[vsnet-grb-info 20963] Konus-Wind observation of the bright nearby GRB 180728A

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  23061
SUBJECT: Konus-Wind observation of the bright nearby GRB 180728A
DATE:    18/07/30 12:21:03 GMT
FROM:    Dmitry Frederiks at Ioffe Institute  <fred at 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, very bright GRB 180728A
(Swift-BAT detection: Starling et al., GCN 23046;
Fermi-GBM detection: Veres et al., GCN 23053)
triggered Konus-Wind (KW) at T0=62954.093 s UT (17:29:14.093).

The burst light curve shows a smooth pulse with a total duration
(T100) of ~17 s. The emission is seen up to ~10 MeV.

As observed by Konus-Wind, the burst had a fluence of
(5.14 ± 0.29)x10^-5 erg/cm2 and a 64-ms peak energy flux,
measured from T0+15.168, of (1.94 ± 0.15)x10^-5 erg/cm2
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+19.2 s)
is best fit in the 30 keV - 15 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -1.48 (-0.10,+0.12),
the high energy photon index beta = -2.57 (-0.18,+0.12),
the peak energy Ep = 97 (-6,+7) keV,
chi2 = 94/96 dof.

The spectrum near the peak count rate (measured from T0+1.280 s
to T0+1.792 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 = -1.44 (-0.15,+0.15),
the high energy photon index beta = -2.81 (-1.44,+0.24),
the peak energy Ep = 127 (-13,+18) keV,
chi2 = 54/56 dof.

Assuming the redshift z=0.117 (Rossi et al., GCN 23055)
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 following rest-frame parameters:
the isotropic energy release E_iso is (2.33 ± 0.10)x10^51 erg,
the peak luminosity L_iso is (9.51 ± 0.58)x10^50 erg/s
(both in the 1-10000 keV rest-frame band).
The rest-frame peak energy of the time-integrated spectrum,
Epi,z, is 108(-7, 8) keV, and the rest-frame peak energy of
the 'peak' spectrum, Epp,z, is 142(-15, 20) keV.
With these energetics, the burst lies on the upper
edges of 68% 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).

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB180728_T62954/

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