Disk-jet coupling across the spectral transition in supermassive black holes

Abstract Accretion flows in both stellar and supermassive black holes show a distinct spectral transition. This is seen directly in binaries and changing look AGN, and also in a recent sample of eROSITA X-ray selected, unobscured AGN where the stacked spectral energy distributions (SEDs) for a single black hole mass bin (log M/M⊙ = 8 − 8.5) clearly show the UV bright disk appearing as the luminosity increases. In binaries, this transition is associated with a change in radio jet, from coupling to the X-ray hot flow with $L_{\rm R}\propto L_{\rm X}^{0.7}$ (Fundamental Plane relation), to collapsing when the X-ray hot flow collapses into a disc. We explore the radio behaviour across the transition in our AGN sample by stacking VLASS images. We significantly detect weak radio emission even after subtracting the contribution from star formation in the host galaxy. The residual radio emission remains relatively constant across the transition, despite the mean mass accretion rate changing by a factor 6 and UV flux changing by a factor 100. However, the X-rays change by only a factor 2, giving a constant radio to X-ray flux ratio as predicted by the ‘fundamental plane’. We show that this is consistent with these AGN having the same compact radio jet coupling to the X-ray hot flow (not the disc) as in the binaries. The most significant difference is the persistance of the coronal X-rays across the spectral transition in AGN, whereas in binaries the coronal X-rays can be very weak in the disc dominated state.