TITLE:   GCN CIRCULAR
NUMBER:  6330
SUBJECT: GRB 070420: Swift detection of a burst with optical afterglow
DATE:    07/04/20 06:56:57 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

M. Stamatikos (NASA/ORAU), D. N. Burrows (PSU),
G. Cusumano (INAF-IASFPA), P. A. Evans (U Leicester),
C. Gronwall (PSU), V. Mangano (INAF-IASFPA),
C. B. Markwardt (GSFC/UMD), P. T. O'Brien (U Leicester),
D. M. Palmer (LANL), A. M. Parsons (GSFC), M. Perri (ASDC) and
G. Stratta (ASDC) report on behalf of the Swift Team:

At 06:18:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070420 (trigger=276321).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 121.253, -45.561 which is 
   RA(J2000) = 08h 05m 01s
   Dec(J2000) = -45d 33' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multiply-peaked
structure with a duration of at least 70 sec starting at T-50s. 
The peak count rate was ~4000 counts/sec (15-350 keV), at ~7 sec 
after the trigger. 

XRT began observing the field at 06:19:52 UT, 99 seconds after the BAT 
trigger. XRT found a bright, fading, previously uncatalogued  X-ray source 
in the field of view at the following coordinates:
  RA(J2000) =  08h 04m 55.3s
  Dec(J2000)= -45d 33' 23.2"
with an uncertainty of 4 arcseconds radius (90% containment). This is a 
ground calculated position based on prompt downlinked data. This position 
lies 62 arcseconds from the center of the BAT error circle. 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm) filter starting 108 seconds after the BAT trigger.  An
afterglow candidate has been found in the initial data products.  The
2.7'x2.7' region for the list of sources generated on-board covers
100% of the XRT error circle. A source is detected at   
 RA(J2000) = 08h 04m 55.25s 
 Dec(J2000)= -45d 33' 21.0"   
with an estimated uncertainty of +/-0.5 arcsec.  This position is 
2.0 arcsec from the XRT position.  The estimated magnitude in the 
White filter is 17.3. No correction has been made for the expected 
extinction correspondingto E(B-V) of 0.52.