[vsnet-grb-info 13946] Konus-Wind observation of GRB 130925A

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  15260
SUBJECT: Konus-Wind observation of GRB 130925A
DATE:    13/09/26 15:36:02 GMT
FROM:    Dmitry Frederiks at Ioffe Institute  <fred at mail.ioffe.ru>

S. Golenetskii, R.Aptekar, D. Frederiks, V. Pal'shin,
P. Oleynik, M. Ulanov, D. Svinkin, and T. Cline,
on behalf of the Konus-Wind team, report:

The very long GRB 130925A
(Swift-BAT trigger #571830: Lien et al., GCN 15246;
Markwardt et al., GCN 15257;
MAXI/GSC detection: Suzuki et al., GCN 15248;
Fermi/GBM detection: Fitzpatrick, GCN 15255;
INTEGRAL/SPI-ACS observation: Savchenko et al., GCN 15259)
was detected by Konus-Wind in the waiting mode.

The light curve shows several multi-peaked pulses separated
by long periods of a low-level emission with a total duration of ~4500 s.
At least three major emission episodes can be distingushed:
the first one, which triggered BAT, started at ~T0(BAT)-130 s
and lasted until ~T0(BAT)+170 s; the second, more fluent episode,
from ~T0(BAT)+1750 s to ~T0(BAT)+2950 s;
and the third, the weakest one, which onset corresponds
to the MAXI detection, from ~T0(BAT)+3730 s to ~T0(BAT)+4360 s.

The possible precursor detected by GBM is clearly seen
at ~T0(BAT)-900 s in the softest KW energy band.

As observed by Konus-Wind, the burst had a fluence of
(5.0 ± 0.1)x10^-4 erg/cm2 and a 2.944-s peak energy flux,
measured from T0(BAT)+2548.858, of (1.0 ± 0.03)x10^-6 erg/cm2
(both in the 20 - 10000 keV energy range).

The emission shows no signs of strong spectral variability.
Modeling the KW 3-channel time-integrated spectrum
(from T0(BAT)-120 s to T0(BAT)+4340 s) by a power law
with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
yields alpha = -1.42 ± 0.04, and Ep = 181 ± 10 keV.

Assuming z = 0.347 (Vreeswijk et al., GCN 15249) and a
standard cosmology model with H_0 = 71 km/s/Mpc, Omega_M = 0.27,
Omega_Lambda = 0.73,
the isotropic energy release E_iso = (1.50 ± 0.03)x10^53 erg,
the peak isotropic luminosity L_iso = (4.0 ± 0.1)x10^50 erg,
and Ep_rest is 244 ±  13 keV.

Thus, the prompt gamma-ray emission properties of this GRB: fluence, Ep,
and E_iso are similar to those observed in other long energetic GRBs;
the only outstanding feature of the burst is its huge duration.
In that, it resembles other ultra-long bursts: GRB 111209A at z=0.677
(with total duration of ~10000 s: Golenetskii et al., GCN 12663)
and GRB 121027A at z=1.77 (with a total duration of >4000 s: Starling et al.,
in preparation).
Therefore, we suggest that the extremely rapid and dramatic X-ray flaring
observed over the first 10^4 s after the BAT trigger (Evans et al., GCN 15254)
at least partially corresponds to the burst prompt emission.

All the quoted errors are estimated at the 1 sigma confidence level.
All the presented results are preliminary.

The K-W light curve of this burst is available at
http://www.ioffe.rssi.ru/LEA/GRBs/GRB130925A/