GCN Circular 15145

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 long-duration soft-spectrum GRB 130831A
(Swift-BAT trigger 568849: Hagen, et al., GCN 15139
Not found
 
 
)
triggered Konus-Wind at T0=47062.044 s UT (13:04:22.044)

The light curve shows a smooth pulse peaked at ~T0+1.6 s
followed by a weaker emission till ~T0+35~s.
The emission is seen up to ~6 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB130831_T47062/

As observed by Konus-Wind, the burst
had a fluence of (7.6 �� 0.4)x10-6 erg/cm2,
and a 64-ms peak flux, measured from T0+1.600 s,
of (2.5 �� 0.3)x10-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum (measured from T0 to T0+24.832 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -1.61 �� 0.06,
the high energy photon index beta = -3.3 �� 0.3,
the peak energy Ep = 55 �� 4 keV,
chi2 = 118/97 dof.

The spectrum at the maximum count rate (measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -1.51 �� 0.10,
the high energy photon index beta = -2.8 �� 0.1,
the peak energy Ep = 67 �� 4 keV,
chi2 = 91/96 dof.

Assuming Gemini-North redshift z=0.4791 (Cucchiara & Perley, GCN 15144
Not found
 
 
),
and a standard cosmology model with H_0 = 70 km/s/Mpc,
Omega_M = 0.27, and Omega_Lambda = 0.73,
we estimate the following rest-frame parameters of GRB 130831A:
the isotropic energy release E_iso is (4.6 �� 0.2)x10^51 erg,
the peak luminosity (L_iso)_max is (2.7 �� 0.3)x10^51 erg/s,
and the rest-frame peak energy Ep,i = (81 �� 6) keV

All the quoted results are preliminary.