Eccentric Accretion Disks in Active Galactic Nuclei

Abstract

We report that moderately eccentric flows around supermassive black holes (SMBHs), formed via either circumnuclear gas accretion or tidal disruption events, generate eccentricity cascades (from >0.8 to 0.2 outward), explaining multiwavelength emission and variability in active galactic nuclei (AGNs). The flows' non-axisymmetric temperature structure explains non-axisymmetric dust sublimation fronts, distinct broad emission-line components, and their radial motions. The innermost broad-line region (BLR) links to the SMBH vicinity through highly eccentric streams that produce soft X-rays at periapsis. General relativistic precession further compresses these flows, generating a hard X-ray continuum near the SMBH. Precession of the eccentric flow drives optical/X-ray variability, reproducing the observed X-ray power spectral density and occasional X- ray quasi-periodic eruptions. We thus propose eccentric accretion disks as a physical AGN model that unifies the elusive BLRs and X-ray corona. This model will enable detailed anatomy of AGNs and maximize their potential as cosmological standard candles.

Figures (15)

• Figure 1: Formation of highly eccentric flow and the resulting extreme compression. Panels a--f show the midplane density (code units) for the scale-free simulation A3E05-SF and the A3E05 simulation with GR precession (Table \ref{['tab:1']}). Time is measured in units of the dynamical timescale at $a_0$, i.e., $\Omega^{-1}(a_0)$, where $a_0=100\,R_g$ for A3E05. The red circles denote the innermost stable circular orbit at $6\,R_g$. The highly eccentric flow undergoes strong compression and heating.
• Figure 2: Highly eccentric flow and extreme compression in a compact disk. Panels a--f show midplane density maps analogous to Fig. \ref{['fig:r3']}, but for A2E05-SF and A2E05 (Table \ref{['tab:1']}). In contrast to A3E05 in Fig. \ref{['fig:r3']}, A2E05 reaches a quasi-global precessing state after $100\,\Omega^{-1}(100\,R_g)$.
• Figure 3: Eccentricity cascade in low- and moderate-eccentricity disks. Panels a and b show the evolution of the eccentricity profile and argument of periapsis for A2E02-SF. The eccentricity oscillates with minimal AMD damping every $200\,\Omega^{-1}(a_0)$ between a flat-eccentricity state and a state with a negative eccentricity gradient (the black dashed line in panel a shows a uniformly precessing model for reference; see Appendix \ref{['ap:a2']}). The twists are minimal in the two end-member states. However, the A2E05-SF model in panels c and d shows no such oscillations; instead, it exhibits rapid accretion, as indicated by the evolving material distribution.
• Figure 4: Eccentricity profiles for A3E05 and A2E05. Panels a and b show the eccentricity and argument of periapsis of the flow in A3E05 (see also Fig. \ref{['fig:r3']}). We note that A3E05 starts from A3E05-SF (Table \ref{['tab:1']}; Fig. \ref{['fig:r3']}b). Panels c and d show the corresponding quantities for A2E05, which starts from A2E05-SF (Fig. \ref{['fig:r2']}b).
• Figure 5: Extreme vertical compression in eccentric flows. The plot shows the distribution of Lagrangian fluid elements in $[1.18a_0,1.185a_0]$ for the eccentric flow in Fig. \ref{['fig:r2']}b, where the measured local eccentricity, dimensionless eccentricity gradient, and twist are $(e, ae_a, a\omega_a)=(0.62,-0.76,-0.64)$. The disk is compressed by a factor of 127 near periapsis relative to apoapsis (true anomaly is shown in the plot), as predicted by eccentric-disk theory (see Appendix \ref{['ap:a1']}) and accurately captured by our Lagrangian method. Even more extreme compression is expected in inner AGN accretion disks, where high-eccentricity flow is common.