TITLE: GCN CIRCULAR NUMBER: 8944 SUBJECT: GRB 090306B: Burst detection from Swift-BAT slew data DATE: 09/03/07 16:50:40 GMT FROM: Antonio Copete at Harvard U A. Copete, J. Grindlay (Harvard) S. Barthelmy, C. Markwardt, N. Gehrels (NASA/GSFC) We report the detection of a bright GRB by the BAT Slew Survey (BATSS) in the slew that started at 23:07:02 and lasted 66 sec. The ground- calculated position is RA, Dec = 231.204, -6.958 deg, which is RA (J2000) = 15h 24m 49s Dec(J2000) = -06d 57' 29" with an uncertainty of 2.87 arcmin (90% confidence, including systematics). The detection was triggered by simultaneous independent detections of 35.6 sigma and 20.2 sigma from imaging in the 15-50 keV and 50-150 keV energy bands, respectively. The burst mask-tagged lightcurve lightcurve in the 15-150 keV band shows a strong and broad emission peak starting at T+21sec, lasting 29 sec, and peaking twice at T+25sec and T+30sec. T90 = 20.4sec and T50 = 8.0sec. The time-averaged spectrum over the 55 sec exposure time of the source is best fit by a power law with an exponential cutoff, with photon index 1.02 and high-energy cut at 133 keV. The fluence in the 15-150 keV band is 3.1 x 10^-6 erg/cm^2, and and the peak 1-sec flux is 2.5 10^-7 erg/cm^2/sec at T+30sec. A follow-up Swift ToO observation for this burst has been scheduled.