2014MNRAS.442.1090H

Microlensing observations indicate that quasar accretion discs have half-light radii larger than standard theories predict. The observations of Blackburne and colleagues suggest that these half-light radii may have a weak wavelength dependence. We consider disc temperature profiles that might match these observations. Nixon and colleagues have suggested that misaligned discs around spinning black holes can have annuli torn off them at radii where the Lense-Thirring torque overcomes the disc's viscosity. These annuli precess, spread radially, and intersect with the remaining disc, heating the disc at potentially large radii. However, unless the intersections occur at an angle of <1°, highly supersonic collisions will shock heat intersecting gas to T ∼ 10⁷ K. Discs with such shock-heated regions have significantly larger half-light radii, but are overluminous in X-rays. If instead heating occurs through intermittent weak shocks, the X-ray luminosities are consistent with observations but the half-light radii are not significantly larger. We also study two phenomenological disc temperature profiles. Discs with a temperature spike at relatively large radii and lowered temperatures at radii inside the spike yield improved and acceptable fits to microlensing sizes in most cases. Such temperature profiles could in principle occur in sub-Keplerian discs partially supported by magnetic pressure. However, such discs overpredict the fluxes from quasars studied with microlensing except in the limit of negligible continuum emission from radii inside the temperature spike.