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Register a new userClumpy accretion onto black holes. I. Clumpy-ADAF structure and radiation
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In this paper, we investigate the dynamics of clumps embedded in and confined by the advection-dominated accretion flows (ADAF), in which collisions among the clumps are neglected. We start from the collisionless Boltzmann equation and assume that interaction between the clumps and the ADAF is responsible for transporting angular momentum of clumps outward. The inner edge of the clumpy-ADAF is set to be the tidal radius of the clumps. We consider strong and weak coupling cases, in which the averaged properties of clumps follow the ADAF dynamics and mainly determined by the black hole potential, respectively. We get the analytical solution of the dynamics of clumps for the two cases. The velocity dispersion of clumps is one magnitude higher than the ADAF for the strong coupling case. For the weak coupling case, we find that the mean radial velocity of clumps is linearly proportional to the coefficient of the drag force. We show that the tidally disrupted clumps would lead to accumulation of the debris to form a debris disk in the Shakura-Sunyaev regime. The entire hot ADAF will be efficiently cooled down by photons from the debris disk, giving rise to collapse of the ADAF and quench the clumpy accretion. Subsequently, evaporation of the collapsed ADAF drives resuscitate of a new clumpy-ADAF, resulting in an oscillation of the global clumpy-ADAF. Applications of the present model are briefly discussed to X-ray binaries, ionization nuclear emission regions (LINERs) and BL Lac objects.
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Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds - a departure from the hot mode accretion model - although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z=0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities can precipitate from this hot gas, producing a rain of cold clouds that fall toward the galaxys centre, sustaining star formation amid a kiloparsec-scale molecular nebula that inhabits its core. The observations show that these cold clouds also fuel black hole accretion, revealing shadows cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole in the galaxy centre, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.
We study the dynamics of massive black hole pairs in clumpy gaseous circumnuclear disks. We track the orbital decay of the light, secondary black hole $M_{bullet2}$ orbiting around the more massive primary at the center of the disk, using $N$-body/smoothed particle hydrodynamic simulations. We find that the gravitational interaction of $M_{bullet2}$ with massive clumps $M_{rm cl}$ erratically perturbs the otherwise smooth orbital decay. In close encounters with massive clumps, gravitational slingshots can kick the secondary black hole out of the disk plane. The black hole moving on an inclined orbit then experiences the weaker dynamical friction of the stellar background, resulting in a longer orbital decay timescale. Interactions between clumps can also favor orbital decay when the black hole is captured by a massive clump which is segregating toward the center of the disk. The stochastic behavior of the black hole orbit emerges mainly when the ratio $M_{bullet2}/M_{rm cl}$ falls below unity, with decay timescales ranging from $sim1$ to $sim50$ Myr. This suggests that describing the cold clumpy phase of the inter-stellar medium in self-consistent simulations of galaxy mergers, albeit so far neglected, is important to predict the black hole dynamics in galaxy merger remnants.
The Unruhs thermal state in the vicinity of the event horizon of the black hole provides conditions where impinging particles can radiate other particles. The subsequent decays may eventually lead to observable radiation of photons and neutrinos induced even by massive particles with gravitational interaction only. The hadronic particles will induce $sim 30$ MeV gamma radiation from $pi^{0}$ decays.
These notes resulted from a series of lectures at the IAC winter school. They are designed to help students, especially those just starting in subject, to get hold of the fundamental tools used to study accretion powered sources. As such, the references give a place to start reading, rather than representing a complete survey of work done in the field. I outline Compton scattering and blackbody radiation as the two predominant radiation mechanisms for accreting black holes, producing the hard X-ray tail and disc spectral components, respectively. The interaction of this radiation with matter can result in photo-electric absorption and/or reflection. While the basic processes can be found in any textbook, here I focus on how these can be used as a toolkit to interpret the spectra and variability of black hole binaries (hereafter BHB) and Active Galactic Nuclei (AGN). I also discuss how to use these to physically interpret real data using the publicly available XSPEC spectral fitting package (Arnaud et al 1996), and how this has led to current models (and controversies) of the accretion flow in both BHB and AGN.
Spectral formation in steady state, spherical accretion onto neutron stars and black holes is examined by solving numerically and analytically the equation of radiative transfer. The photons escape diffusively and their energy gains come from their scattering off thermal electrons in the converging flow of the accreting gas. We show that the bulk motion of the flow is more efficient in upscattering photons than thermal Comptonization in the range of non-relativistic electron temperatures. The spectrum observed at infinity is a power law with an exponential turnover at energies of order the electron rest mass. Especially in the case of accretion into a black hole, the spectral energy power-law index is distributed around 1.5. Because bulk motion near the horizon (1-5 Schwarzschild radii) is most likely a necessary characteristic of accretion into a black hole, we claim that observations of an extended power law up to about the electron rest mass, formed as a result of bulk motion Comptonization, is a real observational evidence for the existence of an underlying black hole.
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