TITLE:   GCN CIRCULAR
NUMBER:  14487
SUBJECT: Konus-Wind observation of GRB 130427A
DATE:    13/04/28 10:22:50 GMT
FROM:    Dmitry Frederiks at Ioffe Institute  <fred@mail.ioffe.ru>

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

The long extremely intense GRB 130427A (Swift-BAT trigger #554620:
Maselli et al., GCN 14448; Barthelmy et al., GCN 14470;
Fermi-LAT detection: Zhu et al., GCN 14471;
Fermi-GBM observation: von Kienlin, GCN 14473;
SPI-ACS/INTEGRAL observations: Pozanenko et al., GCN 14484)
triggered Konus-Wind (K-W) at T0=28029.501s UT (07:47:09.501)
~50 s before the BAT trigger.

The K-W light curve shows a huge mult-peaked emission complex
started at ~T0, peaked at ~T0+8 s, and having a duration of ~20 s.
The emission is seen up to ~12 Mev.
This extremely bright phase of the event passes into a weaker decaying
tail out to ~T0+120s, when the second emission episode started.
It shows a FRED-like pulse, ~100 times weaker in a peak count
rate than the initial complex.
The decaying emission is detectable by K-W in the 20-1200 keV band
out to ~T0+250s, when the instrument switched into the data readout mode.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB130427_T28029/

As observed by Konus-Wind, the main phase of the burst had
a fluence of (2.68 ± 0.01)x10^-3 erg/cm2 and a 16-ms peak flux,
measured from T0+7.774 s, of (6.9 ± 0.1)x10^-4 erg/cm2/s
(both in the 20 - 1200 keV energy range).
The fluence of GRB 130427A is the highest observed by K-W
for ~18 years of GRB observations and its peak flux
is only ~30% lower than measured by K-W for the
ultra-luminous GRB 110918A.

The time-averaged spectrum of the main phase of the burst
(measured from T0 to T0+18.688 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 = -0.958 ± 0.006,
the high energy photon index beta = -4.17 ± 0.16,
the peak energy Ep = 1028 ± 8 keV,
chi2 = 124/96 dof.

The spectrum at the maximum count rate (measured from T0+7.680 to T0+7.936 s)
is best fit in the 20 keV - 15 MeV range
by a power law with exponential cutoff model, for which:
the photon index alpha = -0.57 ± 0.02, and
the peak energy Ep = 1213 ± 31 keV,
chi2 = 63/84 dof.

Modelling the 3-channel time-averaged spectrum
of the second emission episode (from T0+120 s to T0+250 s)
by a power law with exponential cutoff model
yiels the photon index alpha~-1.6 and
the peak energy Ep ~ 240 keV.
The 20-1200 keV energy fluence measured at this
phase of the event is ~9x10^-5 erg/cm2.

Assuming z=0.34 (Levan et al., GCN 14455; Xu et al. GCN 14478)
and a standard cosmology model with H_0 = 70 km/s/Mpc,
Omega_M = 0.27, and Omega_Lambda = 0.73,
the isotropic energy release E_iso is ~8.5x10^53 erg,
and the peak luminosity (L_iso)_max is ~2.7x10^53 erg/s.

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