TITLE:   GCN CIRCULAR
NUMBER:  30657
SUBJECT: Konus-Wind detection of GRB 210818A
DATE:    21/08/18 18:23:06 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 210818A (Swift detection: Beardmore et al., GCN 30652;
AGILE detection: Ursi et al., GCN 30654)
triggered Konus-Wind (KW) at T0=3729.710 s UT (01:02:09.710).

The burst light curve started at ~T0 with a bright, short (<0.5s),
hard-spectrum initial pulse, which was followed, after a ~0.5 s interval,
by a similarly bright and hard, multi-peaked emission, which gradually decays until ~T0+90 s.
We also note a QPO-like behavior of the light curve in the interval
from ~T0+1.6 s to ~T0+3.2 s, with the quasi period of ~0.21 s.

The emission is seen up to ~7 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB210818_T03729/

As observed by Konus-Wind, the burst had
a fluence of (2.67 ± 0.12)x10^-4 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 0.128 s,
of (5.12 ± 0.54)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst (measured from T0 to T0+80.640 s)
is best fit in the 20 keV - 20 MeV range
by a power law with exponential cutoff (CPL) model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.99(-0.03,+0.03) and Ep = 1107(-71,+78) keV (chi2 = 117/98 dof).
Fitting this spectrum by a Band function yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.2 (chi2 = 117/97 dof).

The spectrum near the peak count rate (measured from T0 to T0+0.256 s,
that corresponds to the short initial pulse)
is best fit in the 20 keV - 20 MeV range by the CPL model
with alpha = -0.28(-0.13,+0.14) and Ep = 1247(-133,+47) keV (chi2 = 38/58 dof).
Fitting this spectrum by a Band function yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.4 (chi2 = 38/57 dof).

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