[vsnet-grb-info 13941] Fermi GBM detection of GRB 130925A and a possible precursor

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  15255
SUBJECT: Fermi GBM detection of GRB 130925A and a possible precursor
DATE:    13/09/25 16:13:06 GMT
FROM:    Gerard Fitzpatrick at UCD  <gerard.fitzpatrick at ucdconnect.ie>

Gerard Fitzpatrick (UCD) reports on behalf of the Fermi GBM Team:

At 04:09:26.73 UT on 25 September 2013, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB130925A (trigger 401774969/130925173),
which was also detected approximately 2 minutes later by Swift (Lien et
al., GCN 15246). The on-ground
calculated location is consistent with the Swift location. The GBM data
show that the event
was still in progress at the time of the Swift trigger. The burst was
sufficiently
bright that a Fermi Automatic Repointing Request (ARR) was triggered. This
event may
not actually be a GRB, as indicated by Burrows et al. (GCN 15253). Downlink
of the full data set for
this event was delayed due to the ARR; more details will be provided when
the data are available.

At 03:56:23.29 UT, 15 minutes prior to the GBM detection of GRB130925A,
GBM triggered and located
GBM trigger 401774186/130925164.  The on-ground calculated location, using
the GBM trigger
data, is RA = 48.3 , DEC = -21.8  (J2000 degrees,
equivalent to 03h 13m, -21d 42.0'), with an uncertainty
of 12.8  degrees (radius, 1-sigma containment,
statistical only; there is additionally a systematic
error which is currently estimated to be 2 to 3 degrees).
This is also consistent with the Swift location of GRB130925A
(Lien et al., GCN 15246). The temporal and positional coincidence indicate
that this may be
a precursor pulse to GRB130925A.

The GBM light curve for this possible precursor consists of a single peak
with a duration (T90) of about 6.6 s (50-300 keV).
The time-averaged spectrum from T0-4 s to T0+2.5 s is
adequately fit by a simple power law function with index -2.25 +/- 0.07

The event fluence (10-1000 keV) in this time interval is
(6.9 +/- 0.6)E-07 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-3 s in the 10-1000 keV band is 3.0 +/- 0.2 ph/s/cm^2.