TITLE:   GCN CIRCULAR
NUMBER:  42448
SUBJECT: Konus-Wind detection of GRB 251016B
DATE:    25/10/25 20:36:05 GMT
FROM:    Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>

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 251016B
(Ferm-GBM detection: The Fermi GBM team, GCN 42316;
Godwin and Meegan, GCN 42383;
CALET-GBM detection: Yamaoka et al., GCN 42320;
AstroSat-CZTI detection: Harsha et al., GCN 42324;
IPN triangulation: Kozyrev et al., GCN 42403)
triggered Konus-Wind at T0=86340.495 s UT (23:59:00.495).

The burst light curve shows a single pulse
which starts at ~T0-0.5 s and has a total duration of ~18.5 s.
The emission is seen up to ~6 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB251016_T86340/

As observed by Konus-Wind, the burst
had a fluence of 3.50(-0.18,+0.18)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+2.320 s,
of 1.22(-0.11,+0.11)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+11.520 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 = -0.98(-0.06,+0.06),
the high energy photon index beta = -2.29(-0.10,+0.08),
the peak energy Ep = 226(-18,+19) keV
(chi2 = 93/85 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+3.328 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 = -0.85(-0.05,+0.06),
the high energy photon index beta = -2.41(-0.11,+0.09),
the peak energy Ep = 294(-21,+21) keV
(chi2 = 67/67 dof).

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.