GCN Circular 10628

W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
M. M. Chester (PSU), N. Gehrels (NASA/GSFC), C. Guidorzi (U Ferrara),
E. A. Hoversten (PSU), O. M. Littlejohns (U Leicester),
R. Margutti (INAF-OAB), C. B. Markwardt (CRESST/GSFC/UMD),
P. T. O'Brien (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), A. Rowlinson (U Leicester), M. H. Siegel (PSU)
and E. Troja (NASA/GSFC/ORAU) report on behalf of the Swift Team:

At 05:22:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100420A (trigger=419932).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 296.127, +55.793 which is 
   RA(J2000) = 19h 44m 31s
   Dec(J2000) = +55d 47' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for an image trigger, there is nothing of
note in the BAT light curve. 

The XRT began observing the field at 05:24:52.8 UT, 130.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 296.12646, 55.76885 which
is equivalent to:
   RA(J2000)  = 19h 44m 30.35s
   Dec(J2000) = +55d 46' 07.9"
with an uncertainty of 4.0 arcseconds (radius, 90% containment). This
location is 86 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
1.19e+21 cm^-2 (Kalberla et al. 2005). 


UVOT took a finding chart exposure of 150 seconds with the White filter
starting 134 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers none of
the XRT error circle. The coverage of the XRT error circle by the 8'x8' region
for the list of sources generated on-board is uncertain because the large
number of sources filled the available telemetry. No correction has been made
for the expected extinction corresponding to E(B-V) of 0.11. 




Burst Advocate for this burst is W. H. Baumgartner (wayne AT milkyway.gsfc.nasa.gov). 
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.)