TITLE:   GCN CIRCULAR
NUMBER:  13208
SUBJECT: GRB 120404A: Swift detection of a burst with an optical counterpart
DATE:    12/04/04 13:01:42 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

G. Stratta (ASDC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester),
S. Campana (INAF-OAB), V. D'Elia (ASDC), P. A. Evans (U Leicester),
C. Guidorzi (U Ferrara), S. T. Holland (STScI), E. A. Hoversten (PSU),
J. A. Kennea (PSU), A. Y. Lien (NASA/GSFC/ORAU),
V. Mangano (INAF-IASFPA), F. E. Marshall (NASA/GSFC),
A. Maselli (INAF-IASFPA), A. Melandri (INAF-OAB),
P. T. O'Brien (U Leicester), C. Pagani (U Leicester),
K. L. Page (U Leicester), P. Romano (INAF-IASFPA),
R. L. C. Starling (U Leicester), M. C. Stroh (PSU) and
B.-B. Zhang (PSU) report on behalf of the Swift Team:

At 12:51:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120404A (trigger=519380).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 235.012, +12.882 which is 
   RA(J2000)  =  15h 40m 03s
   Dec(J2000) = +12d 52' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single symmetric peak
with a duration of about 35 sec.  The peak count rate
was ~1800 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 12:53:12.4 UT, 130.0 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 235.00884, 12.88446 which
is equivalent to:
   RA(J2000)  = 15h 40m 02.12s
   Dec(J2000) = +12d 53' 04.1"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 14 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 3.42
x 10^20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 7.34e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 138 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	15:40:02.29 = 235.00956
  DEC(J2000) = +12:53:06.3  =  12.88508
with a 90%-confidence error radius of about 0.65 arc sec. This position is 4.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.37 with a 1-sigma error of about  0.16. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.05. 

Burst Advocate for this burst is G. Stratta (giulia.stratta AT asdc.asi.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.)