TITLE:   GCN CIRCULAR
NUMBER:  41957
SUBJECT: Konus-Wind detection of GRB 250920C
DATE:    25/09/23 19:12:38 GMT
FROM:    Valentina Panteleeva at Ioffe Institute <panteleevav228@gmail.com>

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

The long-duration GRB 250920C (Fermi-GBM detection:
The Fermi GBM team, GCN 41903; Mailyan & Meegan, GCN 41917;
Swift-BAT detection: Gupta et al., GCN 41904; Gupta et al., GCN 41924; 
SVOM-GRM observaion: Wang et al., GCN 41929)
triggered Konus-Wind at T0=55519.312 s UT (15:25:19.312).

The burst light curve shows a multipeaked structure
which starts at ~T0-0.4 s and has a total duration of ~41 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/GRB250920_T55519/

As observed by Konus-Wind, the burst
had the total fluence of 2.91(-0.21,+0.29)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0+25.904 s,
of 8.27(-0.78,+0.78)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+41.216 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.22(-0.07,+0.09),
the high energy photon index beta = -3.19(-6.81,+0.51),
the peak energy Ep = 160(-12,+9) keV
(chi2 = 104/97 dof).

The spectrum near the maximum count rate
(measured from T0+24.832 to T0+33.024 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 = -1.14(-0.07,+0.07)
and Ep = 220(-14,+16) keV (chi2 = 96/81 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.15
(chi2 = 97/80 dof).

Assuming the redshift z=1.399 (Sanchez-Ramirez et al., GCN 41928;
Izzo et al., GCN 41955)
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 following rest-frame parameters:
the isotropic energy release E_iso is 1.54(-0.11,+0.16)x10^53 erg,
the peak luminosity L_iso is 1.05(-0.10,+0.10)x10^53 erg/s,
the rest-frame peak energy of the time-averaged spectrum
Ep,i,z is 383(-30,+22) keV and the spectrum near the maximum count rate
Ep,p,z is 528(-33,+38) keV.

With the obtained estimates, GRB 250920C is inside 68% prediction bands for
both the 'Amati' and the 'Yonetoku' relations 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/GRB250920_T55519/GRB250920C_rest_frame.pdf

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.