[vsnet-grb-info 13919] GRB 130913A detected in ground analysis of Swift-BAT data

[GCN Circulars]

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  <james.r.cummings at nasa.gov>

J. R. Cummings (UMBC at 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.