GCN Circular 40746
Subject
Konus-Wind detection of GRB 250612C (short/hard with extended emission)
Event
Date
2025-06-16T11:33:32Z (5 months 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 bright GRB 250612C (Fermi GBM detection: Fermi GBM team, GCN 40704;
Neights & Meegan, GCN 40713; SVOM/GRM observation: Wang et al., GCN 40708;
GRID detection: Yang et al., GCN 40732)
triggered Konus-Wind (KW) at T0=53591.512 s UT (14:53:11.512).
The burst light curve shows a very bright, multi-peaked emission episode,
which starts at ~T0-0.05 s and has a duration of ~1.3 s.
This episode is followed by a much weaker extended emission tail visible to ~T0+25 s.
The emission is seen up to ~15 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250612_T53591/
As observed by Konus-Wind, the burst had the total fluence
of (6.92 ± 0.50)x10^-5 erg/cm^2 and a 16-ms peak energy flux,
measured from T0+0.016, of (3.01 ± 0.38)x10^-4 erg/cm^2/s
(both in the 20 keV - 10 MeV energy range).
The time-integrated spectrum of the initial episode (measured from T0 to T0+1.280 s)
is best fit in the 20 keV - 20 MeV range by a power law with exponential
cutoff (CPL) model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.90(-0.03,+0.02) and Ep = 2084 (-97,+103) keV (chi2 = 91.8/74 dof).
Fitting this spectrum by a Band function yields similar, within errors,
values of alpha and Ep, and an upper limit on the high energy photon index beta
of -4.0 (chi2 = 91.7/73 dof).
The spectrum near the peak count rate (measured from T0 to T0+0.064 s)
is best fit in the 20 keV - 20 MeV range by a CPL model with
alpha = -0.14(-0.13,+0.14) and Ep = 2073 (-214,+263) keV (chi2 = 38.7/35 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 -2.0 (chi2 = 38.4/34 dof).
The spectrum of the extended emission, measured from T0+1.280 s to T0+25.856 s,
is best fit in the 20 keV - 20 MeV range by a simple power law (PL) function with
the photon PL index of -1.68 ± 0.12.
All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.