GCN Circular 45038
Subject
Konus-Wind detection of GRB 260618B
Event
Date
2026-06-24T17:14:43Z (14 days ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
A. Tsvetkova, M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:
The long-duration GRB 260618B
(Fermi-GBM detection: Fermi GBM team, GCN 44979;
Veres & Meegan, GCN 45010;
Fermi-LAT detection: Gupta et al., GCN 44983)
triggered Konus-Wind at T0=60907.637 s UT (16:55:07.637).
The burst light curve shows a multipeaked structure
which starts at ~T0-1.6 s and has a total duration of ~61.2 s.
The emission is seen up to ~4 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260618_T60907/
As observed by Konus-Wind, the burst
had the total fluence of 2.26(-0.34,+0.43)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0+0.378 s,
of 4.90(-0.98,+1.01)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst
(measured from T0 to T0+57.600 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.83(-0.16,+0.19)
and Ep = 1066(-241,+365) keV (chi2 = 94/98 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.7
(chi2 = 94/97 dof).
The spectrum near the maximum count rate
(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.39(-0.14,+0.15)
and Ep = 817(-97,+119) keV (chi2 = 96/88 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.2
(chi2 = 96/87 dof).
All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.