Photon Bubbles and the Vertical Structure of Accretion Disks

We consider the effects of "photon bubble" shock trains on the verticalstructure of radiation pressure-dominated accretion disks. These densityinhomogeneities are expected to develop spontaneously in radiation-dominatedaccretion disks where magnetic pressure exceeds gas pressure, even in thepresence of magnetorotational instability. They increase the rate at whichradiation escapes from the disk, and may allow disks to exceed the Eddingtonlimit by a substantial factor. We first generalize the theory of photon bubblesto include the effects of finite optical depths and radiation damping.Modifications to the diffusion law at low optical depth tend to fill in thelow-density regions of photon bubbles, while radiation damping inhibits theformation of photon bubbles at large radii, small accretion rates, and smallheights above the equatorial plane. Accretion disks dominated by photon bubbletransport may reach luminosities of 10 to >100 times the Eddington limit (L_E),depending on the mass of the central object, while remaining geometricallythin. However, photon bubble-dominated disks with alpha-viscosity are subjectto the same thermal and viscous instabilities that plague standard radiationpressure-dominated disks, suggesting that they may be intrinsically unsteady.Photon bubbles can lead to a "core-halo" vertical disk structure. Insuper-Eddington disks the halo forms the base of a wind, which carries awaysubstantial energy and mass, but not enough to prevent the luminosity fromexceeding L_E. Photon bubble-dominated disks may have smaller color correctionsthan standard accretion disks of the same luminosity. They remain viablecontenders for some ultraluminous X-ray sources and may play a role in therapid growth of supermassive black holes at high redshift.Comment: 38 pages, 2 figures, accepted for publication in The Astrophysical Journa