GCN Circular 39655
Subject
Konus-Wind detection of GRB 250309B
Date
2025-03-10T17:17:33Z (4 days ago)
From
Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>
Via
email
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.