Mass-scaling as a method to constrain outflows and particle acceleration from low-luminosity accreting black holes

The ‘Fundamental Plane of black hole accretion’ (FP), a relation between the radio luminosities (L_R), X-ray luminosities (L_X) and masses (M_BH) of hard/quiescent state black hole binaries and low-luminosity active galactic nuclei, suggests some aspects of black hole accretion may be scale invariant. However, key questions still exist concerning the relationship between the inflow/outflow behaviour in the ‘classic’ hard state and quiescence, which may impact this scaling. We show that the broad-band spectra of A0620-00 and Sgr A* (the least luminous stellar mass/supermassive black holes on the FP) can be modelled simultaneously with a physically motivated outflow-dominated model where the jet power and all distances are scaled by the black hole mass. We find we can explain the data of both A0620-00 and Sgr A* (in its non-thermal flaring state) in the context of two outflow-model scenarios: (1) a synchrotron-self-Compton dominated state in which the jet plasma reaches highly sub-equipartition conditions (for the magnetic field with respect to that of the radiating particles), and (2) a synchrotron-dominated state in the fast-cooling regime in which particle acceleration occurs within the inner few gravitational radii of the black hole and plasma is close to equipartition. We show that it may be possible to further discriminate between models (1) and (2) through future monitoring of Sgr A*'s submm/infrared/X-ray emission, in particular via time lags between the variable emission in these bands.