TITLE:   GCN CIRCULAR
NUMBER:  14874
SUBJECT: GRB 130612A: Swift detection of a burst with an optical counterpart
DATE:    13/06/12 03:44:00 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

J. L. Racusin (NASA/GSFC), J. R. Cummings (NASA/UMBC),
N. Gehrels (NASA/GSFC), J. A. Kennea (PSU),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC),
C. Pagani (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 03:22:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 130612A (trigger=557976).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 259.804, +16.706 which is 
   RA(J2000) = 17h 19m 13s
   Dec(J2000) = +16d 42' 21"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 5 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 03:23:49.1 UT, 87.0 seconds after
the BAT trigger. The position determined from promptly downlinked data
differs significantly from the on-board position, suggesting that the
XRT may have centroided on a cosmic ray; the initial XRT position
notice should be treated with caution. Using promptly downlinked data
we find an uncatalogued X-ray source with an enhanced position: RA, Dec
259.7940, 16.7205 which is equivalent to:
   RA(J2000)  = 17h 19m 10.55s
   Dec(J2000) = +16d 43' 13.9"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 62 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 in excess of the Galactic value (5.96 x
10^20 cm^-2, Kalberla et al. 2005), with an excess column of 7.8
(+9.63/-6.58) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of  91 seconds with the White filter
starting 94 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	17:19:10.59 = 259.79411
  DEC(J2000) = +16:43:11.7  =  16.71991
with a 90%-confidence error radius of about 0.64 arc sec. This position is 2.3
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.80 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.08. 

Burst Advocate for this burst is J. L. Racusin (judith.racusin AT 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.)