Black hole accretion flows can be divided into two broad classes: cold and hot. Whereas cold accretion flows consist of cool optically thick gas and are found at relatively high mass accretion rates, hot accretion flows, the topic of this review, are virially hot and optically thin, and occur at lower mass accretion rates. They are described by accretion solutions such as the advection-dominated accretion flow and luminous hot accretion flow. Because of energy advection, the radiative efficiency of these flows is in general lower than that of a standard thin accretion disk. Moreover, the efficiency decreases with decreasing mass accretion rate. Observations show that hot accretion flows are associated with jets. In addition, theoretical arguments suggest that hot flows should produce strong winds. Hot accretion flows are believed to be present in low-luminosity active galactic nuclei and in black hole X-ray binaries in the hard and quiescent states. The prototype is Sgr A*, the ultralow-luminosity supermassive black hole at our Galactic center. The jet, wind, and radiation from a supermassive black hole with a hot accretion flow can interact with the external interstellar medium and modify the evolution of the host galaxy.

Hot Accretion Flows Around Black Holes

We study accretion processes for tidally disrupted stars approaching supermassive black holes on bound orbits, by performing three dimensional Smoothed Particle Hydrodynamics simulations with a pseudo-Newtonian potential We find that there is a critical value of the orbital eccentricity below which all the stellar debris remains bound to the black hole For high but sub-critical eccentricities, all the stellar mass is accreted onto the black hole in a finite time, causing a significant deviation from the canonical t 5/3 mass fallback rate When a star is on a moderately eccentric orbit and its pericenter distance is deeply inside the tidal disruption radius, there can be several orbit crossings of the debris streams due to relativistic precession This dissipates orbital energy in shocks, allowing for rapid circularization of the debris streams and formation of an accretion disk The resultant accretion rate greatly exceeds the Eddington rate and differs strongly from the canonical rate of t 5/3  By contrast, there is little dissipation due to orbital crossings for the equivalent simulation with a purely Newtonian potential This shows that general relativistic precession is crucial for accretion disk formation via circularization of stellar debris from stars on moderately eccentric orbits

Finite, Intense Accretion Bursts from Tidal Disruption of Stars on Bound Orbits

Author(s): Stephan, Alexander P; Naoz, Smadar; Ghez, Andrea M; Witzel, Gunther; Sitarski, Breann N; Do, Tuan; Kocsis, Bence | Abstract: Most, if not all, stars in the field are born in binary configurations or higher multiplicity systems. In dense stellar environment such as the Galactic Center (GC), many stars are expected to be in binary configurations as well. These binaries form hierarchical triple body systems, with the massive black hole (MBH) as the third, distant object. The stellar binaries are expected to undergo large amplitude eccentricity and inclination oscillations via the so-called "eccentric Kozai-Lidov" (EKL) mechanism. These eccentricity excitations, combined with post main sequence stellar evolution, can drive the inner stellar binaries to merge. We study the mergers of stellar binaries in the inner 0.1 pc of the GC caused by gravitational perturbations due to the MBH. We run a large set of Monte Carlo simulations that include the secular evolution of the orbits, general relativistic precession, tides, and post-main-sequence stellar evolution. We find that about 13 % of the initial binary population will have merged after a few million years and about 29 % after a few billion years. These expected merged systems represent a new class of objects at the GC and we speculate that they are connected to G2-like objects and the young stellar population.

/pdf/merging-binaries-in-the-galactic-center-the-eccentric-kozai-4txnywrxvs.pdf

Merging binaries in the Galactic Center: the eccentric Kozai-Lidov mechanism with stellar evolution

The center of our Galaxy hosts the best constrained supermassive black hole in the universe, Sagittarius A* (Sgr A*). Its mass and distance have been accurately determined from stellar orbits and proper motion studies, respectively, and its high-frequency radio, and highly variable near-infrared and X-ray emission originate from within a few Schwarzschild radii of the event horizon. The theory of general relativity (GR) predicts the appearance of a black hole shadow, which is a lensed image of the event horizon. This shadow can be resolved by very long baseline radio interferometry and test basic predictions of GR and alternatives thereof. In this paper we review our current understanding of the physical properties of Sgr A*, with a particular emphasis on the radio properties, the black hole shadow, and models for the emission and appearance of the source. We argue that the Galactic Center holds enormous potential for experimental tests of black hole accretion and theories of gravitation in their strong limits.

