TITLE:   GCN CIRCULAR
NUMBER:  41027
SUBJECT: GRB 250706B/C: Konus-Wind analysis of the prompt emission
DATE:    25/07/08 13:55:05 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:

We report on a further analysis of the KW detection
of the very bright, long-duration GRB 250706C
(Frederiks et al., GCN 41013), which common origin with
the SVOM/ECLAIRs GRB 250706B (Palmerio et al., GCN 40989)
was confirmed from the FERMI/LAT observation (Longo et al., GCN 41019).

GRB 250706C triggered Konus-Wind (KW) at T0=60322.626 s UT (16:45:22.626),
~1200 s before the SVOM/ECLAIRs trigger on GRB 250706B.
The burst light curve shows a very bright, multi-peaked
emission pulse with the duration of ~50 s (20-1500 keV).
This pulse is followed by a weaker, smoothly decaying emission tail,
visible up to the end of the KW triggered data record (~T0+250 s).
The emission in the main pulse is seen up to ~15 MeV.

The KW light curve is available at
http://www.ioffe.ru/LEA/GRBs/GRB250706_T60322/

As observed by Konus-Wind, the burst had
the total fluence of (5.12 ± 0.06)x10^-4 erg/cm^2 and
a 16-ms peak energy flux, measured from T0 + 36.800 s,
of (4.16 ± 0.22)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+51.968 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -1.00 (-0.01,+0.01),
the high energy photon index beta = -3.65 (-0.44,+0.23),
the peak energy Ep = 850(-15,+15) keV,
chi2 = 125/96 dof.

The spectrum near the peak count rate (measured from T0+36.608 s to T0+37.120 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -1.10 (-0.04,+0.04),
the high energy photon index beta = -2.91 (-0.60,+0.30),
the peak energy Ep = 1005(-91,+104) keV,
chi2 = 69/58 dof.

The spectrum of the decaying tail (measured from T0+51.968 s to T0+232.192)
is best fit by a simple power law (PL) function with the PL photon index
of (-2.0 ± 0.06), chi2 = 92/99 dof.

Assuming the redshift z=0.942 (Palmerio et al., GCN 41022)
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 (1.26 ± 0.02)x10^54 erg,
the isotropic peak luminosity L_iso to (2.00 ± 0.11)x10^53 erg/s, and
the rest-frame peak spectral energy Epi,z to 1650(-30,+30) keV.
With the obtained estimates, GRB 250706B/C 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/GRB250706_T60322/GRB250706BC_rest_frame.pdf

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