TITLE:   GCN CIRCULAR
NUMBER:  11941
SUBJECT: GRB 110420A: Swift detection of a burst with optical counterpart
DATE:    11/04/20 11:17:43 GMT
FROM:    David Burrows at PSU/Swift  <burrows@astro.psu.edu>

V. Mangano (INAF-IASFPA), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), S. Campana (INAF-OAB),
J. R. Cummings (NASA/UMBC), P. A. Evans (U Leicester),
J. M. Gelbord (PSU), B. Gendre (ASDC), C. Guidorzi (U Ferrara),
S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU),
P. T. O\'Brien (U Leicester), C. Pagani (U Leicester),
P. Romano (INAF-IASFPA), A. Rowlinson (U Leicester),
B. Sbarufatti (INAF-OAB/IASFPA), M. H. Siegel (PSU),
M. Stamatikos (OSU/NASA/GSFC) and R. L. C. Starling (U Leicester)
report on behalf of the Swift Team:

At 11:02:24 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 110420A (trigger=451757).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 2.165, -37.855 which is 
   RA(J2000) = 00h 08m 40s
   Dec(J2000) = -37d 51' 17"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
structure with a duration of about 20 sec.  The peak count rate
was 4000 counts/sec (15-350 keV), at 10 sec after the trigger. 

The XRT began observing the field at 11:03:51.6 UT, 87.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 2.16337, -37.88696 which
is equivalent to:
    RA(J2000)  = 00h 08m 39.21s
    Dec(J2000) = -37d 53' 13.1"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 115 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 in excess of the Galactic value (1.26 x
10^20 cm^-2, Kalberla et al. 2005), with an excess column of 2.8
(+2.08/-1.84) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 8.02e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 97 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	00:08:39.27 =	2.16364
  DEC(J2000) = -37:53:12.0  = -37.88667
with a 90%-confidence error radius of about 0.75 arc sec. This position is 1.3
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.77 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 V. Mangano (vanessa AT ifc.inaf.it). 
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.)