TITLE:   GCN CIRCULAR
NUMBER:  41905
SUBJECT: Konus-Wind detection of GRB 250919A
DATE:    25/09/20 15:50:37 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:

The long GRB 250919A (Fermi-GBM detection:
The Fermi GBM team, GCN 41874; Mukherjee & Meegan, GCN 41890;
EP-WXT detection: Liang et al., GCN 41879;
SVOM-GRM observaion: Wang et al., GCN 41882),
Fermi-LAT detection: Holzmann Airasca et al., GCN 41884;
Glowbug detection: Cheung et al., GCN 41891)
triggered Konus-Wind (KW) at T0=1741.403 s UT (00:29:01.403).

The burst consists of two separated emission pulses
and had the total duration of ~240 s.
The emission is seen up to ~10 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250919_T01741/

As observed by Konus-Wind, the burst had
a fluence of (4.24 ± 0.25)x10^-4 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 23.168 s,
of (1.33 ± 0.09)x10^-4 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+218 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -0.94 (-0.05,+0.05),
the high energy photon index beta = -2.32 (-0.17,+0.11),
the peak energy Ep = 452 (-33,+39) keV,
chi2 = 101/97 dof.

The spectrum near the peak count rate (measured from T0+23.04 to T0+23.296 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -0.07 (-0.09,+0.10),
the high energy photon index beta = -2.15 (-0.07,+0.06),
the peak energy Ep = 515 (-38,+42) keV,
chi2 = 63/62 dof.

Assuming the redshift z=1.145 (Levan et al., GCN 41883)
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 (1.53 ± 0.09)x10^54 erg,
the isotropic peak luminosity L_iso to (1.03 ± 0.07)x10^54 erg/s, and
the rest-frame peak spectral energy Ep,z to (970 ± 80) keV.
With the obtained estimates, GRB 250919A is consistent with 68% prediction bands
of both 'Amati' and 'Yonetoku' relations 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/GRB250919_T01741/GRB250919A_rest_frame.pdf

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