The contribution of spin to jet-disk coupling in black holes
  The spin of supermassive black holes could power jets from active galactic
nuclei (AGN), although direct observational evidence for this conjecture is
sparse. The accretion disk luminosity and jet power, on the other hand, have
long been observed to follow a linear correlation. If jet power is coupled to
black hole spin, deviations from the jet-disk correlation for a sample of AGN
can be used to probe the dispersion of the spin parameter within this sample.
To exploit this idea, we matched double-lobed radio sources from the FIRST
survey to spectroscopically confirmed quasars from SDSS. We obtain 763 FR-II
quasars with a median redshift of z=1.2 A tight correlation between the optical
luminosity of the accretion disk and the lobe radio luminosity is observed. We
estimate that 5-20% of the bolometric disk luminosity is converted to jet
power. Most of the scatter to the optical-radio correlation is due to
environment; deviations from jet-disk coupling due to internal factors (e.g.,
spin) contribute at most 0.2 dex. Under the assumption that the
Blandford-Znajek mechanism operates in AGN, we obtain an upper limit of 0.1 dex
to the dispersion of the product of the spin and the magnetic flux threading
the horizon. If black hole spin determines the AGN jet efficiency, then our
observations imply that all FR-II quasars have very similar spin. In addition,
the quasar spin distribution needs to have a wide gap to explain the
radio-quiet population. The alternative, and perhaps more likely,
interpretation of the tight jet-disk correlation is that black hole spin is not
relevant for powering AGN jets.
