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The models that seek to explain the reflection spectrum in black holebinaries usually invoke a point-like primary source of hard X-rays. This sourceilluminates the accretion disk and gives rise to the discrete (lines) andcontinuum-reflected components. The main goal of this work is to investigatewhether the extended, mildly relativistic jet that is present in black holebinaries in the hard and hard-intermediate states is the hard X-ray source thatilluminates the accretion disk. We use a Monte Carlo code that simulates theprocess of inverse Compton scattering in a mildly relativistic jet. Blackbodyphotons from the thin accretion disk are injected at the base of the jet andinteract with the energetic electrons that move outward. Despite the fact thatthe jet moves away from the disk at a mildly relativistic speed, we find thatapproximately $15-20$\% of the input soft photons are scattered back toward theaccretion disk. The vast majority of the Comptonized, back-scattered photonsescape very close to the black hole ($h\lesssim 6 r_g$, where $r_g$ is thegravitational radius), but a non-negligible amount escape at a wide range ofheights. At high heights, $h\sim 500-2000\,r_g$, the distribution falls offrapidly. The high-height cutoff strongly depends on the width of the jet at itsbase and is almost insensitive to the optical depth. The disk illuminationspectrum is softer than the direct jet spectrum of the radiation that escapesin directions that do not encounter the disk. We conclude that an extended jetis an excellent candidate source of hard photons in reflection models.

Illumination of the accretion disk in black hole binaries: An extended jet as the primary source of hard X-rays