TITLE:   GCN CIRCULAR
NUMBER:  29415
SUBJECT: Konus-Wind detection of GRB 210204A / ZTF21aagwbjr
DATE:    21/02/07 14:27:40 GMT
FROM:    Dmitry Frederiks at Ioffe Institute  <fred@mail.ioffe.ru>

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

The long GRB 210204A (Fermi GBM detection:  Bissaldi, GCN 29393;
GECAM detection: Li et al, GCN 29392;
IPN triangulation: Hurley et al., GCN 24908;
AstroSat CZTI Detection: Waratkar et al., GCN 29410),
associated with the fast optical transient ZTF21aagwbjr/AT2021buv
(Kool et al., GCN 29405),
triggered Konus-Wind (KW) at T0=23570.576 s UT (06:32:50.576).

The burst light shows a bright, multi-peaked pulse, which starts
at ~T0-30 s and has a duration of ~80 s.
A weaker precursor is clearly visible in the KW waiting
mode data around ~T0-165 s.
The emission in the main pulse is seen up to ~3 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB210204_T23570/

As observed by Konus-Wind, the burst had
a fluence of (5.7 ± 0.9)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0+2.304,
of (5.0 ± 0.7)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+49.408 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 = -1.04 (-0.12,+0.18),
the high energy photon index beta = -2.43 (-0.18,+0.31),
the peak energy Ep = 137 (-25,+22) keV,
chi2 = 79/97 dof.

The spectrum near the peak count rate (measured from T0
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 = -0.28 (-0.54,+0.64),
the high energy photon index beta = -2.12 (-0.24,+0.11),
the peak energy Ep = 123 (-24,+49) keV,
chi2 = 109/97 dof.

Assuming the redshift z=1.466 (Izzo et al., GCN 29411)
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 ~2.3x10^53 erg,
the isotropic luminosity L_iso to ~7.1x10^52 erg/s,
and the rest-frame peak energy of the time-integrated spectrum Ep,z to ~340 keV.
With these values, GRB 210204A is within 68% prediction bands
for both 'Amati' and 'Yonetoku' relations for the sample of >300 long KW GRBs
with known redshifts (Tsvetkova et al., ApJ, in press),
see http://www.ioffe.ru/LEA/GRBs/GRB210204_T23570/GRB210204A_rest_frame.pdf



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