Friday, November 4, 2011

1111.0878 (Andrzej A. Zdziarski et al.)

The gamma-ray emitting region of the jet in Cyg X-3    [PDF]

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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