TITLE:   GCN CIRCULAR
NUMBER:  43214
SUBJECT: Konus-Wind detection of GRB 251222A / EP251222b
DATE:    25/12/23 12:47:21 GMT
FROM:    Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>

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 251222A (Fermi-GBM detection:
The Fermi GBM team, GCN 43186; SVOM detection:
Yang et al., GCN 43188, Wang et al., GCN 43202),
associated with the Einstein Probe X-ray transient
EP251222b (Guo et al., GCN 43201),
triggered Konus-Wind (KW) at T0=61605.670 s UT (17:06:45.670).

The burst shows multiple emission pulses in the interval from
T0-50 s to T0+33 s and has a total duration of ~83 s.
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB251222_T61605/

As observed by Konus-Wind, the burst had
the fluence of (9.92 ± 0.94)x10^-6 erg/cm^2 and
the 64-ms peak energy flux, measured from T0 + 4.864 s,
of (2.51 ± 0.32)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the brightest part of the burst
(measured from T0 to T0+33.024 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 = -1.45 (-0.21, + 0.24)
and Ep = 102(-16,+17) keV, chi^2 = 94/98 dof.

The spectrum near the peak count rate (measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 15 MeV range by a CPL model
with alpha = -1.23 (-0.20, + 0.23)and Ep = 131(-14,+18) keV, chi^2 = 69/83 dof.

Assuming the redshift z=3.171 (Saccardi et al., GCN 43204)
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 (2.21 ± 0.21)x10^53 erg,
the isotropic peak luminosity L_iso to (2.33 ± 0.30)x10^53 erg/s,
the rest-frame peak energy of the time-integrated spectrum Ep,i,z to (425 ± 69) keV,
and the rest-frame peak energy at the peak of the emission Ep,p,z to (546 ± 68) keV.
With the obtained estimates, GRB 251222A is consistent with 68% prediction bands
of both 'Amati' and 'Yonetoku' relations 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/GRB251222_T61605/GRB251222A_rest_frame.pdf

All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.