Thermodynamics of an Accretion Disk Annulus with Comparable Radiation and Gas Pressure

We explore the thermodynamic and global structural properties of a local patch of an accretion disk whose parameters were chosen so that radiation pressure and gas pressure would be comparable in magnitude. Heating, radiative transport, and cooling are computed self-consistently with the structure by solving the equations of radiation MHD in the shearing-box approximation. Using a fully 3-d and energy-conserving code, we compute the structure and energy balance of this disk segment over a span of more than forty cooling times. As is also true when gas pressure dominates, the disk's upper atmosphere is magnetically-supported. However, unlike the gas-dominated case, no steady-state is reached; instead, the total (i.e., radiation plus gas) energy content fluctuates by factors of 3--4 over timescales of several tens of orbits, with no secular trend. Because the radiation pressure varies much more than the gas pressure, the ratio of radiation pressure to gas pressure varies over the approximate range 0.5--2. The volume-integrated dissipation rate generally increases with increasing total energy, but the mean trend is somewhat slower than linear, and the instantaneous dissipation rate is often a factor of two larger or smaller than the mean for that total energy level. Locally, the dissipation rate per unit volume scales approximately in proportion to the current density; the time-average dissipation rate per unit mass is proportional to m^{-1/2}, where m is the horizontally-averaged mass column density to the nearer of the top or bottom surface. As in our earlier study of a gas-dominated shearing-box, we find that energy transport is completely dominated by radiative diffusion, with Poynting flux carrying less than 1% of the energy lost from the box.Comment: ApJ, in pres

Continuum Spectra of Quasar Accretion Disk Models

We have calculated the spectrum and polarization of a standard thin accretion disk with parameters appropriate for a bright quasar. This model improves upon previous work by including ultraviolet metal line opacities, assumed for now to be in LTE. Though not yet fully self-consistent, our calculations demonstrate that metal lines can change the spectral slope, reduce the polarization, and reduce the Lyman edge feature in accretion disk spectra. Some observational differences between quasar spectra and accretion disk models might be reconciled with the inclusion of metal lines.Comment: 4 pages, 3 figures, to appear in "Accretion Processes in Astrophysical Systems: Some Like it Hot," proceedings of the 8th Annual October Astrophysics Conference in Marylan

High-Frequency and Type-C QPOs from Oscillating, Precessing Hot, Thick Flow

Motivated by recent studies showing an apparent correlation between the high-frequency quasi-periodic oscillations (QPOs) and the low-frequency, type-C QPO in low-mass, black hole X-ray binaries (LMXBs), we explore a model that explains all three QPOs in terms of an oscillating, precessing hot flow in the truncated-disk geometry. Our model favors attributing the two high-frequency QPOs, often occurring in a near 3:2 frequency ratio, to the breathing and vertical epicyclic frequency modes of the hot, thick flow, although we can not rule out the Keplerian and m=-1 radial epicyclic modes. In either case, the type-C QPO is attributed to precession. The correlation of the QPOs comes from the fact that all three frequencies are associated with the same geometrical structure. While the exact QPO frequencies are sensitive to the black hole mass and spin, their evolution over the course of an outburst is mainly tied to the truncation radius between the geometrically thin, optically thick disk and the inner, hot flow. We show that, in the case of the LMXB GRO J1655-40, this model can explain the one simultaneous observation of all three QPOs and that an extrapolation of the model appears to match lower frequency observations where only two of the three components are seen. Thus, this model may be able to unify multiple QPO observations using the properties of a single, simple, geometrical model.Comment: 7 pages, 4 figures, accepted to MNRA

The buried Balmer-edge signatures from quasars

In our previous paper, we have reported the detection of a Balmer edge absorption feature in the polarized flux of one quasar (Ton 202). We have now found similar Balmer edge features in the polarized flux of four more quasars (4C09.72, 3C95, B2 1208+32, 3C323.1), and possibly a few more, out of 14 newly observed with the VLT and Keck telescopes. In addition, we also re-observed Ton 202, but we did not detect such a dramatic feature, apparently due to polarization variability (the two observations are one-year apart). The polarization measurements of some quasars are affected by an interstellar polarization in our Galaxy, but the measurements have been corrected for this effect reasonably well. Since the broad emission lines are essentially unpolarized and the polarization is confined only to the continuum in the five quasars including Ton 202 in both epochs, the polarized flux is considered to originate interior to the broad emission line region. The Balmer edge feature seen in the polarized flux is most simply interpreted as an intrinsic spectral feature of the quasar UV/optical continuum, or the ``Big Blue Bump'' emission. In this case, the edge feature seen in absorption indeed indicates the thermal and optically-thick nature of the continuum emitted. However, we also discuss other possible interpretations.Comment: Accepted for publication in MNRAS; 31 pages, 38 figures with reduced resolutions; the paper with a full resolution is at http://www.roe.ac.uk/~mk/papers/04Ba_vk.ps.g