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GCN Circular 12794

Subject
GRB 120102A: Swift detection of a burst with an optical counterpart
Date
2012-01-02T02:28:38Z (12 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
F. E. Marshall (NASA/GSFC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
N. Gehrels (NASA/GSFC), E. A. Hoversten (PSU),
A. Y. Lien (NASA/GSFC/ORAU), C. B. Markwardt (NASA/GSFC),
T. Sakamoto (NASA/UMBC), C. J. Saxton (UCL-MSSL), C. A. Swenson (PSU)
and T. N. Ukwatta (MSU) report on behalf of the Swift Team:

At 02:15:55 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120102A (trigger=510922).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 276.201, +24.732 which is 
   RA(J2000)  =  18h 24m 48s
   Dec(J2000) = +24d 43' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows an initial small peak
and then a much larger peak 32 sec later with a toal duration of about 50 sec. 
The peak count rate was ~15,000 counts/sec (15-350 keV), at ~33 sec
after the trigger. 

The XRT began observing the field at 02:17:47.9 UT, 112.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 276.22495,
24.71375 which is equivalent to:
   RA(J2000)  = 18h 24m 53.99s
   Dec(J2000) = +24d 42' 49.5"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 102 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 in excess of the Galactic value (1.03 x
10^21 cm^-2, Kalberla et al. 2005), with an excess column of 4.1
(+2.17/-1.89) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 4.02e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 121 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	18:24:53.90 = 276.22457
  DEC(J2000) = +24:42:47.3  =  24.71314
with a 90%-confidence error radius of about 0.62 arc sec. This position is 3.3
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.04 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.13. 

Burst Advocate for this burst is F. E. Marshall (marshall 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.)
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