TITLE: GCN CIRCULAR NUMBER: 10699 SUBJECT: GRB 100427A detected in ground analysis of Swift BAT data DATE: 10/04/28 16:25:05 GMT FROM: Jay R. Cummings at NASA/GSFC/Swift J. R. Cummings (GSFC/UMBC), H. A. Krimm (GSFC/USRA), and R. L. C. Starling (U Leicester) on behalf of the Swift team: At 08:31:55 UT Swift-BAT detected a rate increase. Onboard analysis was terminated prematurely by a pre-planned slew, but in later ground analysis a significant source was found at RA, Dec 89.171, -3.461, which is RA (J2000) 05h 56m 41.1s Dec (J2000) -03d 27' 40" with an estimated uncertainty of 2.5 arcmin radius (90% containment). The burst had two main peaks, the first much weaker, from about T-3 to T+3 sec and from T+11 to T+23 sec. We have full data for only the first, weaker peak. The peak non-maskweighted count rate (15-350 keV) was about 3000 counts/sec at 16 seconds after the trigger. Since the source was not detected onboard, there are no automated data products for this burst. A Swift TOO has started.