TITLE: GCN CIRCULAR NUMBER: 39978 SUBJECT: Konus-Wind detection of GRB 250327B DATE: 25/03/31 16:40:41 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 250327B (SVOM-ECLAIRs detection: Bouchet et al., GCN 39888) was detected by Konus-Wind (KW) in the waiting mode at T0 = T0(ECLAIRs) = 76287 s UT (21:11:27).
A Bayesian block analysis of the KW waiting mode data in the 20-1200 keV band reveals a ~20 sigma count rate increase in the interval from ~T0-12 s to ~T0+424 s.
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB250327B/
The total burst fluence is 2.31(-0.22,+0.26)x10^-5 erg/cm^2, and the 2.944 s peak energy flux, measured from T0+85 s, is 2.43(-0.52,+0.55)x10^-7 erg/cm^2. (both in the 20 keV - 10 MeV energy range).
The time-integrated spectrum of the burst, measured from T0-12 s to T0+424 s, can be described by a power law with exponential cutoff (CPL) model dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -1.11(-0.17,+0.21) and Ep = 212(-38,+48) keV.
The spectrum near the peak count rate, measured from T0+76 s to T0+91 s, can be described by a CPL model with alpha = -0.82(-0.31,+0.43) and Ep = 203(-49,+64) keV.
Assuming the redshift z=3.035 (Malesani et al., GCN 39893) 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 4.78(-0.45,+0.54)x10^53 erg, the peak luminosity L_iso is 2.03(-0.44,+0.46)x10^52 erg/s, the rest-frame peak energy of the time-averaged spectrum Ep,i,z is 856(-152,+195) keV and the spectrum near the maximum count rate Ep,p,z is 818(-198,+257) keV.
With the obtained estimates, GRB 250327B is inside 68% prediction band for the 'Amati' and 90% prediction band for 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/GRB250327B/GRB250327B_rest_frame.pdf
All the quoted errors are estimated at the 68% confidence level. All the presented results are preliminary.
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