GCN Circular 17938

S. Golenetskii, R.Aptekar, D. Frederiks, V. Pal'shin, P. Oleynik,
M. Ulanov, D. Svinkin, A. Tsvetkova, A.Lysenko, and T. Cline
on behalf of the Konus-Wind team, report:

On June 15, 2015 Konus-Wind detected a hard X-ray transient
activity lasting from ~65300 s UT (18:08:20) to
~69000 s UT (19:10:00).
Taking into account the proximity of this burst
to the Swift/BAT trigger due to V404 Cygni
(T0=T0(BAT)=18:31:38 UT; Barthelmy et al., GCN 17929
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 ),
two subsequent Fermi/GBM triggers due to
the same source (Younes, GCN 17932
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 ), and
consistency of the KW ecliptic latitude response with
the position of V404 Cygni we suggest that KW observed
the ~1 hour-long flare from this Galactic black hole candidate.

The KW light curve recorded in the waiting mode shows
a multi-peaked complex which started at ~T0(BAT)-1400 s,
peaked at ~T0(BAT)+1640 s, and lasted until ~T0+2300 s.
The emission is visible in the instrument's
G1 (20-75 keV) and G2 (75-300 keV) bands.

The K-W light curve is available at
http://www.ioffe.rssi.ru/LEA/V404_Cyg/KW150615_V404_Cyg/

As observed by Konus-Wind, the burst had a fluence of
(1.8 �� 0.1)x10^-4 erg/cm2, and a 2.944-s peak flux,
measured from ~T0+1642 s, of (2.3 �� 0.4)x10^-7 erg/cm2/s
(both in the 20 - 1200 keV energy range).

Modeling the 3-channel time-integrated spectrum
(from T0-1394.283 s to T0+2315.157 s) by a power law
with exponential cutoff (CPL) model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
yields alpha = -1.28 �� 0.16, and Ep = 122 �� 9 keV.

Assuming the distance 2.4 kpc (Miller-Jones et al. 2009)
the energy release in the flare is E ~ 8.9x10^40 erg,
and the peak luminosity L is ~1.1x10^38 erg/s.

Further KW observations of the activity of the source will
be reported (there are at lest two episodes detected
by the instrument on June 16, 2015).

All the quoted errors are estimated at the 1 sigma confidence level.
All the presented results are preliminary.