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GCN Circular 23011

Subject
Konus-Wind observation of GRB 180720B
Date
2018-07-22T13:43:39Z (6 years ago)
From
Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>
D. Frederiks, S. Golenetskii, R.Aptekar, A. Kozlova,
A.Lysenko,  D. Svinkin, A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long, extremely bright GRB 180720B
(Swift-BAT detection: Siegel et al., GCN 22973, GCN 22975;
Fermi-LAT detection: Bissaldi and Racusin, GCN 22980;
Fermi-GBM detection: Roberts et al., GCN 22981)
triggered Konus-Wind at T0=51705.261 s UT (14:21:45.261).

The burst light curve shows a multi-peaked structure
with a total duration (T100) of ~125 s.
The emission is seen up to ~15 MeV.

As observed by Konus-Wind, the burst had a fluence of
(5.43 �� 0.26)x10^-4 erg/cm2 and a 64-ms peak energy flux,
measured from T0+15.168, of (9.70 �� 0.52)x10^-5 erg/cm2
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+119.808 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 = -1.01 (-0.06,+0.06),
the high energy photon index beta = -2.07 (-0.08,+0.07),
the peak energy Ep = 451 (-45,+52) keV,
chi2 = 102/97 dof.

The spectrum near the peak count rate (measured from T0+14.592 s
to T0+15.360 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 = -0.82 (-0.09,+0.10),
the high energy photon index beta = -2.27 (-0.22,+0.13),
the peak energy Ep = 779 (-127,+169) keV,
chi2 = 92/76 dof.

Assuming the redshift z=0.654 (Vreeswijk et al., GCN 22996)
and a standard cosmology model with H_0 = 70 km/s/Mpc,
Omega_M = 0.30, and Omega_Lambda = 0.70,
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is ~6.0x10^53 erg,
the peak luminosity L_iso is ~1.8x10^53 erg/s,
and the rest-frame peak energy of the time-integrated spectrum,
Ep,z, is ~746 keV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB180720_T51705/

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