TITLE:   GCN CIRCULAR
NUMBER:  14417
SUBJECT: Konus-Wind observation of GRB 130418A
DATE:    13/04/20 17:42:33 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 GRB 130418A (Swift-BAT trigger #553847:
De Pasquale et al., GCN 14377; Cummings et al., GCN 14392)
was observed by Konus-Wind (K-W) in the waiting mode.

K-W light curve shows a soft pulse started ~215 s before
the BAT trigger (T0(BAT)=19:00:53).
A total duration of the pulse is ~120 s.
The decay phase of the burst which had triggered Swift-BAT
is virtually indistinguishable from the K-W background.
Since the K-W ecliptic latitude response to the burst
is consistent with the BAT localization, we suggest
the pulse observed by K-W is an initial phase of GRB 130418A.

As observed by Konus-Wind, this part of the burst had
a fluence of (1.57 ± 0.25)x10^-5 erg/cm2 and a 3-s peak flux,
measured from T0(BAT)-186 s, of (2.5 ± 0.4)x10^-7 erg/cm2/s
(both in the 20 - 1200 keV energy range).

Fitting the K-W 3-channel time-averaged spectrum
(from T0(BAT)-215 s to T0(BAT)-95 s) by a simple
power-law model yields a photon index of 2.12 ± 0.09,
which is consistent with the slope reported by BAT
for the burst tail.

Assuming z=1.218 (de Ugarte Postigo et al., GCN 14380;
Kruehler et al. GCN 14390) and a standard cosmology model
with H_0 = 70 km/s/Mpc, Omega_M = 0.27, Omega_Lambda = 0.73,
the isotropic energy release E_iso is (6.3 ± 1.0)x10^52 erg,
and the peak luminosity (L_iso)_max is (1.2 ± 0.2)x10^51 erg/s.

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

The K-W light curve of this burst is available at
http://www.ioffe.rssi.ru/LEA/GRBs/GRB130418A/