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

Jia-Lai Kang, Chris Done, Scott Hagen, Mai Liao, Matthew J. Temple, John D. Silverman, Junyao Li, Jun-Xian Wang

First listed 2025-04-10 | Last updated 2025-09-04

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 $M/M_{\odot} =8-8.5$) 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 $L_R \propto L_X^{0.7}$ (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 persistence 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.

Short digest

Stacks of VLASS images for eROSITA-selected, unobscured AGN in a single mass bin (log M/M☉=8–8.5) track radio behavior across the spectral transition identified from optical/UV–X-ray SEDs. After subtracting matched host-galaxy star-formation radio, the residual core radio stays roughly constant through the transition while the mean accretion rate rises by ~6× and the UV disk brightens by ~100×; the X-rays change by only ~2×, yielding an approximately constant radio/X-ray ratio consistent with the Fundamental Plane. The result favors a persistent, compact jet coupled to the X-ray hot flow (not the thin disk), explaining why radio-loudness flips mainly reflect optical/UV changes rather than jet quenching. Compared to binaries, AGN retain strong coronal X-rays in the disk-dominated state, accounting for the continued radio–X coupling.

Key figures to inspect

• Figure 1: Compare the stacked VLASS radio point to the three SED bins and the agnsed components to see the UV disk emerge while the coronal X-ray component—and the radio point tied to it—remain comparable across bins.
• Figure 2: Inspect AGN versus control stacks to verify a compact radio excess in AGN after host subtraction, and to gauge how the excess does (not) evolve from faint→bright SED bins.
• Figure 3: Check redshift distributions of quiescent vs star-forming hosts in each luminosity bin to ensure the control subtraction is not biased by differing z or host mix across the transition.
• Figure 4: Use stacked inactive galaxies (quiescent vs star-forming) to quantify the star-formation radio baseline that is removed from the AGN stacks, validating the residual as an AGN-linked core component.