TITLE:   GCN CIRCULAR
NUMBER:  33687
SUBJECT: GRB 230427A: Swift detection of a burst with an optical counterpart
DATE:    23/04/27 10:51:55 GMT
FROM:    K.L. Page at U Leicester <klp5@leicester.ac.uk>


E. Troja (UToV), P. A. Evans (U Leicester),
R. A. J. Eyles-Ferris (U Leicester), C. Gronwall (PSU),
J.D. Gropp (PSU), J. A. Kennea (PSU) and K. L. Page (U Leicester)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 10:31:52 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230427A (trigger=1165948).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 153.324, +48.180 which is 
   RA(J2000) = 10h 13m 18s
   Dec(J2000) = +48d 10' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  No BAT light-curve is available at this time. 

The XRT began observing the field at 10:33:25.9 UT, 94.0 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 153.31199,
48.20404 which is equivalent to:
   RA(J2000)  = 10h 13m 14.88s
   Dec(J2000) = +48d 12' 14.6"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 91 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 (8.35 x
10^19 cm^-2, Willingale et al. 2013), with an excess column of 3.5
(+2.81/-2.63) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 98 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	10:13:15.05 = 153.31270
  DEC(J2000) = +48:12:14.4  =  48.20400
with a 90%-confidence error radius of about 0.74 arc sec. This position is 1.4
arc sec. from the center of the XRT error circle. The estimated magnitude is
20.31 with a 1-sigma error of about  0.20. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.007. 

Burst Advocate for this burst is E. Troja (nora.gsfc 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/)