TITLE:   GCN CIRCULAR
NUMBER:  35449
SUBJECT: GRB 240101B: Swift detection of a burst
DATE:    24/01/01 20:42:28 GMT
FROM:    K.L. Page at U Leicester <klp5@leicester.ac.uk>


S. B. Cenko (GSFC), C. Gronwall (PSU), J.D. Gropp (PSU),
H. A. Krimm (NSF), A. Y. Lien (U Tampa) and K. L. Page (U Leicester)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 20:25:39 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240101B (trigger=1205744).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 352.052, +51.645 which is 
   RA(J2000) = 23h 28m 12s
   Dec(J2000) = +51d 38' 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 20 sec.  The peak count rate
was ~600 counts/sec (15-350 keV), at ~7 sec after the trigger. 

The XRT began observing the field at 20:27:43.2 UT, 123.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 352.0966, 51.6379 which is equivalent to:
   RA(J2000)  = 23h 28m 23.18s
   Dec(J2000) = +51d 38' 16.4"
with an uncertainty of 5.1 arcseconds (radius, 90% containment). This
location is 102 arcseconds from the BAT onboard position, within the
BAT error circle. 

The initial flux in the 2.5 s image was 2.19e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 134 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
0.286. 

Burst Advocate for this burst is S. B. Cenko (brad.cenko 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/)