GCN Circular 19604

D. Svinkin, S. Golenetskii, R. Aptekar, D. Frederiks, P. Oleynik,
M. Ulanov,  A. Tsvetkova, A. Lysenko, A. Kozlova, and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration, extremely bright GRB 160625B
(Fermi-LAT detection: Dirirsa et al., GCN Circ. 19580
Not found
 
 
;
Fermi-GBM detection: Burns, GCN Circ. 19581
Not found
 
 
)
triggered Konus-Wind at T0=81619.875 s UT (22:40:19.875).

The burst light curve starts, at ~T0-0.3 s, with a short soft
initial pulse (precursor) which had a duration of about 1 s.
The precursor is followed, starting at ~T0+180 s, by the main, extremely 
bright and spectrally-hard episode with a duration of
about 40 s. This episode evolves into a weak emission traceable
up to ~T0+680 s.
The emission in the main episode is seen up to ~20 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB160625_T81619/

As observed by Konus-Wind, the burst
had a fluence of 9.44(-0.16,+0.16)x10^-4 erg/cm2,
and a 256-ms peak flux, measured from T0+188.928 s,
of 1.26(-0.10,+0.10)x10^-4 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the initial pulse
(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 1 MeV range
by a simple power law model with
the photon index alpha = -2.34(-0.22,+0.25)
(chi2 = 38/61 dof)

The time-averaged spectrum of the main episode
(measured from T0+180.480 to T0+229.632 s)
is best fit in the 20 keV - 20 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.79(-0.02,+0.02),
the high energy photon index beta = -2.25(-0.04,+0.04),
the peak energy Ep = 554(-19,+20) keV
(chi2 = 353/97 dof)

The spectrum near the maximum count rate
(measured from T0+188.928 to T0+189.184 s)
is best fit in the 20 keV - 20 MeV range
by the Band model with the following parameters:
the low-energy photon index alpha = -0.77(-0.08,+0.10),
the high energy photon index beta = -2.22(-0.25,+0.17),
the peak energy Ep = 937(-179,+195) keV
(chi2 = 62/67 dof)

Assuming the redshift z=1.406 (Xu, et al., GCN Circ. 19600
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:
the isotropic energy release E_iso is ~5.0x10^54 erg,
the peak luminosity L_iso is ~1.6x10^54 erg/s,
and the rest-frame peak energy of the time-integrated spectrum,
Ep,i, is ~1.3 MeV.

This Eiso is the highest one measured by Konus-Wind for
more than 21 years of its GRB observations and, probably,
the highest Eiso recorded for a GRB ever.

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.