[vsnet-grb-info 20346] GRB 180224A: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22442
SUBJECT: GRB 180224A: Swift detection of a burst with an optical counterpart
DATE:    18/02/24 22:32:57 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), J.D. Gropp (PSU),
H. A. Krimm (NSF/USRA), S. J. LaPorte (PSU),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 22:21:07 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180224A (trigger=811561).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 202.661, +38.088 which is 
   RA(J2000)  =  13h 30m 39s
   Dec(J2000) = +38d 05' 17"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single FRED peak
with a duration of about 15 sec.  The peak count rate
was ~2200 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 22:22:28.1 UT, 80.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 202.68324, 38.07881 which is equivalent
to:
   RA(J2000)  = 13h 30m 43.98s
   Dec(J2000) = +38d 04' 43.7"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 71 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 8.24
x 10^19 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 85 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	13:30:44.09 = 202.68370
  DEC(J2000) = +38:04:44.6  =  38.07906
with a 90%-confidence error radius of about 0.63 arc sec. This position is 0.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.94 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.01. 

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.)