TITLE:   GCN CIRCULAR
NUMBER:  42765
SUBJECT: Konus-Wind detection of GRB 251118C (EP251118a)
DATE:    25/11/19 16:58:51 GMT
FROM:    Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>

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

A long-duration GRB 251118C was detected by Konus-Wind (KW)
in the waiting mode at T0=T0(KW)~16:44:49 UT.
A Bayesian block analysis of the KW data in the 20-1500 keV
band reveals a ~17 sigma count rate increase in the interval
from ~T0-0.5 s to ~T0+23 s.

Corrected for the propagation time, the burst started ~140 s
after the start time of the fast X-ray transient EP251118a
(T0(EP)=16:42:29 UT; Jiang et al., GCN 42749).
The burst source is located in the southern ecliptic hemisphere
at a low ecliptic latitude, which is consistent with the EP251118a localization.
The positional and temporal coincidence of GRB 251118C with
the EP251118a supports the conclusion that both events have
a common origin.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB251118C/

The time-integrated spectrum of the burst, measured from T0-0.5 s to T0+23 s,
can be described by a simple power function with the photon index (2.29 ± 0.11).

The total burst fluence is (5.08 ± 0.40)x10^-6 erg/cm^2,
and the 2.944 s peak energy flux, measured from T0-1.96 s,
is (4.84 ± 0.38)x10^-7 erg/cm^2(both in the 20 - 1500 keV energy range).

Assuming the redshift z=1.216 (An et al., GCN 42756)
and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315,
and Omega_Lambda = 0.685 (Planck Collaboration, 2014),
we estimate the burst isotropic energy release E_iso to (2.06 ± 0.16)x10^52 erg,
the isotropic peak luminosity L_iso to (4.34 ± 0.34)x10^51 erg/s.

All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.