[vsnet-grb-info 28591] Konus-Wind detection of GRB 210822A

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30694
SUBJECT: Konus-Wind detection of GRB 210822A
DATE:    21/08/23 12:38:10 GMT
FROM:    Dmitry Frederiks at Ioffe Institute  <fred at 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 210822A (Swift detection: Page et al., GCN 30677;
GECAM detection: Wang et al., GCN 30678;
Fermi-LAT detection: Ohno et al., GCN 30687;
AstroSat CZTI detection: Ohno et al., GCN 30693)
triggered Konus-Wind (KW) at T0=33502.974 s UT (09:18:22.974).

The burst light curve shows a bright, multi-peaked pulse
which starts at ~T0-1 s and has a total duration of ~12 s,
followed by a weaker decaying emission visible up to ~T0+50 s.
The emission in the main pulse is seen up to ~15 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB210822_T33502/

As observed by Konus-Wind, the burst had
a fluence of (1.20 ± 0.11)x10^-4 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 0.320 s,
of (3.25 ± 0.28)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+18.176 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 = -0.64 (-0.06,+0.07),
the high energy photon index beta = -2.39 (-0.15,+0.12),
the peak energy Ep = 398 (-29,+30) keV,
chi2 = 124/96 dof.

The spectrum near the peak count rate (measured from T0+4.096
to T0+7.936 s) is best fit in the 20 keV - 15 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.59 (-0.15,+0.18),
the high energy photon index beta = -2.08 (-0.18,+0.12),
the peak energy Ep = 429 (-83,+108) keV,
chi2 = 75/63 dof.

Assuming the redshift z=1.736 (Zhu et al., GCN 30692)
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 (9.5 ± 0.8)x10^53 erg,
the isotropic luminosity L_iso to (7.0 ± 0.6)x10^53 erg/s,
the rest-frame peak energy of the time-integrated spectrum Epi,z to (1090 ± 80) keV,
and the rest-frame peak energy of the 'peak' spectrum Epp,z to (1175 ± 250) keV.
With these values, GRB 210822A lies within 68% prediction bands
for both 'Amati' and 'Yonetoku' relations for the sample of >300 long KW GRBs
with known redshifts (Tsvetkova et al., 2021, ApJ, 908, 83),
see http://www.ioffe.ru/LEA/GRBs/GRB210702_T68826/GRB210822A_rest_frame.pdf

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