TITLE:   GCN CIRCULAR
NUMBER:  44677
SUBJECT: Konus-Wind detection of GRB 260519C
DATE:    26/05/21 13:10:04 GMT
FROM:    Dmitry Svinkin at Ioffe Institute <dmitrysvinkin@gmail.com>

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

The long-duration GRB 260519C
(NuSTAR-ACS detection: Waratkar et al., GCN 44672)
triggered Konus-Wind at T0=49536.443 s UT (13:45:36.443).

The burst light curve shows a multipeaked structure
which starts at ~T0-3.5 s and has a total duration of ~27 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/GRB260519_T49536/

As observed by Konus-Wind, the burst
had the total fluence of 2.47(-0.15,+0.13)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0+2.192 s,
of 2.45(-0.46,+0.46)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+24.832 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(-0.16,+0.18),
the high energy photon index beta = -3.17(-0.94,+0.28),
the peak energy Ep = 119(-7,+8) keV
(chi2 = 96/96 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+8.448 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 = -0.70(-0.11,+0.11)
and Ep = 209(-10,+11) keV (chi2 = 91/78 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.6
(chi2 = 91/77 dof).

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