Rimmed and rippled accretion disc models to explain AGN continuum lags

We propose a solution to the problem of accretion disc sizes in active galactic nuclei being larger when measured by reverberation mapping than predicted by theory. Considering the disc's exposed-surface thickness profile H(r), our solution invokes a steep rim or rippled structures irradiated by the central lamp-post. We model the continuum lags and the faint and bright disc spectral energy distribution (SED) in the best-studied case NGC 5548 (black hole mass M-center dot = 7 x 10 7 M-circle dot, disc inclination i = 45 degrees). With the lamp-post off, the faint-disc SED fixes a low accretion rate M similar or equal to 0.0014M(circle dot)yr (-1) and high prograde black hole spin a(center dot) similar or equal to 0.93, for which r(in) = 2 G M-center dot/c(2) and L-disc = 0.25M(over dot) c(2). The bright-disc SED then requires a lamp-post luminosity L-LP similar or equal to 5 M c(2) / (1 - A ) for disc albedo A . Reprocessing on the thin disc with T proportional to r(-3/4) gives time lags tau proportional to lambda(4/3) but three times smaller than observed. Introducing a steep H(r) rim, or multiple crests, near r similar to 5 light days, reprocessing on the steep centre-facing slope increases temperatures from similar to 1500 to similar to 6000 K, and this increases optical lags to match the lag data. Most of the disc surface maintains the cooler T proportional to r(-3/4) profile that matches the SED. The bright lamp-post may be powered by magnetic links tapping the black hole spin. The steep rim occurs near the disc's dust sublimation radius as in the failed disc wind model for broad-line clouds'. Lens-Thirring torques aligning the disc and black hole spin may also raise a warp and associated waves. In both scenarios, the small density scale height implied by the inferred value of H(r) suggests possible marginal gravitational instability in the disc.

Automated summary

We propose a solution to the problem of accretion disc sizes in active galactic nuclei being larger when measured by reverberation mapping than predicted by theory, which involves a steep rim or rippled structures irradiated by the central lamp-post.