GCN Circular 15670

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 GRB 131231A
(Fermi-LAT detection: Sonbas, et al., GCN 15640
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 ;
Fermi-GBM observation: Jenke & Xiong, GCN 15644
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 )
triggered Konus-Wind at T0=17132.361 s UT (04:45:32.361).

The light curve shows a broad mult-peaked pulse from ~T0-13 s to ~T0+35 s.
The emission is seen up to ~15 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB131231_T17132/
Note: periodic 'dips' in the count rate are due
to the GRB source occultation by the s/c structure.

As observed by Konus-Wind, the burst
had a fluence of (1.55 � 0.05)x10-4 erg/cm2,
and a 64-ms peak flux, measured from T0+7.488 s,
of (2.14 � 0.13)x10-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum (measured from T0 to T0+34.303 s)
is best fit in the 35 keV - 18 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -1.28 � 0.04,
the high energy photon index beta = -2.47 � 0.05,
the peak energy Ep = 163 � 6 keV,
chi2 = 93.8/95 dof.

The spectrum near the maximum count rate
(measured from T0+5.888 s to T0+8.448 s)
is best fit in the 35 keV - 10 MeV range
by the cutoff power law with the following model parameters:
the photon index alpha = -0.76 � 0.09,
and the peak energy Ep = 226 � 14 keV,
chi2 = 94.3/82 dof.

Assuming the redshift z=0.644 (Xu, et al., GCN 15645
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 ;
Cucchiara, GCN 15652
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 ),
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:
the isotropic energy release E_iso is (1.7 � 0.1)x10^53 erg,
the peak luminosity L_iso is (3.9 � 0.2)x10^52 erg/s,
and the rest-frame peak energy Ep,i = (270 � 10) keV

All the quoted results are preliminary.