:

At 11:42:18 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150530A (trigger=642018).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 327.523, +57.490 which is 
   RA(J2000) = 21h 50m 06s
   Dec(J2000) = +57d 29' 25"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak with
some structure with a duration of about 10 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV). 

The XRT began observing the field at 11:44:00.8 UT, 102.7 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 327.5109, 57.5164 which
is equivalent to:
   RA(J2000)  = 21h 50m 02.61s
   Dec(J2000) = +57d 30' 58.9"
with an uncertainty of 3.3 arcseconds (radius, 90% containment). This
location is 96 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 1.00
x 10^22 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 106 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. 

Burst Advocate for this burst is R. L. C. Starling (rlcs1 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.)