TITLE:   GCN CIRCULAR
NUMBER:  29233
SUBJECT: GRB 210104A: Swift detection of a burst with a bright optical counterpart
DATE:    21/01/04 11:41:25 GMT
FROM:    Kim Page at U.of Leicester  <klp5@leicester.ac.uk>

E. Troja (NASA/GSFC/UMCP), M. G. Bernardini (INAF-OAB),
A. A. Breeveld (UCL-MSSL), P. D'Avanzo (INAF-OAB),
P. A. Evans (U Leicester), C. Gronwall (PSU), J.D. Gropp (PSU),
A. Y. Lien (GSFC/UMBC), A. Melandri (INAF-OAB),
K. L. Page (U Leicester) and T. Sbarrato (INAF-OAB) report on behalf
of the Neil Gehrels Swift Observatory Team:

At 11:26:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210104A (trigger=1015873).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 103.705, +64.661 which is 
   RA(J2000) = 06h 54m 49s
   Dec(J2000) = +64d 39' 39"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 40 sec.  The peak count rate
was ~16000 counts/sec (15-350 keV), at ~21 sec after the trigger. 

The XRT began observing the field at 11:28:01.8 UT, 62.7 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 103.7707, 64.6743 which is equivalent to:
   RA(J2000)  = 06h 55m 4.97s
   Dec(J2000) = +64d 40' 27.5"
with an uncertainty of 5.3 arcseconds (radius, 90% containment). This
location is 111 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. 

The initial flux in the 2.5 s image was 1.41e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of nominal 150.0 seconds with the White
filter starting 70 seconds after the BAT trigger. There is a candidate
afterglow in the list of sources generated on-board at
RA(J2000) = 06:55:05.25 = 103.77188
DEC(J2000) = +64:40:34.0 = 64.67611
with a 90%-confidence error radius of about 1.10 arc sec. This position is 6.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
15.52. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.04. 

Burst Advocate for this burst is E. Troja (eleonora.troja 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/)