Towards the event horizon -- the supermassive black hole in the Galactic Center

Context. There are a number of faint compact infrared excess sources in the central stellar cluster of the Milky Way. Their nature a nd origin is unclear. In addition to several isolated objects o f this kind there is a small but dense cluster of comoving sources (IRS13N) located∼3” west of SgrA* just 0.5” north of the bright IRS13E cluster of Wolf-Rayet and O-type stars. Based on the analysis of their color and brightness, there are two main possibilities: (1) they may be dust-embedded stars older than a few Myr, or (2) very young, dusty stars with ages younger than 1 Myr. Aims. We present a first K s-band identification and proper motions of the IRS13N member s, the high-velocity dusty S-cluster object (DSO, also referred to as G2), and other infrared excess sour ces in the central field. Goal is to constrain the nature of the se source. Methods. The L ′ - (3.8� m) Ks- (2.2� m) and H-band (1.65� m) observations were carried out using the NACO adaptive optics system at the ESO VLT. Proper motions were obtained by linear fitting of the stellar positions extracted by StarFinder as a functi on of time, weighted by positional uncertainties, and by Gaussian fitti ng from high-pass filtered and deconvolved images. We also pr esent results of near-infrared (NIR) H- and Ks-band ESO-SINFONI integral field spectroscopy of the Galact ic Center cluster ISR13N. Results. We show that within the uncertainties, the positions and proper motions of the IRS13N sources in Ks- and L ′ -band are identical. The HK−sL’ colors then indicate that the bright L ′ -band IRS13N sources are indeed dust-enshrouded stars rather than core-less dust clouds. The proper motions also show that the IRS13N sources are not strongly gravitationally bound to each other. Combined with their NIR colors, this implies that they have been formed recently. For the DSO we obtain proper motions and a Ks-L ′ -color. Conclusions. Most of the compact L ′ -band excess emission sources have a compact H- or Ks-band counterpart and therefore are likely stars with dust shells or disks. Our new results and or bital analysis from our previous work favor the hypothesis t hat the infrared excess IRS13N members and other dusty sources close to SgrA* are young dusty stars and that star formation at the Galactic Center (GC) is a continuously ongoing process. For the DSO the color information indicates that it may be a dust cloud or a dust-embedded star.

/pdf/near-infrared-proper-motions-and-spectroscopy-of-infrared-316u5d7t3w.pdf

Near-infrared proper motions and spectroscopy of infrared excess sources at the Galactic center

The origin, structure and evolution of the small gas cloud, G2, is investigated, that is on an orbit almost straight into the Galactic central supermassive black hole (SMBH). G2 is a sensitive probe of the hot accretion zone of Sgr A*, requiring gas temperatures and densities that agree well with models of captured shock-heated stellar winds. Its mass is equal to the critical mass below which cold clumps would be destroyed quickly by evaporation. Its mass is also constrained by the fact that at apocenter its sound crossing timescale was equal to its orbital timescale. Our numerical simulations show that the observed structure and evolution of G2 can be well reproduced if it formed in pressure equilibrium with the surrounding in 1995 at a distance from the SMBH of 7.6e16 cm. If the cloud would have formed at apocenter in the 'clockwise' stellar disk as expected from its orbit, it would be torn into a very elongated spaghetti-like filament by 2011 which is not observed. This problem can be solved if G2 is the head of a larger, shell-like structure that formed at apocenter. Our numerical simulations show that this scenario explains not only G2's observed kinematical and geometrical properties but also the Br_gamma observations of a low surface brightness gas tail that trails the cloud. In 2013, while passing the SMBH G2 will break up into a string of droplets that within the next 30 years mix with the surrounding hot gas and trigger cycles of AGN activity.

Physics of the Galactic Center Cloud G2, on its Way towards the Super-Massive Black Hole

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Physics of the Galactic Center Cloud G2, on its Way towards the Super-Massive Black Hole