TITLE:   GCN CIRCULAR
NUMBER:  35742
SUBJECT: GRB 240218A: Swift detection of a burst
DATE:    24/02/18 02:17:56 GMT
FROM:    K.L. Page at U Leicester <klp5@leicester.ac.uk>


K. L. Page (U Leicester), T. M. Parsotan (GSFC) and M. H. Siegel (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 02:00:00 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240218A (trigger=1215912).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 161.781, +1.304 which is 
   RA(J2000) = 10h 47m 07s
   Dec(J2000) = +01d 18' 14"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a broad single peak
structure after a period of noise with a duration of about 15 sec.  
The peak count rate was ~1400 counts/sec (15-350 keV), at ~22 sec after the trigger. 

The XRT began observing the field at 02:02:30.1 UT, 149.4 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 161.7953, 1.2782 which is equivalent to:
   RA(J2000)  = 10h 47m 10.87s
   Dec(J2000) = +01d 16' 41.5"
with an uncertainty of 4.9 arcseconds (radius, 90% containment). This
location is 106 arcseconds from the BAT onboard position, within the
BAT error circle. No event data are yet available to determine the
column density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 119 seconds with the White
filter  starting 158 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 0.045. 

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.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/)