TITLE: GCN CIRCULAR NUMBER: 39655 SUBJECT: Konus-Wind detection of GRB 250309B DATE: 25/03/10 17:17:33 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-duration GRB 250309B ( Fermi-GBM BALROG localization: Preis and Greiner, GCN 39629; Fermi-GBM detection: The Fermi GBM team, GCN 39635; McDermott et al., GCN 39642; GECAM-B detection: Wang et al., GCN 39648; IPN triangulation: Kozyrev et al., GCN 39652) triggered Konus-Wind (KW) at T0=27513.791 s UT (07:38:33.791).
The burst light curve shows a multi-peaked structure which starts at ~T0-0.5 s and has a total duration of ~8 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/GRB250309_T27513/
As observed by Konus-Wind, the burst had a fluence of (5.79 ± 0.48)x10^-6 erg/cm^2 and a 64-ms peak energy flux, measured from T0 + 1.024 s, of (4.35 ± 0.43)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).
The time-integrated spectrum (measured from T0 to T0+8.448 s) is best fit in the 20 keV - 15 MeV range by a power law with exponential cutoff (CPL) model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -1.03(-0.20,+0.22) and Ep = 106(-7,+8) keV (chi2 = 80/79 dof). Fitting this spectrum by a Band function yields the same values of alpha and Ep, and an upper limit on the high energy photon index beta of -3.1 (chi2 = 79/78 dof).
Assuming the redshift z=1.898 ( Malesani et al., GCN 39647) of ZTF25aaitvjt/AT 2025dws, the candidate counterpart to GRB 250309B (Stein et al., GCN 39639), 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 (5.3 ± 0.4)x10^52 erg, the isotropic peak luminosity L_iso to (1.2 ± 0.12)x10^53 erg/s, and the rest-frame peak spectral energy Ep,z to (306 ± 21) keV. With the obtained estimates, GRB 250309B is consistent with 68% prediction band of the 'Amati' relation and 90% prediction band of the 'Yonetoku' relation 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/GRB250309_T27513/GRB250309B_rest_frame.pdf
All the quoted errors are estimated at the 68% confidence level. All the presented results are preliminary.
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