Friday, July 5, 2013

1307.1309 (Andrzej A. Zdziarski et al.)

Self-consistent jet models for black-hole binaries in the hard state and application to Cyg X-1    [PDF]

Andrzej A. Zdziarski, Patryk Pjanka, Marek Sikora
We study models of compact jets in the hard state of black-hole binaries. Their typical radio spectra are flat, with the spectral energy index of alpha ~0, which appear to be due to superposition of partially self-absorbed synchrotron spectra emitted by relativistic power-law electrons with the shape of their distribution maintained over the length of the jet. This requires a constant dissipation rate per unit logarithmic jet length. We study a model in which we self-consistently solve for both the steady-state electron distribution and the emitted spectrum in both the self-absorbed and optically-thin parts as functions of the height along the jet. We take into account synchrotron emission and absorption, Compton scattering of synchrotron, stellar and accretion-flow photons, adiabatic losses, and absorption of very-high energy gamma-rays by pair production on stellar photons. We find Compton upscattering of blackbody stellar radiation in high-mass X-ray binaries to be a major contributor to their gamma-ray emission. Then measurements and/or upper limits on that emission impose strong constraints on the jet parameters, in particular on their magnetic field. We apply our model to the hard state of Cyg X-1, and find it accounts well for its radio/IR spectrum as well as it can account for its observed GeV-range flux. The model can also account for the MeV-range emission as synchrotron radiation, as implied by recent claims of strong linear polarization in that regime, but only for very strong magnetic fields, which are required in that case to satisfy the GeV-range data, and very hard electron acceleration.
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