TITLE: GCN CIRCULAR NUMBER: 39423 SUBJECT: Konus-Wind detection of GRB 250221A DATE: 25/02/22 12:13:42 GMT FROM: Dmitry Frederiks at Ioffe Institute fred@mail.ioffe.ru

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

GRB 250221A (Swift detection: Caputo et al., GCN 39396) was detected by Konus-Wind (KW) in the waiting mode.

A Bayesian block analysis of the KW waiting mode data in the 20-400 keV band reveals a ~12 sigma count-rate increase in the interval from T0-1.5 s to T0+4.4 s where T0 = T0(BAT) = 03:34:37 UT.

The KW light curve of this burst is available at http://www.ioffe.ru/LEA/GRBs/GRB250221A/

Modeling the time-integrated spectrum of the burst by a power law with exponential cutoff (CPL) model dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) yields alpha = -1.05 (-0.12, + 0.12) and Ep = 242(-22,+26) keV. In the 10 keV -10 MeV band, standard for the KW analysis, the burst fluence is (1.67 ± 0.09)x10^-6 erg/cm^2 and the 2.944 s peak energy flux is (2.97 ± 0.17)x10^-7 erg/cm^2/s.

Assuming the redshift z=0.768 (Palmerio et al., GCN 39418) 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 (2.8 ± 0.2)x10^51 erg, the isotropic peak luminosity L_iso to (8.6 ± 0.5)x10^50 erg/s, and the rest-frame peak spectral energy Ep,z to (428 ± 44) keV. With the obtained estimates, GRB 250221A is a hard-spectrum outlier (lies outside 90% 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/GRB250221A/GRB250221A_rest_frame.pdf

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

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