The contribution of spin to jet-disk coupling in black holes

Context. 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. Aims. 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 (a) within this sample. Methods. To obtain a large sample of radio-loud AGN, we matched double-lobed radio sources identified in Faint Images of the Radio Sky at Twenty-centimeters (FIRST, 1.4 GHz) to spectroscopically confirmed quasars from the Sloan Digital Sky Survey (SDSS). We obtain 763 FR II quasars with a median redshift of z = 1.2. Results. 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. Conclusions. 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.