Abstract
The emission of steady compact jets observed in the hard spectral state of X-ray binaries is likely to be powered by internal shocks caused by fluctuations of the outflow velocity. The dynamics of the internal shocks and the resulting spectral energy distribution (SED) of the jet is very sensitive to the shape of the Power Spectral Density (PSD) of the fluctuations of the jet Lorentz factor. I used both Monte-Carlo simulations and semi-analytical methods to investigate this dependence. It turns out that Lorentz factor fluctuations injected at the base of the jet with a flicker noise power spectrum (i.e. P(f) 1/f) naturally produce the canonical flat SED observed from radio to IR band in X-ray binary systems in the hard state. This model also predicts a strong, wavelength dependent, variability that resembles the observed one. In particular, strong sub-second variability is predicted in the infrared and optical bands. The complex timing correlations observed between the IR/optical light curves and the X-rays can then be used to probe the accretion/ejection connection on short time-scales.
Highlights
Steady compact jets are probably the most common form of jets in X-ray binaries
If the jet is launched by the accretion disc, the variability of the jet Lorentz factor must be related to that of the accretion disc
We assumed an average Lorentz factor of 2 varying with fractional rms amplitude of 30 percent, i.e. comparable to that observed in the X-ray variability of black hole binaries in the hard state
Summary
They appear to be present in all black hole and neutron star binaries when in the hard X-ray spectral state They have an approximatively flat Spectral Energy Distribution (SED) extending from the radio to the mid-IR (e.g. Fender et al 2000; Corbel & Fender 2002; Chaty et al 2003; Migliari et al 2010). These flat spectra are usually ascribed to self-absorbed synchrotron emission from conical compact jets (Blandford & Königl 1979) under the assumption of continuous energy replenishment of the adiabatic losses.
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