TITLE:   GCN CIRCULAR
NUMBER:  34511
SUBJECT: Konus-Wind detection of GRB 230818A
DATE:    23/08/21 15:16:55 GMT
FROM:    Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>

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

The long GRB 230818A (Swift detection: Tohuvavohu et al., GCN 34479;
Fermi GBM observation: Veres et al., GCN 34501)
triggered Konus-Wind (KW) at T0=84458.443 s UT (23:27:38.443).

The burst light curve shows a single FRED-like pulse
which starts at ~T0-0.256 s and has a duration of ~10 s.
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB230818_T84458/

As observed by Konus-Wind, the burst had
a total fluence of (4.91 ± 1.76)x10^-6 erg/cm^2 and
a 64-ms peak energy flux, measured from T0,
of (3.08 ± 0.90)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst (measured from T0 to T0+16.640 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 = -1.00(-0.23,+0.27) and Ep = 251(-49,+81) keV (chi2 = 91/98 dof).
Fitting this spectrum by a Band function yields the same values of alpha and Ep,
and an upper limit on the high energy photon index beta of -2.5 (chi2 = 91/97 dof).

The spectrum near the peak count rate (measured from T0 to T0+0.256 s)
is best fit in the 20 keV - 15 MeV range by a CPL model
with alpha = -0.40(-0.44,+0.56) and Ep = 259(-45,+64) keV (chi2 = 18/17 dof).
Fitting this spectrum by a Band function yields the same values of alpha and Ep,
and an upper limit on the high energy photon index beta of -2.7 (chi2 = 18/16 dof).

Assuming the redshift z=2.42 (Malesani et al., GCN 34485)
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 (6.95 ± 2.49)x10^52 erg,
the isotropic peak luminosity L_iso to (1.49 ± 0.44)x10^53 erg/s,
the rest-frame peak energy of the time-integrated spectrum Ep,i,z to ~860 keV,
and the rest-frame peak energy at the peak of the emission Ep,p,z to ~890 keV.
With the obtained estimates, GRB 230818A is inside 90% prediction band for
the 'Amati' relation and inside 68% prediction band for the 'Yonetoku' relation
derived 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/GRB230818_T84458/GRB230818A_rest_frame.pdf

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