TITLE:   GCN CIRCULAR
NUMBER:  30276
SUBJECT: Konus-Wind detection of GRB 210619B
DATE:    21/06/20 15:56:25 GMT
FROM:    Dmitry Svinkin at Ioffe Institute  <svinkin@mail.ioffe.ru>

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

The very bright, long-duration GRB 210619B
(Swift-BAT detection: D'Avanzo et al., GCN Circ. 30261;
GECAM detection: Zhao et al., GCN Circ. 30264;
Fermi-LAT detection: Axelsson et al., GCN Circ. 30270)
triggered Konus-Wind at T0=86368.157 s UT (23:59:28.157).

The burst light curve shows a bright hard initial episode,
which starts at ~T0 and has a duration of ~10 s,
followed by weaker pulses. The total burst duration is ~80 s.
The emission is seen up to ~15 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB210619_T86368/

As observed by Konus-Wind, the burst
had a fluence of 4.60(-0.13,+0.13)x10^-4 erg/cm2,
and a 64-ms peak flux, measured from T0+0.640 s,
of 1.54(-0.12,+0.12)x10^-4 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+66.816 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.02(-0.03,+0.04),
the high energy photon index beta = -2.10(-0.04,+0.03),
the peak energy Ep = 261(-13,+14) keV
(chi2 = 118/96 dof).

The spectrum near the maximum count rate
(measured from T0+0.512 to T0+0.768 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.41(-0.11,+0.12),
the high energy photon index beta = -2.06(-0.12,+0.09),
the peak energy Ep = 572(-81,+98) keV
(chi2 = 76/60 dof).

Assuming the redshift z=1.937
(de Ugarte Postigo et al., GCN Circ. 30272)
and a standard cosmology model with H_0 = 67.3 km/s/Mpc,
Omega_M = 0.315, and Omega_Lambda = 0.685 (Planck Collaboration, 2014),
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is 4.41(-0.12,+0.12)x10^54 erg,
the peak luminosity L_iso is 4.34(-0.34,+0.34)x10^54 erg/s,
the rest-frame peak energy of the time-integrated spectrum,
Ep,i,z, is 767(-38,+41) keV, and the rest-frame peak energy of
the 'peak' spectrum, Ep,p,z, is 1680(-238,+288) keV.
With these energetics, the burst lies within the 90% prediction bands
for both 'Amati' and 'Yonetoku' relations built for the sample
of 138 long KW GRBs with known redshifts
(Tsvetkova et al., ApJ 850 161, 2017),
see http://www.ioffe.ru/LEA/GRBs/GRB210619_T86368/GRB210619B_rest_frame.pdf.

Among the KW GRB sample, GRB 210619B is within top 4% in the terms
of E_iso and have L_iso comparable to the most luminous GRB 110918A 
(Frederiks et al., ApJ 779, 151, 2013).

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.