GCN Circular 35893

D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB 231118A
(Fermi-GBM detection: The Fermi GBM team, GCN Circ. 35100
Not found
 
 
;
Barra and Meegan, GCN Circ. 35131
Not found
 
 
;
Swift-BAT detection: Laha et al., GCN Circ. 35101
Not found
 
 
;
Moss et al., GCN Circ. 35141
Not found
 
 
;
AstroSat-CZTI detection: Navaneeth et al., GCN Circ. 35116
Not found
 
 
;
VZLUSAT-2 detection: Dafcikova et al., GCN Circ. 35143
Not found
 
 
;
GRBAlpha detection: Dafcikova et al., GCN Circ. 35151
Not found
 
 
)
triggered Konus-Wind at T0=62189.937 s UT (17:16:29.937).

The burst light curve shows the main multi-peaked emission episode
which starts at ~T0-2.7 s and has a total duration of ~6.7 s,
followed by a weaker emission seen up to ~70 s.
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB231118_T62189/

As observed by Konus-Wind, the main episode
had a fluence of 9.65(-0.91,+1.13)x10^-6 erg/cm2,
and a 64-ms peak flux, measured from T0+2.864 s,
of 9.13(-1.69,+1.83)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The spectrum of the main episode
(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.15(-0.17,+0.20)
and Ep = 321(-55,+84) keV (chi2 = 81/80 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.3
(chi2 = 81/79 dof).

Assuming the redshift z=0.8304 (Schneider et al., GCN Circ. 35123
Not found
 
 
)
and a standard cosmology model with H_0 = 67.3 km/s/Mpc,
Omega_M = 0.315, and Omega_Lambda = 0.685 (Planck Collaboration, 2014),
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is 1.86(-0.18,+0.22)x10^52 erg,
the peak luminosity L_iso is 3.22(-0.59,+0.64)x10^52 erg/s,
and the rest-frame peak energy of the time-integrated spectrum,
Ep,i,z is 588(-101,+155) keV.
With the obtained estimates, GRB 231118A is inside 90% prediction band for the 'Amati' relation and inside 68% prediction band for the 'Yonetoku' relation derived for the sample of >300 long KW GRBs with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021), see http://www.ioffe.ru/LEA/GRBs/GRB231118_T62189/GRB231118A_rest_frame.pdf

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