GCN Circular 32668

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

The extraordinary bright GRB 221009A
(Fermi-GBM detection: Veres et al. GCN Circ 32636
Not found
 
 
, Lesage et al. GCN Circ 32642
Not found
 
 
;
Fermi-LAT detection: Bissaldi et al. GCN Circ 32637
Not found
 
 
;
IPN triangulation: Svinkin et al. GCN Circ 32641
Not found
 
 
;
AGILE/MCAL detection detection: Ursi et al., GCN Circ 32650
Not found
 
 
)
triggered Konus-Wind (KW) at T0=47821.648 s UT (13:17:01.648).

The burst light curve starts with a FRED-like initial pulse (IP)
that lasts from ~T0-1 s to ~T0+25 s. The IP is followed by an extremely
bright multi-peaked emission in the interval from ~T0+170 s to ~T0+600 s,
where a preliminary-estimated count rate reaches several hundred counts/s.
The emission at this stage is seen up to at least ~15 MeV.
The pulsed phase of the burst evolves to a steadily decaying emission tail,
which is visible in the KW data for more than 10000 s.

The preliminary Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB221009_T47821/

A time-averaged spectrum of the IP (measured from T0 to T0+28.842 s)
is best fit in the 20 keV - 15 MeV range by a power law with exponential
cutoff (CPL) model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.62(-0.04,+0.05) and Ep = 975(-332,+712) keV (chi2 = 80/98 dof).
The fluence in this time interval is estimated to (2.4 �� 0.3)x10^-05 erg/cm^2
and a 64-ms peak energy flux, measured from T0 + 5.760 s,
to (6.2 �� 0.7)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

A time-averaged spectrum at the onset of the brightest phase of the event
(measured from T0+180.48 to T0+200.064 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 = -1.09 (-0.01,+0.01),
the high energy photon index beta = -2.60 (-0.06,+0.06),
the peak energy Ep = 1060 (-30,+31) keV, chi2 = 161/97 dof.
The fluence in this time interval is estimated to (8.8 �� 0.1)x10^-04 erg/cm^2

The brightness of the main burst episode doesn't allow to perform
the standard KW spectral analysis of the emission at this stage of the event.
However, using the latter spectrum and a count rate light curve in the 80-320 keV range
with preliminary dead-time corrections applied, we obtain a rough estimate
of the fluence of the event in the interval from T0 to T0+600 s of ~0.052 erg/cm^2,
which is the highest value observed for GRBs for almost 28 years of the KW operation.
Further analysis of this extraordinary event is ongoing and
the results will be reported eslsewhere.

Assuming the redshift z=0.151 (de Ugarte Postigo et al., GCN 32648
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 burst isotropic energy release E_iso to a high,
but reasonable value of ~3.0x10^54 erg.
The rest-frame peak energy of the spectrum Ep,z is estimated to ~1150 keV.
With these preliminary values, GRB 221009A perfectly fits the
'Amati' relation 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/GRB221009_T47821/GRB221009A_rest_frame.pdf

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