Andrzej A. Zdziarski, Marek Sikora, Guillaume Dubus, Feng Yuan, Benoit Cerutti, Anna Ogorzalek
We study models of the gamma-ray emission of Cyg X-3 observed by Fermi. We
calculate the average X-ray spectrum, based on Swift data, during the gamma-ray
active periods. Then, we calculate spectra from Compton scattering of a photon
beam into a given direction by isotropic relativistic electrons with a
power-law distribution, both based on the Klein-Nishina cross section and in
the Thomson limit. Applying the results to scattering of stellar blackbody
radiation in the inner jet of Cyg X-3, we find that a low-energy cut-off in the
electron distribution at a Lorentz factor of 10^3 is required by the form of
the observed X-ray/gamma-ray spectrum in order to avoid overproducing the
observed X-ray flux. The electrons giving rise to the observed gamma-rays are
efficiently cooled by Compton scattering, and the power-law index of the
acceleration process is 2.5--3. The presence of the low energy electron cut-off
is well explained by recent shock acceleration models, in which it is related
to the ion/electron mass ratio. The bulk Lorentz factor of the jet and the
kinetic power before the dissipation region depend on the fraction of the
dissipation power supplied to the electrons; if it is about 1/2, the Lorentz
factor is about 2, and the kinetic power is about 10^38 erg/s, which is
comparable to the radiative power of Cyg X-3. A large fraction of the kinetic
power dissipated in the gamma-ray emitting region is radiated, unless the
electrons receive much less energy than the ions. We find the magnetic field in
that region to be relatively strong, implying the synchrotron losses may be not
negligible.
View original:
http://arxiv.org/abs/1111.0878
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