TITLE:   GCN CIRCULAR
NUMBER:  26993
SUBJECT: GRB 200205B: Swift detection of a burst with optical counterpart
DATE:    20/02/05 19:46:01 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

P. A. Evans (U Leicester), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), M. J. Moss (GWU), K. L. Page (U Leicester)
and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels
Swift Observatory Team:

At 19:21:43 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 200205B (trigger=954519 and 954520).  Swift slewed
to the burst after a short delay. 
The BAT on-board calculated location is 
RA, Dec 107.861, -56.506 which is 
   RA(J2000) = 07h 11m 27s
   Dec(J2000) = -56d 30' 21"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a peak about 20 
seconds long (trigger 954519) followed by a brighter more complex 
peak structure 350 s later (trigger 954520) with a peak countrate
of 3500 counts/s.  Due to the double trigger, the complete
BAT lightcurve is not immediately available. 

The XRT began observing the field at 19:27:26.3 UT, 342.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 107.7881,
-56.4875 which is equivalent to:
   RA(J2000)  = 07h 11m 09.14s
   Dec(J2000) = -56d 29' 15.0"
with an uncertainty of 2.8 arcseconds (radius, 90% containment). This
location is 37 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 https://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 1.04
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.26e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 351 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	07:11:09.15 = 107.78811
  DEC(J2000) = -56:29:15.1  = -56.48753
with a 90%-confidence error radius of about 0.61 arc sec. This position is 0.8
arc sec. from the center of the XRT error circle. The estimated magnitude is
16.43 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.11. 

Burst Advocate for this burst is P. A. Evans (pae9 AT star.le.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/)