GRB 080420, GRB 080420A

B. E. Schaefer (Louisiana State), F. Yuan (U Mich), W. Rujopakarn 
(Steward), report on behalf of the ROTSE collaboration:

ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB
080420 (Swift trigger 309665; Vetere, L. et al., GCN 7632
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 ) a possible
short burst, producing images beginning 8.1 s after the GCN notice time.
An automated response took the first image at 04:13:10.2 UT, 23.0 s after
the burst, under fair conditions.  These observations were affected by the
Full Moon 75 degrees away. We took 10 5-sec, and 60 20-sec exposures.
These unfiltered images are calibrated relative to USNO A2.0 (R). Imaging
is on going.

Comparison to the DSS (second epoch) reveals no new sources within the 
3-sigma Swift/BAT error circle; the field is not crowded. Individual 
images have limiting magnitudes ranging from 16.1-17.1; we set the 
following specific limits.

start UT       end UT      t_exp(s)   mlim   t_start-tGRB(s)  Coadd?
--------------------------------------------------------------------
04:13:10.2   04:13:15.2         5     16.1           23.0       N
04:13:10.2   04:14:17.3        67     18.1           23.0       Y
04:24:09.3   04:28:51.6       282     18.5          682.1       Y

L. Vetere (PSU), D. N. Burrows (PSU), M. M. Chester (PSU),
D. Grupe (PSU), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA),
C. B. Markwardt (CRESST/GSFC/UMD), C. Pagani (PSU),
D. M. Palmer (LANL), J. L. Racusin (PSU), M. Stamatikos (NASA/ORAU)
and M. C. Stroh (PSU) report on behalf of the Swift Team:

At 04:12:47 UT, the Swift Burst Alert Telescope (BAT) triggered and
located possible GRB 080420 (trigger=309665).  Swift slewed immediately 
to the calculated location.  The BAT on-board calculated location is 
RA, Dec 152.639, +43.242 which is 
   RA(J2000) = 10h 10m 33s
   Dec(J2000) = +43d 14' 33"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single spike
structure in a single 64 msec bin.  The peak count rate
was ~1300 counts/sec (15-350 keV), at ~0 sec after the trigger. 

Because of the low significance of the detection (6.6 sigma) we
cannot positively confirm that this is a real event until we
receive the downlinked data from Malindi around 10:00 UT. 

The XRT began observing the source at 04:13:42 UT, 54 s after the BAT trigger. 
The on-board detection algorithm did not centroid on a source due to 
insufficient counts so no prompt X-ray position is available at the moment. 
No source is seen in 300 seconds of promptly downloaded data; therefore 
any X-ray source is extremely faint.  No further data will be available until
about 10 UT. 

UVOT took a finding chart exposure of 100 seconds with the White 
(160-650 nm)  filter starting 57 seconds after the BAT trigger, and 
an exposure of 400 seconds with the V filter starting 163 seconds 
after the BAT trigger. No afterglow candidate has  been found in  the
initial data products. The 2.7'x2.7' sub-image covers 25% of  the  BAT
error circle. The typical 3-sigma upper limit has been about 18.5 
mag. The 8'x8' region for the list of sources generated on-board  
covers 100% of the BAT error circle. The list of sources is typically
 complete  to about 18 mag. No correction has been made for the 
expected extinction  corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is L. Vetere (vetere AT astro.psu.edu). 
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.)

[GCN OPS NOTE(21apr08): Per author's request, the "~2000 counts/sec"
in the first paragraph was changed to "~1300 counts/sec".]