[vsnet-grb-info 3129] GRB 060418: Swift detection of a burst with bright x-ray and optical afterglow

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4966
SUBJECT: GRB 060418: Swift detection of a burst with bright x-ray and optical afterglow
DATE:    06/04/18 03:23:03 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <Scott at lheamail.gsfc.nasa.gov>

A. D. Falcone (PSU), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
J. R. Cummings (NASA/ORAU), N. Gehrels (NASA/GSFC),
S. D. Hunsberger (PSU), J. A. Kennea (PSU) and D. M. Palmer (LANL)
report on behalf of the Swift Team:

At 03:06:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 060418 (trigger=205851).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA,Dec 236.420, -3.643 {15h 45m 41s, -03d 38' 35"} (J2000)
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a broad smooth peak
starting at T-8 sec.  It decays almost to background, and then a bright
peak at T+27 sec (FWHM ~3 sec).  There is possibly emission at T+130 sec. 
The peak count rate was ~6000 counts/sec (15-350 keV), at ~27 sec
after the trigger. 

The XRT began observing the field at 03:07:26 UT, 78 seconds after the
BAT trigger. XRT found a bright, variable, uncatalogued X-ray source
located at RA(J2000) = 15h 45m 42.8s, Dec(J2000) = -03d 38' 26.1",
with an estimated uncertainty of 5.8 arcseconds (90% confidence
radius). This location is 32 arcseconds from the BAT on-board
position, within the BAT error circle. The initial flux in the 0.1s
image was 1.6e-08 erg/cm2/s (0.2-10 keV). 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm) filter starting 88 seconds after the BAT trigger. There
is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image
at (RA,DEC) (J2000) of (236.4275,-3.6389) or
(15h45m42.60s,-03o38'20.0") with a 1-sigma error radius of about 0.5
arc sec. This position is 7.2 arc sec. from the center of the XRT
error circle. The estimated V magnitude is 14.5 with a 1-sigma error
of about 0.5 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.22.