[vsnet-grb-info 3724] GRB 060912: Swift detection of a burst with an optical afterglow

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5558
SUBJECT: GRB 060912: Swift detection of a burst with an optical afterglow
DATE:    06/09/12 14:24:06 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at lheamail.gsfc.nasa.gov>

C. P. Hurkett (U Leicester), S. D. Barthelmy (GSFC),
P. J. Brown (PSU), D. N. Burrows (PSU), M. M. Chester (PSU),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC),
O. Godet (U Leicester), C. Guidorzi (Univ Bicocca&INAF-OAB),
S. T. Holland (GSFC/USRA), S. Immler (GSFC/USRA), J. A. Kennea (PSU),
H. A. Krimm (GSFC/USRA), F. E. Marshall (NASA/GSFC),
K. M. McLean (LANL/UTD), P. T. O'Brien (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL), A. M. Parsons (GSFC),
P. Romano (INAF-OAB), T. Sakamoto (NASA/ORAU), G. Sato (GSFC/ISAS),
M. Stamatikos (NASA/ORAU) and H. Ziaeepour (UCL-MSSL) report on
behalf of the Swift Team:

At 13:55:54 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 060912 (trigger=229185).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA,Dec 5.286, +20.971 {00h 21m 09s, +20d 58' 17"} (J2000)
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single FRED-like peak
(FWHM ~3 sec) with a total duration of about 7 sec.  The peak count rate
was ~6000 counts/sec (15-350 keV), at ~1 sec after the trigger.  We note that the 
spectrum is hard and the main emission is confined to ~3 sec.  It is possible that 
the burst is short, although it appears with existing data to be long.  A lag 
analysis will be done with the event data in ~3 hours. 

The XRT began taking data at 13:57:43 UT, 109 seconds after the BAT
trigger. The XRT on-board centroid algorithm did not find a source in
the image but ground analysis of flight data reveals a fading source
at Ra, Dec 00h 21m 08.2 +20 58 18.6 (J2000), with an uncertainty of
3.9" (radius, 90% containment). This is 7 arcseconds from the BAT
position and 1 arcsecond from the UVOT position. 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm) filter starting 113 seconds after the BAT trigger. There
is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image
at RA(J2000) = 00:21:08.16 (5.2840) DEC(J2000) = +20:58:17.8
(20.9716) with a 1-sigma error radius of about 0.5 arc sec. This
position is 7.0 arc sec. from the center of the BAT error circle. The
estimated magnitude is 16.1 with a 1-sigma error of about 0.5 mag. The
afterglow fades to about 18th magnitude in the subsequent white
finding chart. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.05. 

We note that there is a 2MASS galaxy in the Hyperleda catalog
at  00:21:08.6 20:58:08 (PGC # 1639821), 11.5 arc sec from the UVOT 
position.  The diameter of the galaxy is 0.36 arcmin.