GRB060105: Swift detection of a bright long burst

TITLE: GCN GRB OBSERVATION REPORT
NUMBER: 4429
SUBJECT: GRB060105: Swift detection of a bright long burst
DATE: 06/01/05 07:56:10 GMT
FROM: David Burrows at PSU/Swift dxb15@psu.edu

H. Ziaeepour (UCL-MSSL), A. Blustin (UCL-MSSL), D. Burrows (PSU), J. Cummings (GSFC/ORAU), N. Gehrels (GSFC), O. Godet (U. Leicester), J. Kennea (PSU), C. Markwardt (GSFC/UMd), K. Page (U. Leicester), and D. Palmer (LANL) on behalf of the Swift team:

At 06:49:28 UT, Swift-BAT triggered and located GRB 060105 (trigger=175942). The spacecraft slewed immediately. The BAT on-board calculated location is RA,Dec 297.474d, +46.362d {19h 49m 54s, +46d 21′ 45″} (J2000), with an uncertainty of 3 arcmin (radius, 90% containment, stat+sys). The BAT light curve shows a multi-peaked structure with a total duration of 60 sec, beginning around T-20s. The peak count rate is ~20000 counts/sec (15-350 keV), at ~10 seconds after the trigger.

XRT began observing the field at 06:50:55.3 UT, 87 sec after the BAT trigger. Onboard centroiding found a bright fading uncatalogued X-ray source in the field of view at the following coordinates:

RA(J2000): 19h 50m 01.1s
Dec(J2000): +46d 21′ 01.0″

We estimate the uncertainty of this position to be 7 arcsec (radius, 90% containment). This uncertainty includes a systematic error of about 5 arcseconds in the on-board calculated positions due to the XRT boresight offset. This position is 87 arcsec from the BAT position above. The initial source flux is 6.8e-9 erg/cm2/sec. We note that the automated GCN notice identified this as a probable cosmic ray. The XRT image at the position given above is definitely a bright celestial source, not a cosmic ray.

UVOT took a finding chart exposure of 200 seconds with the V filter starting 91 seconds after the BAT trigger. The 2.7’x2.7′ sub-image covers 100% of the XRT error circle. The 8’x8′ region for the list of sources generated on-board covers 100% of the XRT error circle. No unambiguous afterglow candidate has been found in the initial data products, although there is a potential low-significance UVOT counterpart on the edge of the XRT error circle, at RA=19h 50m 01s DEC=+46d 20m 56s, with a V magnitude of ~19. The typical 3-sigma upper limit has been about 18th mag. No correction has been made for the expected visual extinction of about 0.6 magnitudes. The XRT position is very close to a bright (B mag ~ 12.8) star.

We are currently in the portion of the orbits where the spacecraft does not pass over the Malindi downlink station. Therefore, it will be ~8 hours before we have access to the full data set for the refined analyses.