TITLE:   GCN CIRCULAR
NUMBER:  14406
SUBJECT: GRB 130420A: Swift detection of a burst with an optical counterpart
DATE:    13/04/20 07:50:15 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

M. J. Page (UCL-MSSL), W. H. Baumgartner (GSFC/UMBC),
A. P. Beardmore (U Leicester), M. M. Chester (PSU),
J. R. Cummings (NASA/UMBC), S. R. Oates (UCL-MSSL) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 07:28:29 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 130420A (trigger=553977).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 196.100, +59.424 which is 
   RA(J2000) = 13h 04m 24s
   Dec(J2000) = +59d 25' 25"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a very gradual beginning 
and a large peak at T+100 seconds.  The total duration was about 200 sec.  
Because of the gradual beginning, the trigger was an image rather than a rate
trigger.  The peak count rate was ~3985 counts/sec (15-350 keV), at ~110 sec 
after the trigger. 

The XRT began observing the field at 07:40:44.8 UT, 735.3 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 196.1061, 59.4241 which
is equivalent to:
   RA(J2000)  = 13h 04m 25.46s
   Dec(J2000) = +59d 25' 26.8"
with an uncertainty of 2.6 arcseconds (radius, 90% containment). This
location is 11 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 http://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 1.26
x 10^20 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 738 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	13:04:25.57 = 196.10654
  DEC(J2000) = +59:25:26.9  =  59.42415
with a 90%-confidence error radius of about 0.75 arc sec. This position is 5.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.22 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is M. J. Page (m.page AT ucl.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/too.html.)