We present numerical models of the gas dynamics in the inner parsec of the Galactic centre. We follow the gas from its origin as stellar winds of several observed young massive stars, until it is either captured by the central black hole, or leaves the system. Unlike our previous models, we include an outflow from the inner accretion flow. Two different kinds of outflows are modelled: (i) an instantaneous-response feedback mode, in which the outflow rate is directly proportional to the current black hole gas capture rate; and (ii) an outburst mode, which is stronger but lasts for a limited time. The latter situation may be particularly relevant to Sgr A*, since there is evidence that Sgr A* was much brighter in the recent past. We find that both types of outflow perturb the gas dynamics near the Bondi radius and the black hole capture rate significantly. The effects persist longer than the outflow itself. We also compare the effects of spherically symmetric and collimated outflows, and find that the latter are far less efficient in transferring its energy to the surrounding gas near the capture radius. Our results imply that accretion feedback is important for non-radiative accretion flows not only within but also outside the capture radius. Steady-state Bondi accretion rate estimates that do not account for feedback outflows over-predict not only the accretion rate onto the black hole but also the capture rate at the Bondi radius itself. Finally, the steady-state assumption under which non-radiative flows have been routinely studied in the literature may have to be abandoned if accretion feedback is bursty in nature.
An important parameter in the theory of hot accretion flows around black holes is $δ$, which describes the fraction of ``viscously'' dissipated energy in the accretion flow that goes directly into heating electrons. For a given mass accretion rate, the radiative efficiency of a hot accretion flow is determined by $δ$. Unfortunately, the value of $δ$ is hard to determine from first principles. The recent Event Horizon Telescope Collaboration (EHTC) results on M87* and Sgr A* provide us with a different way of constraining $δ$. By combining the mass accretion rates in M87* and Sgr A* estimated b…
We briefly summarize the method of simulating Sgr A* polarized sub-mm spectra from the accretion flow and fitting the observed spectrum. The dynamical flow model is based on three-dimensional general relativistic magneto hydrodynamic simulations. Fully self-consistent radiative transfer of polarized cyclo-synchrotron emission is performed. We compile a mean sub-mm spectrum of Sgr A* and fit it with the mean simulated spectra. We estimate the ranges of inclination angle theta=42-75deg, mass accretion rate Mdot=(1.4-7.0)*10^{-8}Msun/yr, and electron temperature Te=(3-4)*10^{10}K at 6M. We discus…
A possibility of time-delayed radio brightenings of Sgr A* triggered by the pericenter passage of the G2 cloud is studied by carrying out global three-dimensional magnetohydrodynamic simulations taking into account the radiative cooling of the tidal debris of the G2 cloud. Magnetic fields in the accretion flow are strongly perturbed and re-organized after the passage of G2. We have found that the magnetic energy in the accretion flow increases by a factor 3-4 in 5-10 years after the pericenter passage of G2 by a dynamo mechanism driven by the magneto-rotational instability. Since this B-field …
Radiatively inefficient accretion flows (RIAFs) are believed to power supermassive black holes (SMBH) in the underluminous cores of galaxies. Such black holes are typically accompanied by flat-spectrum radio cores indicating the presence of moderately relativistic jets. One of the best constrained RIAFs is associated with the SMBH in the Galactic center, Sgr A*. Since the plasma in RIAFs is only weakly collisional, the dynamics and the radiative properties of these systems are very uncertain. Here we want to study the impact of varying electron temperature on the appearance of accretion flows …
The candidate supermassive black hole in the Galactic Centre, Sagittarius A* (Sgr A*), is known to be fed by a radiatively inefficient accretion flow (RIAF), inferred by its low accretion rate. Consequently, radiative cooling has in general been overlooked in the study of Sgr A*. However, the radiative properties of the plasma in RIAFs are poorly understood. In this work, using full 3D general-relativistic magneto-hydrodynamical simulations, we study the impact of radiative cooling on the dynamical evolution of the accreting plasma, presenting spectral energy distributions and synthetic sub-mi…
We review our current understanding to the accretion and ejection processes in Sgr A*. Roughly speaking, they correspond to the quiescent and flare states of the source respectively. The high-resolution {\it Chandra} observations to the gas at the Bondi radius combined with the Bondi accretion theory, the spectral energy distribution from radio to X-ray, and the radio polarization provide us strict constraints and abundant information to the theory of accretion. We review these observational results and describe how the advection-dominated accretion flow model explains these observations. Rece…
The massive black hole in our galactic center, Sgr A*, accretes only a small fraction of the gas available at its Bondi radius. The physical processes determining this accretion rate remain unknown, partly due to a lack of observational constraints on the gas at distances between ~10 and ~10$^5$ Schwarzschild radii (Rs) from the black hole. Recent infrared observations identify low-mass gas clouds, G1 and G2, moving on highly eccentric, nearly co-planar orbits through the accretion flow around Sgr A*. Although it is not yet clear whether these objects contain embedded stars, their extended gas…
