GCN Circular 43013
Subject
Konus-Wind detection of GRB 251118A
Event
Date
2025-12-06T16:38:47Z (10 months ago)
From
Valentina Panteleeva at Ioffe Institute <panteleevav228@gmail.com>
Via
Web form
V. Panteleeva, 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 251118A
(Fermi-GBM detection: The Fermi GBM team, GCN 42743;
Palafox and Meegan, GCN 42783;
BALROG localization: Preis and Greiner, GCN 42745;
Fermi-LAT detection: Gupta and Khalil, GCN 42752;
GECAM-B detection: Ren et al., GCN 42779;
Glowbug detection: Cheung et al., GCN 42784)
triggered Konus-Wind at T0=73904.449 s UT (20:31:44.449).
The burst light curve starts, at ~T0-20.5 s, with a short soft
initial pulse (precursor) which had a duration of about 25.8 s.
The precursor is followed, starting at ~T0+32 s,
by the main episode, lasting for ~25 s.
The total duration of the burst is ~48 s.
The emission in the main episode is seen up to ~3 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB251118_T73904/
As observed by Konus-Wind, the burst
had the total fluence of 1.07(-0.03,+0.03)x10^-4 erg/cm2,
and the 64-ms peak flux, measured from T0+35.776 s,
of 2.03(-0.15,+0.15)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the initial pulse
(measured from T0 to T0+8.448 s)
is best described 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 = -1.94(-0.03,+0.12)
and Ep < 41 keV (chi2 = 89/85 dof).
Fitting this spectrum by a GRB (Band) model results
in the same estimates of alpha and Ep, and gives
only an upper limit on the high energy photon index
beta of -2.8 (chi2 = 89/84 dof).
The time-averaged spectrum of the main episode
(measured from T0+33.024 to T0+65.536 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.29(-0.03,+0.04),
the high energy photon index beta = -2.56(-0.08,+0.07),
the peak energy Ep = 154(-5,+5) keV
(chi2 = 110/97 dof).
The spectrum near the maximum count rate
(measured from T0+33.024 to T0+36.608 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.74(-0.05,+0.06),
the high energy photon index beta = -2.53(-0.10,+0.08),
the peak energy Ep = 251(-12,+12) keV
(chi2 = 68/69 dof).
All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.