1207.6984 (Emma Gardner et al.)

Jets and the Accretion Flow in Low Luminosity Black Holes    [PDF]

Emma Gardner, Chris Done

We present a broadband (radio - X-rays) spectral model for black hole binaries (BHBs) in the low/hard state (LHS). We model the accretion flow as a disc which is replaced in the inner regions by a radiatively inefficient hot flow. The flow emits cyclo-synchrotron radiation, which produces another source of seed photons for Compton scattering as well as those intercepted from the outer disc. We balance heating with Compton cooling to derive the electron temperature self-consistently, and find the dominant source of seed photons changes from disc to cyclo-synchrotron emission as accretion rate drops and the disc recedes. This reproduces the observed behaviour of the X-ray spectral index, which initially hardens with decreasing accretion rate, and then softens again. We include a conical synchrotron jet of constant bulk Lorentz factor with parameters based on those observed from a bright low/hard state in GX339-4. This model reproduces the observed $L_R\propto L_X^{0.7}$ radio-X-ray correlation as expected for a jet where the magnetic energy density and particle density scale with $\propto\dot{m}$. The kinetic energy of the jet also scales as $\propto\dot{m}$, but its optically thin synchrotron luminosity scales as $\propto\dot{m}^2$ as it is determined by the product of the electron and magnetic energy densities. Thus the X-rays from the jet are as radiatively inefficient as the X-rays from the hot flow and there is no transition from an accretion flow dominated state to a jet dominated state in the X-ray emission in these models.

View original: http://arxiv.org/abs/1207.6984