[vsnet-grb-info 16032] GRB 150120B: Swift detection of a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17314
SUBJECT: GRB 150120B: Swift detection of a burst
DATE:    15/01/20 07:51:54 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

V. D'Elia (ASDC), L. Izzo (URoma/ICRA), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL)
and M. H. Siegel (PSU) report on behalf of the Swift Team:

At 07:21:55 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150120B (trigger=627170).  Swift slewed to this burst
after a short delay due the Earth limb constraint. 
The BAT on-board calculated location is 
RA, Dec 39.294, +8.079 which is 
   RA(J2000) = 02h 37m 11s
   Dec(J2000) = +08d 04' 43"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 30 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 07:29:01.2 UT, 425.7 seconds after
the BAT trigger. The position determined from promptly downlinked data
differs significantly from the on-board position, suggesting that the
XRT may have centroided on a cosmic ray; the initial XRT position
notice should be treated with caution. Using promptly downlinked data
we find an uncatalogued X-ray source with an enhanced position: RA, Dec
39.2927, 8.0781 which is equivalent to:
   RA(J2000)  = 02h 37m 10.25s
   Dec(J2000) = +08d 04' 41.2"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 5.7 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 1.16
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 432 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 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.20. 

Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.it). 
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.)