TITLE: GCN CIRCULAR NUMBER: 9912 SUBJECT: GRB 090915 Swift-BAT detection of a burst in ground analysis DATE: 09/09/16 03:11:44 GMT FROM: Jay R. Cummings at NASA/GSFC/Swift J. R. Cummings (NASA/GSFC/CRESST) and C. B. Markwardt (NASA/GSFC/CRESST) on behalf of the Swift-BAT team At 15:35:36 Swift-BAT triggered on GRB 090915. This was probably the same event as Fermi-GBM trigger 274721737. No source was found onboard. A weak but significant source was found in ground analysis, 15 degrees from the GBM ground- calculated (2nd version) position as reported at http://gcn.gsfc.nasa.gov/fermi_grbs.html. The GBM position was also in the BAT FOV. The BAT ground-calculated position is RA, Dec = 237.990, +15.480 deg which is RA(J2000) = 15h 51m 57.6s Dec(J2000) = +15d 28' 48" with an uncertainty of 3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 12%. The mask-weighted light curve shows a single weak peak. T90 (15-350 keV) is 8 +- 2 sec (estimated error including systematics). The time-averaged spectrum from T-3 to T+5 sec is best fit by a power law. This fit gives a photon index 1.0 +- 0.2. For this model the total fluence in the 15-150 keV band is 5 +- 2 x 10-7 erg/cm2 and the 1-sec peak flux measured from T+2 sec in the 15-150 keV band is 0.8 +- 0.3 ph/cm2/sec. All the quoted errors are at the 90% confidence level. A Swift TOO has been requested and approved. The narrow- field instruments will observe the location approximately 13 hours after the burst.