TITLE:   GCN CIRCULAR
NUMBER:  27518
SUBJECT: GRB 200410A: Swift detection of a burst with a possible optical counterpart
DATE:    20/04/10 02:49:58 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

K. K. Simpson (PSU), S. D. Barthelmy (GSFC), J.D. Gropp (PSU),
J. A. Kennea (PSU), N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), M. H. Siegel (PSU)
and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels
Swift Observatory Team:

At 02:25:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 200410A (trigger=965638).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 299.637, +51.763, which is 
   RA(J2000)  =  19h 58m 33s
   Dec(J2000) = +51d 45' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple peaks
with a total duration of about 45 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 02:26:51.7 UT, 101.3 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 299.6456,
51.7813 which is equivalent to:
   RA(J2000)  = 19h 58m 34.94s
   Dec(J2000) = +51d 46' 52.5"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 68 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (2.15 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 2.3
(+2.17/-1.95) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 104 seconds after the BAT trigger. There is a possible candidate
afterglow in the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	19:58:35.32 = 299.64717
  DEC(J2000) = +51:46:53.0  = +51.78138
with a 90%-confidence error radius of about 0.6 arc sec. This position is 3.5
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.71 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.15. 

Burst Advocate for this burst is K. K. Simpson (kira.simpson1984 AT gmail.com). 
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/)