TITLE: GCN CIRCULAR NUMBER: 15233 SUBJECT: GRB 130913A detected in ground analysis of Swift-BAT data DATE: 13/09/19 18:34:54 GMT FROM: Jay R. Cummings at NASA/GSFC/Swift J. R. Cummings (UMBC@NASA GSFC), D. M. Palmer (LANL), A. Y. Lien (NASA/GSFC/ORAU), and C. B. Markwardt (GSFC) report on behalf of the Swift science team: On 2013-09-13 at 00:28:21 Swift-BAT detected a preliminary rate trigger (Trigger #570531), now designated GRB 130913A. On-board analysis was terminated prematurely by a preplanned slew maneuver. No significant source was found on-board. In post-processing on the ground, a significant source was located at RA, Dec 341.960, +1.294, which is RA(J2000) = 22h 47m 50.4s Dec(J2000) = +01d 17' 38.0" with an estimated 90% error radius of 3.3 arcmin. The lightcurve shows a single FRED peak about 10 seconds long (T100). Determining a formal T90 is difficult because of the weakness of the burst. The time-averaged spectrum from T-2 to T+8 sec is fit by a simple power law model. This fit gives a photon index of 0.9 +- 0.3 (68% confidence). The fluence in the 15-150 keV band was (1.3 +- 0.4) x 10^-7 ergs/cm^2 (90% confidence). We note the presence of a galaxy cluster at z ~ 0.123 centered within 5 arcmin of the BAT position. The source went into Moon constraint for Swift very shortly after discovery, so no TOO was possible, and there will be no narrow-field instrument data for this GRB.