GCN Circular 30804

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

The very bright burst from SGR 1935+2154
(IPN triangulation: Ridnaia et al., GCN Circ. 30803
Not found
 
 
)
triggered Konus-Wind at T0=2750.499 s UT (00:45:50.499)
on 2021 September 10.

The light curve shows a single pulse with a sharp(<10 ms) rise
and a total duration of ~1.4 s.
The emission is seen up to ~200 keV.

The Konus-Wind light curve of this burst is available at
http://www.ioffe.ru/LEA/SGRs/210910_T02750/

As observed by Konus-Wind, the burst
had a fluence of 2.51(-0.04,+0.04)x10^-5 erg/cm2,
and a 16-ms peak flux, measured from T0+0.322 s,
of 3.09(-0.18,+0.18)x10^-5 erg/cm2/s
(both in the 20 - 500 keV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+0.512 s)
is best fit in the 20 - 200 keV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = 0.21(-0.18,+0.19)
and Ep = 52(-1,+1) keV (chi2 = 37/22 dof).

The rather long duration of the burst along with the
large measured energy fluence put the burst in the class
of "intermediate" SGR bursts. The event is as fluent as
the April 12, 2015 Intermediate Flare
(Kozlova et al. 2016, MNRAS 460, 2008);
the spectrum of the burst is characterized by the
higher peak energy than of the April 12, 2015 IF (Ep~35 keV).

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.