TITLE:   GCN CIRCULAR
NUMBER:  27591
SUBJECT: GRB 200416A: Swift detection of a burst with an optical afterglow
DATE:    20/04/16 07:19:36 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

P. A. Evans (U Leicester), V. D'Elia (SSDC), J.D. Gropp (PSU),
N. J. Klingler (PSU), S. Laha (GSFC/UMBC/CRESST),
A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL)
and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels
Swift Observatory Team:

At 07:05:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 200416A (trigger=966554).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 335.691, -7.530 which is 
   RA(J2000) = 22h 22m 46s
   Dec(J2000) = -07d 31' 47"
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 10 sec.  The peak count rate
was ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 07:06:57.1 UT, 99.9 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 335.69906, -7.51816 which
is equivalent to:
   RA(J2000)  = 22h 22m 47.77s
   Dec(J2000) = -07d 31' 05.4"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
location is 51 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 (6.54 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.2
(+3.00/-2.56) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 102 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	22:22:47.83 = 335.69929
  DEC(J2000) = -07:31:05.9  =  -7.51831
with a 90%-confidence error radius of about 0.75 arc sec. This position is 1.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
16.47 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.06. 

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/)