Using INTEGRAL to observe the high-energy emission of Cygnus X-3
Cygnus X-3 is one of the first discovered X-ray binaries and the only
bright compact binary system known to host a Wolf-Rayet star as a
companion. The intense stellar wind created by this companion is one of
the reasons why Cygnus X-3 exhibits a very peculiar spectral behavior.
Indeed, it shows a wider variety of states than the two canonical ones
usually observed in other standard X-ray binaries. Despite the fact that
the source is well known since many years, very little is known about
its spectral behavior beyond 50 keV. This lack of knowledge is
especially due to the source being very faint at these energies. Thanks
to INTEGRAL, it is possible to explore more than 16 years of
observations, and so to probe the sky region of Cygnus X-3 with the best
sensitivity ever. As shown at the top of the figure, one can detect the
source up to 200 keV. Moreover, one can create six X-ray spectra from 3
to 200 keV, one for each observed spectral state of the source.
A first phenomenological spectral fitting clearly reveals the presence
of an additional component at higher than 50 keV in addition to the
component usually interpreted as thermal Comptonization. This
non-thermal component can either be due to a non-thermalized population
of electrons in a hot plasma very close to the compact object or due to
synchrotron emission from the jets.
The first scenario was investigated by using a well known physical model
and a good agreement with the data was found. Besides, the electron
acceleration seems to be higher in states where major ejections are
observed pointing to a modification of the mechanism responsible for the
electron acceleration through state transition.
The investigation of the second scenario is especially interesting for
states where compact radio jets are observed. At the bottom of the
figure the Cygnus X-3 spectral energy distribution from radio energies
to 1000 keV are shown; the blue crosses are part of the spectrum of the
source in a state where compact jets are observed. Red represents the
50000 K black-body emission (typical of a Wolf-Rayet star). This
component is totally consistent with the infrared points, showing that
all measured infrared emission comes from the companion star. This
allowed to place a rough constraint or limit on the contribution of the
jet-synchrotron emission in the infrared (shown with a green arrow),
which is necessarily negligible compared to the emission from the star.
If one considers the range of the infrared synchrotron break observed in
the case of other black hole binaries, for example GX 339-4 (in grey),
one can extrapolate the then supposed synchrotron emission to the X-
rays (green dotted line). To reach high energies, the synchrotron
power-law index would need then to have an index of 1.8, slightly harder
than what one obtains from the spectral fits. Alternatively,
extrapolating the high-energy tail down to the infrared domain (blue
dotted line) results in a much higher infrared flux than measured.
However, the plot shows that the possible synchrotron extension in light
green could contribute to the high-energy emission one observes in
X-rays, implying that synchrotron emission could also be a plausible
scenario.
Reference:
INTEGRAL discovery of a high-energy tail in the microquasar Cygnus X-3,
F. Cangemi, J. Rodriguez, V. Grinberg, R. Belmont, P. Laurent and J. Wilms,
Astronomy and Astrophysics, in press;
https://arxiv.org/abs/2011.06863
