[vsnet-grb-info 26047] Konus-Wind observation of GRB 200716C

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28147
SUBJECT: Konus-Wind observation of GRB 200716C
DATE:    20/07/19 23:32:33 GMT
FROM:    Dmitry Svinkin at Ioffe Institute  <svinkin at mail.ioffe.ru>

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

The GRB 200716C (Swift-BAT detection: Ukwatta et al., GCN 28124;
Fermi-LAT detection: Ohno et al., GCN 28130;
AGILE-MCAL detection: Ursi et al., GCN 28133;
Fermi-GBM detection: Veres & Meegan, GCN 28135;
CALET-GBM detection: Torii et al., GCN 28139;
Insight-HXMT/HE detection: Xue et al., GCN 28145)
triggered Konus-Wind (KW) at T0=82658.337 s UT (22:57:38.337).

The burst light curve shows two bright, short, hard-spectrum pulses
in the interval from ~T0-0.5 s to ~T0+2.5 s.
Both pulses are ~0.5 s wide and they are peaked
at ~T0-0.128 s and ~T0+2.096 s, respectively.
A much weaker post-burst emission is visible
in the KW 20-80 keV band up to ~T0+75 s.
The emission in the main pulses is seen up to ~7 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB200716_T82658/

As observed by Konus-Wind, the burst had
a fluence of (1.2 +/- 0.2)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0+0.128 s,
of (3.7 +/- 0.6)x10^-5 erg/cm^2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+8.448 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 = -1.07(-0.12,+0.14)
and Ep = 652(-154,+241) keV (chi2 = 91/97 dof).
Fitting by a GRB (Band) model yields similar alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.0
(chi2 =91/96 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 the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.51 (-0.19,+0.31),
the high energy photon index beta = -2.23 (-0.58,+0.29),
the peak energy Ep = 616 (-200,+226) keV,
(chi2 = 48/42 dof).

Assuming the photo z=0.348 of a possible host galaxy (D'Avanzo, GCN 28132)
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 ~ 3.9x10^51 erg,
the peak luminosity L_iso ~ 1.6x10^52 erg/s,
and the rest-frame peak energy of the time-integrated (peak)
spectrum Ep,z ~ 880 keV (~830 keV).

With these values, GRB 200716C is a clear outlier in the 'Amati' relation
built for 138 KW GRBs with known z (Tsvetkova et al., ApJ 850 161, 2017).
Meanwile, in both Eiso-Ep,z and Liso-Ep,z planes, the GRB 200716C position
is consistent with short-hard (Type I) GRB population,
see http://www.ioffe.ru/LEA/GRBs/GRB200716_T82658/GRB200716C.pdf

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