The non-linear dependence of flux on black hole mass and accretion rate in core-dominated jets

Abstract

We derive the non-linear relation between the core flux Fv of accretion-powered jets at a given frequency and the mass M of the central compact object. For scale-invariant jet models, the mathematical structure of the equations describing the synchrotron emission from jets enables us to cancel out the model-dependent complications of jet dynamics, retaining only a simple, model-independent algebraic relation between Fv and M. This approach allows us to derive the FV-M relation for any accretion disc scenario that provides a set of input boundary conditions for the magnetic field and the relativistic particle pressure in the jet, such as standard and advection-dominated accretion flow (ADAF) disc solutions. Surprisingly, the mass dependence of Fv is very similar in different accretion scenarios. For typical flat-spectrum core-dominated radio jets and standard accretion scenarios, we find Fv ∼ M17/12. The 7-9 orders of magnitude difference in black hole mass between microquasars and active galactic nuclei (AGN) jets imply that AGN jets must be about 3-4 orders of magnitude more radioloud than microquasars, i.e. the ratio of radio to bolometric luminosity is much smaller in microquasars than in AGN jets. Because of the generality of these results, measurements of this FV-M dependence are a powerful probe of jet and accretion physics. We show how our analysis can be extended to derive a similar scaling relation between the accretion rate m and Fv for different accretion disc models. For radiatively inefficient accretion modes, we find that the flat-spectrum emission follows Fv ∝ (Mm)17/12.