Scale-invariant radio jets and varying black hole spin

Abstract

Context. Compact radio cores associated with relativistic jets are often observed in both active galactic nuclei and X-ray binaries. Their radiative properties follow some general scaling laws that primarily depend on their masses and accretion rates. However, it has been suggested that black hole spin can also strongly influence the power and radio flux of these. Aims. We attempt to estimate the dependency of the radio luminosity of steady jets launched by accretion disks on black hole mass, accretion rate, and spin using numerical simulations. Methods. We make use of three-dimensional general relativistic magnetohydrodynamical simulations of accretion disks around low-luminosity black holes in which the jet radio emission is produced by the jet sheath. Results. We find that the radio flux increases roughly by a factor of 6 as the black hole spin increases from aâ- â 0 to aâ- = 0.98. This is comparable to the increase in accretion power with spin, meaning that the ratio between radio jet and accretion power hardly changes. Although our jet spine power scales as expected for the Blandford-Znajek process, the dependency of jet radio luminosity on the black hole spin is somewhat weaker. Also weakly rotating black holes can produce visible radio jets. The overall scaling of the radio emission with black hole mass and accretion rate is consistent with the scale-invariant analytical models used to explain the fundamental plane of black hole activity. Spin does not introduce a significant scatter in this model. Conclusions. The jet-sheath model can describe well-resolved accreting systems, such as Sgr A∗ and M 87, as well as the general scaling behavior of low-luminosity black holes. Hence the model should be applicable to a wide range of radio jets in sub-Eddington black holes. The black hole spin has an effect on the production of visible radio jet, but it may not be the main driver to produce visible radio jets. An extension of our findings to powerful quasars remains speculative.