TITLE:   GCN CIRCULAR
NUMBER:  39542
SUBJECT: Konus-Wind detection of GRB 250226A
DATE:    25/02/28 22:10:03 GMT
FROM:    Dmitry Svinkin at Ioffe Institute <svinkin@mail.ioffe.ru>

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 250226A
(Fermi-GBM detection: The Fermi GBM team, GCN 39479;
Pathak and Meegan, GCN 39530;
EP-WXT detection: Jiang et al., GCN 39482;
INTEGRAL SPI-ACS and PICsIT detection: Thakur et al., GCN 39518)
triggered Konus-Wind at T0=23700.486 s UT (06:35:00.486).

The burst light curve shows a single emission episode
which starts at ~T0-5 s and has a total duration of ~32 s.
The emission is seen up to ~1 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250226_T23700/

As observed by Konus-Wind, the burst
had a fluence of 1.17(-0.21,+0.28)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+0.438 s,
of 3.45(-1.09,+1.18)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+24.832 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 = -0.16(-0.47,+0.68),
the high energy photon index beta = -2.25(-0.38,+0.24),
the peak energy Ep = 143(-28,+35) keV
(chi2 = 64/97 dof).

The spectrum near the maximum count rate
(measured from T0 to T0+8.448 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 = -0.60(-0.36,+0.54),
the high energy photon index beta = -2.00(-8,+0.17),
the peak energy Ep = 190(-53,+331) keV
(chi2 = 86/97 dof).

Assuming the redshift z=3.315 (Zhu et al. GCN 39487)
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 2.80(-0.51,+0.67)x10^53 erg,
the peak luminosity L_iso is 3.56(-1.12,+1.22)x10^53 erg/s,
the rest-frame peak energy of the time-averaged spectrum
Ep,i,z is 616(-119,+151) keV and the spectrum near the maximum count rate
Ep,p,z is 822(-229,+1427) keV.

With the obtained estimates, GRB 250226A 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/GRB250226_T23700/GRB250226A_rest_frame.pdf

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