TITLE: GCN CIRCULAR NUMBER: 10602 SUBJECT: GRB 100413B: Swift Confirmation of Burst DATE: 10/04/16 20:56:27 GMT FROM: Judith Racusin at GSFC J. L. Racusin (NASA/GSFC), S. D. Barthelmy (NASA/GSFC), J. Cummings (GSFC/UMBC), P. A. Evans (U. Leicester), N. Gehrels (NASA/GSFC), W. B. Landsman (NASA/GSFC), O. M. Littlejohns (U. Leicester), F. .E. Marshall (NASA/GSFC), K. L. Page (U. Leicester), D. Palmer (LANL) report on behalf of the Swift team: After further analysis of the BAT ground detected GRB 100413B (Racusin et al., GCN 10593), we can now confirm that it is a real burst. It was detected at the level of 12.4 sigma in 10 seconds of data. The light curve shows a peak beginning at ~T-7 sec, and lasting until at least T+18 sec, when the spacecraft slewed away from the burst due to a pre-planned target. Our best position is the BAT ground calculated RA/Dec = 356.8264, +51.2704 deg, which is RA(J2000) = 23h 47m 18.3s Dec(J2000) = +51d 16' 13.4" with an uncertainty of 2 arcmin (radius, 90% containment, including systematic uncertainty). The time-averaged spectrum from T-2 to T+8 sec is best fit by a power- law with an exponential cutoff. The fit gives a power law index of -0.5+/-0.4, and an Epeak of 28+/-6 keV. The fluence in the 15-150 keV band is (4.3 +/- 0.4) x10^-7 ergs cm-2. All the quoted errors are at the 90% confidence level. GRB 100413B was also detected by Konus-Wind in the waiting mode (V. Pal'shin, private communication). A Target of Opportunity observation began ~31.5 hours after the BAT trigger. In 16.5 ks of XRT data, no X-ray afterglow was detected down to a limit of 1.6 x 10^-3 counts s-1 (3-sigma). Assuming an average observed flux to counts conversion factor of 3.8 x 10^-11 (Evans et al., 2009, MNRAS, 397, 1177), this corresponds to a flux limit of 6 x 10^-14 erg cm-2 s-1. Given the late start to follow-up observations and exposure time, the lack of a X-ray counterpart is not surprising. Preliminary 3-sigma upper limits using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) from a 2538 sec u band image taken 31.4 hours after the burst shows no new source within the BAT error circle with a limit of u > 21.4. This value is not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.19 in the direction of the burst (Schlegel et al. 1998).