GCN Circular 26576

D. Frederiks, S. Golenetskii, R.Aptekar, A.Lysenko,
A. Ridnaia, D. Svinkin, A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long GRB 191221B (Swift detection:  Laha et al., GCN 26534
Not found
 
 
;
AGILE/MCAL detection: Longo et al., GCN 26549
Not found
 
 
;
AstroSat CZTI detection: Gaikwad et al., GCN 26567
Not found
 
 
)
triggered Konus-Wind (KW) at T0=74349.647 s UT (20:39:09.647).

The burst light curve shows a bright pulse with a complex, multi-peaked structure.
The total duration of the burst is ~35 s.
The emission is seen up to ~15 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB191221_T74349/

As observed by Konus-Wind, the burst had
a fluence of (1.0 �� 0.1)x10^-4 erg/cm^2 and
a 64-ms peak energy flux, measured from T0+8.064,
of (1.9 �� 0.2)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+21.760 s)
is best fit in the 20 keV - 20 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.81 (-0.06,+0.06),
the high energy photon index beta = -2.47 (-0.24,+0.16),
the peak energy Ep = 377 (-29,+30) keV,
chi2 = 99/97 dof.

The spectrum near the peak count rate (measured from T0+6.912 s
to T0+8.448 s) is best fit in the 20 keV - 20 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.51 (-0.11,+0.13),
the high energy photon index beta = -2.72 (-0.50,+0.26),
the peak energy Ep = 338 (-36,+38) keV,
chi2 = 64/66 dof.

Assuming the redshift z=1.148 (Vielfaure et al., GCN 26553
Not found
 
 
)
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 following rest-frame parameters:
the isotropic energy release E_iso = (3.6 �� 0.4)x10^53 erg,
the peak luminosity L_iso = (1.5 �� 0.16)x10^53 erg/s,
and the rest-frame peak energy of the time-integrated spectrum Ep,z=(810 �� 65) keV.
With these values, GRB 191221B is within 68% prediction bands
for both 'Amati' and 'Yonetoku' relations built for the sample
of 138 long KW GRBs with known redshifts (Tsvetkova et al., ApJ 850 161, 2017),
see http://www.ioffe.ru/LEA/GRBs/GRB191221_T74349/GRB191221B.pdf

All the quoted errors are estimated at the 90% confidence level.
All the presented results are preliminary.