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The Galactic low-mass X-ray binary GS 2023+338 was discovered during
the burst on May 22, 1989 with the Japanise satellite Ginga (aka Galaxy).
The flux reached 17 crabs (this unit of flux measurement is an bright
X-ray source, Crab Nebula) at 10-35 keV.
At the time, the object increased its brightness by about seven magnitudes
and reached a brightness of 11.6mag in the filter V ; thus the object could
be observed with an amateur telescope with a diameter of 20-30 cm.
This object proved to be a recurrent nova V404 Cygni (a variable star in
the Cygnus constellation) which has already the burst in 1938. The system
is dynamically resolved due to optical spectroscopic measurements. The mass
function turned out to be very high: f(M)=6.0, which at once sets the lower
limit of six solar masses on the mass of an invisible compact object.
There are two stellar objects in the binary system on orbit with a period
of 6.5 days: a black hole with a mass of about 10.6 solar masses and an
orange sub-giant, a K0 star with a mass of about 0.3 solar masses
(Cherepashchuk et al.) and a photospheric temperature of about 4700 K.
Apparently we deal with a system where a subgiant fills its Roche lobe
(at a distance of about 60 million kilometres from the black hole) and
transfers the matter onto the accreting black hole; as a result, the hot
accretion disk forms and the system becomes very bright in the X-rays.
The distance to the star of 2.39 kpc (7400 light-years) was exactly
determined from the measurements of the annual parallax of the weak
quiet radio emission (0.3 mJy) from V404 Cyg using the interferometric
system (VLA+VLBA+GBT) in USA (Miller-Jones+, 2009).
Even the measurements of 1989 clearly show that it is an X-ray binary
with irregular relativistic ejections of the matter from the inner parts
near the black hole (Han and Hjellming, 1992). These objects are called
microquasars in modern classification. Korbel et al. (2008) showed that
the X-ray and radio flux from microquasars are well correlated, thus we
may expect high radio fluxes during the X-ray bursts and low fluxes
between those bursts.
According to the BAT-data from the Swift space observatory the X-ray
flux in the range 15-50 keV reached 40 crabs in the period from June
15 and 26, 2015 for the first time from 1989; from the measurements
of the INTEGRAL observatory (60-300 keV) it was even stronger, up to
50 crabs, i.e., several thousand times higher than in the quiescent
state that lasted for 26 years.
We have daily detected radio emission at five frequencies
(2.3, 4.6, 8.5, 11.2, and 21.7 GHz) from June 18 to July 12 with
the RATAN-600 radio telescope. No other radio telescope worldwide
carried out multi-frequency measurements. Unfortunately, the VLBI
observations were conducted before maximal radio brightness and did
not reveal the object's extended structure. At the beginning of the
activity period the measured fluxes varied from 50 to 500 mJy with
the spectral index of synchrotron emission varying from +0.65 to -0.8
from day to day. On June 26, several hours after the bright X-ray
outburst, the radio flux from V404 Cyg increased 30-40 times from
0.05 Jy in the previous day to almost 4 Jy exceeding the maximum
flux in the burst of 1989 several times. It should be noticed that
the level of 4 Jy is very high for this kind of objects and especially
for radio stars. The bursts greater 4 Jy occur only in the microquasar
Cyg X-3 (the record is 25 Jy). The radio spectrum of this radio burst
corresponded to optically thin synchrotron radiation at frequencies
higher than 4 GHz (the spectral index is -0.3) and to optically thick
synchrotron radiation in selfabsorption (the spectral index is -2.5)
at lower frequencies. Probably, we detected the very beginning of the
formation the relativistic jet from the poles of accretion disk around
the black hole. Upon 14 days the radio flux fell below the detection
level at the RATAN-600 in relation to the decrease of the X-ray flux.
It is remarkable that in December 2015 we again detected the radio
flux at the level of 30-70 mJy from the object that became brighter
first in the X-ray (to 5 crabs) and then by 2-3 magnitudes in the
optical range.
(Adapted from the Astronomer's Telegrams (ATels): #7667, #7716, #8454,
and #8482 and from the report at the "High Energy Astrophysics" Conference,
IKI, December 21-25, 2015).
S.A.Trushkin, N.A.Nizhelskij, P.G.Tsybulev
Contact - S.A.Trushkin
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Fig.1.
Light curves of V404 Сyg from June 18 to July 12, 2015 from the Swift/BAT
data in the range of 15-50 keV, RATAN-600 at two frequencies, 4.6 and
8.2 GHz, and Tokyo radio telescope at 1.4 GHz. We marked the measurement
point at 5 GHz (VLA) and measurement times with the SMA interferometer
in the range of 230 GHz, when the flux varied from 60 mJy to 6 Jy at
times less than 2 hours
Fig.2.
Daily radio spectra during measurements at RATAN-600. The characteristic
fitting by a power law usual for optically thin synchrotron spectrum of
relativistic electrons moving in the magnetic field are showed
Fig.3.
Spectrum of the object at the flux maximum on June 26. The most suitable
spectrum approximation with the dependence for synchrotron radiation
with low-frequency turn over is noted. It is the obtained spectral
index equal to +2.5 (5/2) that indicates this slope is caused by
self-absorption, i.e., the radio fotons generated by relativistic
electrons by synchrotron mechanism are absorbed by the same electrons.
This can occur if the relativistic electrons locate in a small volume
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