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.
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