Do stellar-mass and super-massive black holes have similar dining habits?

Over the years, numerous attempts have been made to connect the phenomenology and physics of mass accretion onto stellar-mass and super-massive black holes in a scale-invariant fashion. In this paper, we explore this connection at the radiatively efficient (and non-jetted) end of accretion modes by comparing the relationship between the luminosity of the accretion disc and corona in the two source classes. Motivated by the apparently tight relationship between these two quantities in active galactic nuclei (AGNs), we analyse 458 RXTE-PCA archival observations of the X-ray binary (XRB) GX 339-4, using this object as an exemplar for the properties of XRBs in general. We focus on the soft and soft-intermediate states, which have been suggested to be analogous to radiatively efficient AGNs. The observed scatter in the log L_disc - log L_corona relationship of GX 339-4 is high (∼0.43 dex) and significantly larger than in a representative sample of radiatively efficient, non- or weakly jetted AGNs (∼0.30 dex). At first glance, this would appear contrary to the hypothesis that the systems simply scale with mass. On the other hand, we also find that GX 339-4 and our AGN sample show different accretion rate and power-law index distributions, with the latter in particular being broader in GX 339-4 (dispersion of ∼0.16 cf. ∼0.08 for AGN). GX 339-4 also shows an overall softer slope, with a mean value of ∼2.20 as opposed to ∼2.07 for the AGN sample. Remarkably, once similarly broad Γ and ṁ distributions are selected, the AGN sample overlaps nicely with GX 339-4 observations in the mass-normalised log L_disc - log L_corona plane, with a scatter of ∼0.30 - 0.33 dex in both cases. This indicates that a mass-scaling of properties might hold after all, with our results being consistent with the disc-corona systems in AGNs and XRBs exhibiting the same physical processes, albeit under different conditions for instance in terms of temperature, optical depth and/or electron energy distribution in the corona, heating-cooling balance, coronal geometry and/or black hole spin.