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Register a new userUsing Megamaser Disks to Probe Black Hole Accretion
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Jenny E. Greene
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We examine the alignment between H_2O megamaser disks on sub-pc scales with circumnuclear disks and bars on <500 pc scales observed with HST/WFC3. The HST imaging reveals young stars, indicating the presence of gas. The megamaser disks are not well aligned with the circumnuclear bars or disks as traced by stars in the HST images. We speculate on the implications of the observed misalignments for fueling supermassive black holes in gas-rich spiral galaxies. In contrast, we find a strong preference for the rotation axes of the megamaser disks to align with radio continuum jets observed on >50 pc scales, in those galaxies for which radio continuum detections are available. Sub-arcsecond observations of molecular gas with ALMA will enable a more complete understanding of the interplay between circumnuclear structures.
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Megamaser disks provide the most precise and accurate extragalactic supermassive black hole masses. Here we describe a search for megamasers in nearby galaxies using the Green Bank Telescope (GBT). We focus on galaxies where we believe that we can resolve the gravitational sphere of influence of the black hole and derive a stellar or gas dynamical measurement with optical or NIR observations. Since there are only a handful of super massive black holes (SMBH) that have direct black hole mass measurements from more than one method, even a single galaxy with a megamaser disk and a stellar dynamical black hole mass would provide necessary checks on the stellar dynamical methods. We targeted 87 objects from the Hobby-Eberly Telescope Massive Galaxy Survey, and detected no new maser disks. Most of the targeted objects are elliptical galaxies with typical stellar velocity dispersions of 250 km/s and distances within 130 Mpc. We discuss the implications of our non-detections, whether they imply a threshold X-ray luminosity required for masing, or possibly reflect the difficulty of maintaining a masing disk around much more massive (>10^8 Msun) black holes at low Eddington ratio. Given the power of maser disks at probing black hole accretion and demographics, we suggest that future maser searches should endeavour to remove remaining sample biases, in order to sort out the importance of these covariant effects.
We discuss the issues of stability of accretion disks that may undergo the limit-cycle oscillations due to the two main types of thermal-viscous instabilities. These are induced either by the domination of radiation pressure in the innermost regions close to the central black hole, or by the partial ionization of hydrogen in the zone of appropriate temperatures. These physical processes may lead to the intermittent activity in AGN on timescales between hundreds and millions of years. We list a number of observational facts that support the idea of the cyclic activity in high accretion rate sources. We conclude however that the observed features of quasars may provide only indirect signatures of the underlying instabilities. Also, the support from the sources with stellar mass black holes, whose variability timescales are observationally feasible, is limited to a few cases of the microquasars. Therefore we consider a number of plausible mechanisms of stabilization of the limit cycle oscillations in high accretion rate accretion disks. The newly found is the stabilizing effect of the stochastic viscosity fluctuations.
Several active galactic nuclei and microquasars are observed to eject plasmoids that move at relativistic speeds. We envisage the plasmoids as pre-existing current carrying magnetic flux ropes that were initially anchored in the accretion disk-corona. The plasmoids are ejected outwards via a mechanism called the toroidal instability (TI). The TI, which was originally explored in the context of laboratory tokamak plasmas, has been very successful in explaining coronal mass ejections from the Sun. Our model predictions for plasmoid trajectories compare favorably with a representative set of multi-epoch observations of radio emitting knots from the radio galaxy 3C120, which were preceded by dips in Xray intensity.
We study the prospects of using the low-redshift and high-redshift black hole shadows as new cosmological standard rulers for measuring cosmological parameters. We show that, using the low-redshift observation of the black hole shadow of M87$^star$, the Hubble constant can be independently determined with a precision of about $13%$ as $H_0=70pm 9$ km s$^{-1}$ Mpc$^{-1}$. The high-redshift observations of super-massive black hole shadows may accurately determine a combination of parameters $H_0$ and ${Omega_{rm m}}$, and we show by a simple simulation that combining them with the type Ia supernovae observations would give precise measurements of the cosmological parameters.
The exact time-dependent solution is obtained for a magnetic field growth during a spherically symmetric accretion into a black hole (BH) with a Schwarzschild metric. Magnetic field is increasing with time, changing from the initially uniform into a quasi-radial field. Equipartition between magnetic and kinetic energies in the falling gas is established in the developed stages of the flow. Estimates of the synchrotron radiation intensity are presented for the stationary flow. The main part of the radiation is formed in the region $r leq 7 r_g$, here $r_g$ is a BH gravitational radius. The two-dimensional stationary self-similar magnetohydrodynamic solution is obtained for the matter accretion into BH, in a presence of a large-scale magnetic field, when the magnetic field far from the BH is homogeneous and does not influence the flow. At the symmetry plane perpendicular to the direction of the distant magnetic field, the quasi-stationary disk is formed around BH, which structure is determined by dissipation processes. Parameters of the shock forming due to matter infall onto the disk are obtained. The radiation spectrum of the disk and the shock are obtained for the $10,, M_odot$ BH. The luminosity of such object is about the solar one, for a characteristic galactic gas density, with possibility of observation at distances less than 1 kpc. The spectra of a laminar and a turbulent disk structure around BH are very different. The turbulent disk emits a large part of its flux in the infrared. It may occur that some of the galactic infrared star-like sources are a single BH in the turbulent accretion state. The radiative efficiency of the magnetized disk is very high, reaching $sim 0.5,dot M,c^2$ so it was called recently as a magnetically arrested disk (MAD). Numerical simulations of MAD, and its appearance during accretion into neutron stars are considered and discussed.
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