[vsnet-grb-info 17167] Konus-Wind observation of GRB 151027A

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  18516
SUBJECT: Konus-Wind observation of GRB 151027A
DATE:    15/10/28 14:43:29 GMT
FROM:    Dmitry Svinkin at Ioffe Institute  <svinkin at mail.ioffe.ru>

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

The long-duration GRB 151027A
(Swift-BAT trigger #661775: Maselli et al., GCN Circ. 18478,
Palmer et al., GCN Circ. 18496;
Fermi GBM detection: Toelge et al., GCN Circ. 18492)
triggered Konus-Wind at T0=14304.154 s UT (03:58:24.154).

The burst light curve consists of at least three pulses
with a total duration of ~120 s.
The emission is seen up to ~5 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB151027_T14304/

As observed by Konus-Wind, the burst
had a fluence of 1.42(-0.21,+0.37)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+0.432 s,
of 2.22(-0.61,+0.62)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+131.328 s)
is best fit in the 20 keV - 5 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.44(-0.21,+0.24)
and Ep = 173(-46,+135) keV (chi2 = 78/77 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.0
(chi2 = 77/76 dof)

The spectrum near the maximum count rate
(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.96(-0.28,+0.32)
and Ep = 91(-11,+14) keV (chi2 = 56/61 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.7
(chi2 = 56/60 dof)

Assuming the redshift z=0.81 (Perley et al., GCN Circ. 18487)
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 ~2.4x10^52 erg,
the peak luminosity L_iso is ~7x10^51 erg/s,
and the rest-frame peak energy of the time-integrated spectrum,
Ep,i, is ~310 keV.

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