TITLE:   GCN CIRCULAR
NUMBER:  44968
SUBJECT: Konus-Wind detection of GRB 260616B/EP260616a
DATE:    26/06/17 15:37:56 GMT
FROM:    Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>

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

The long-duration GRB 260616B
(BALROG localization: Preis & Greiner, GCN 44951;
Fermi-GBM detection: Malacaria et al., GCN 44953;
EP-WXT detection: Li et al., GCN 44956;
CALET-GBM detection: Asaoka et al., GCN 44966)
triggered Konus-Wind at T0=25522.793 s UT (07:05:22.793).

The burst light curve shows two multi-peaked emission pulses.
The total duration of the burst is ~13 s.
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260616_T25522/

As observed by Konus-Wind, the burst
had the total fluence of 2.52(-0.29,+0.30)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0+9.760 s,
of 8.62(-1.09,+1.12)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+16.832 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.78(-0.18,+0.23),
the high energy photon index beta = -2.25(-0.28,+0.16),
the peak energy Ep = 279(-49,+62) keV
(chi2 = 101/97 dof).

The spectrum near the maximum count rate
(measured from T0+8.640 to T0+16.832 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -1.05(-0.10,+0.11)
and Ep = 382(-41,+52) keV (chi2 = 87/85 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.0
(chi2 = 87/84 dof).

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.