TITLE:   GCN CIRCULAR
NUMBER:  29150
SUBJECT: GRB 201221A: Konus-Wind detection and joint Konus-Wind + Swift-BAT spectral analysis
DATE:    20/12/23 15:51:43 GMT
FROM:    Dmitry Frederiks at Ioffe Institute  <fred@mail.ioffe.ru>

D. Frederiks, S. Golenetskii, R.Aptekar, A. Kozlova, A.Lysenko,
D. Svinkin, A. Tsvetkova,  M. Ulanov, T. Cline, on behalf of the Konus-Wind team,
and A. Lien (GSFC/UMBC), report:

The long GRB 201221A (Swift-BAT trigger #1013852, T0 = T0(BAT)= 07:09:01.4258 UT:
Page et al., GCN 29096; Breadmore et al., GCN 29108;
Fermi GBM Sub-Threshold Detection: Veres, GCN 29110, 29111)
was detected by Konus-Wind (KW) in the waiting mode.

A Bayesian block analysis of the KW waiting mode data in the 20-400 keV band
reveals a ~11 sigma count rate increase over background in the interval
from ~T0(BAT)-10 s to ~T0(BAT)+20 s.
The KW light curve of this burst is available
at http://www.ioffe.ru/LEA/GRBs/GRB201221A/

To derive the broad-band spectral parameters of this burst, we performed
a joint spectral analysis of the Swift/BAT data (15-150 keV) and the Konus-Wind
3-channel spectral data, which cover the energy range from ~20 keV to ~1.5 MeV.

The time-averaged spectrum, measured from T0(BAT)-9.6 s to T0(BAT)+19.8 s,
is best fit in the 15 keV - 1.5 MeV range by the GRB (Band)
function with the following model parameters:
the low-energy photon index alpha = -0.95 (-0.22,+0.31),
the high energy photon index beta = -2.39 (-0.89,+0.38),
and the peak energy Ep = 194 (-63,+81) keV, chi2 = 65/58 dof.

In the 15-1500 keV band, the total burst fluence S is 4.3(-0.4, + 0.7)x10^-6 erg/cm^2,
and the 2.944 s peak energy flux Fp is 2.0 (-0.2, + 0.3)x10^-7 erg/cm^2.
In the 10-10000 keV band, standard for the KW analysis, S = 5.3(-1.0, + 2.3)x10^-6 erg/cm^2
and Fp = 2.4 (-0.4, + 1.0)x10^-7 erg/cm^2.

Assuming the redshift z=5.70 (Malesani et al., GCN 29100)
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 isotropic energy release E_iso to 3.0(-0.5,+1.3)x10^53 erg,
the isotropic luminosity L_iso to 9.0(-1.6,+3.9)x10^52 erg/s,
and the rest-frame peak energy of the time-integrated spectrum Ep,z to 1300(-420,+540) keV.
With these values, GRB 201221A is within 90% prediction bands
for both 'Amati' and 'Yonetoku' relations for the sample of >300 long KW GRBs
with known redshifts (part I: Tsvetkova et al., ApJ 850 161, 2017;
part II: Tsvetkova et al., ApJ, accepted),
see http://www.ioffe.ru/LEA/GRBs/GRB201221A/GRB201221A_rest_frame.pdf

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