High-frequency quasi-periodic oscillations (QPOs) appear in general-relativistic magnetohydrodynamic simulations of magnetically choked accretion flows around rapidly rotating black holes (BHs). We perform polarized radiative transfer calculations with ASTRORAY code to explore the manifestations of these QPOs for SgrA*. We construct a simulation-based model of a radiatively inefficient accretion flow and find model parameters by fitting the mean polarized source spectrum. The simulated QPOs have a total sub-mm flux amplitude up to 5% and a linearly polarized flux amplitude up to 2%. The oscill…
The interplay between supermassive black holes (SMBHs) and their environments is believed to command an essential role in galaxy evolution. The majority of these SMBHs are in the radiative inefficient accretion phase where this interplay remains elusive, but suggestively important, due to few observational constraints. To remedy this, we directly fit 2-D hydrodynamic simulations to Chandra observations of Sgr A* with Markov Chain Monte Carlo sampling, self-consistently modelling the 2-D inflow-outflow solution for the first time. We find the temperature and density at flow onset are consistent…
Radiatively inefficient accretion flow models have been shown to accurately account for the spectrum and luminosity observed from Sgr A* in the X-ray regime down to mm wavelengths. However, observations at a few GHz cannot be explained by thermal electrons alone but require the presence of an additional non-thermal particle population. Here, we propose a model for the origin of such a population in the accretion flow via means of a pulsar orbiting the supermassive black hole in our Galaxy. Interactions between the relativistic pulsar wind with the disc lead to the formation of a bow shock in t…
Millimeter polarimetry of Sgr A* probes the linearly polarized emission region on a scale of $\sim 10$ Schwarzschild radii ($R_S$) as well as the dense, magnetized accretion flow on scales out to the Bondi radius ($\sim 10^5 R_S$) through Faraday rotation. We present here multi-epoch ALMA Band 6 (230 GHz) polarimetry of Sgr A*. The results confirm a mean rotation measure, ${\rm RM} \approx -5 \times 10^5 {\rm\ rad\ m^{-2}}$, consistent with measurements over the past 20 years and support an interpretation of the RM as originating from a radiatively inefficient accretion flow (RIAF) with $\dot{…
We present the combination of dynamical accretion model based on 3D GRMHD simulations and general relativistic (GR) polarized radiative transfer. We write down the formalism of and perform the GR ray-tracing of cyclo-synchrotron radiation through the model of accretion flow in Sagittarius A*. GR polarimetric imaging is presented as well as the results for spectrum for a probable set of spins and orientations. Precise fitting formulae for Faraday rotation and Faraday conversion coefficients are employed for thermal plasma. The axisymmetic flow pattern and the magnetic field geometry correspond …
The supermassive black hole in the center of the Milky Way, Sgr A*, displays a nearly flat radio spectrum that is typical for jets in active galactic nuclei. Indeed, time-dependent magnetized models of radiatively inefficient accretion flows (RIAFs), which are commonly used to explain the millimeter, near-infrared, and X-ray emission of Sgr A*, often also produce jet-like outflows. However, the emission from these models has so far failed to reproduce the flat radio spectrum. Aims: We investigate whether current accretion simulations can produce the compact flat spectrum emission by simply usi…
Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disk, the ram and thermal pressures of the disk terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulati…
The recent image of our galaxy's supermassive black hole Sgr A* derived from the 7 April 2017 data of the Event Horizon Telescope Collaboration shows multiple hot spots in its accretion disk. Using the analytical framework, we demonstrate that the observed hot spots may not be disjoint elements but causally linked components ("petals") of one rotating quasi-stationary macro-structure formed in the thermo-vorticial field within the accretion disk.
The observed rotation measures (RMs) towards the galactic centre magnetar and towards Sagittarius A* provide a strong constraint on MHD models of the galactic centre accretion flow, probing distances from the black hole separated by many orders of magnitude. We show, using 3D simulations of accretion via magnetized stellar winds of the Wolf-Rayet stars orbiting the black hole, that the large, time-variable RM observed for the pulsar PSR J1745-2900 can be explained by magnetized wind-wind shocks of nearby stars in the clockwise stellar disc. In the same simulation, both the total X-ray luminosi…
Sgr A* represents a unique laboratory for the detailed study of accretion processes around a low-luminosity supermassive black hole (SMBH). Recent X-ray observations have allowed for spatially resolved modeling of the emission from the accretion flow around the SMBH, placing tight constraints on the flux and spectral shape of the accretion from the inner region with $r<10^3~r_g$, where $r_g$ is the gravitational radius of the black hole. We present here the first modeling of the multi-band spectral energy distribution (SED) of this inner region to better constrain the physical condition of the…
The long-term accretion luminosity of V4641 Sgr through binary evolution simulations: implications for its ultrahigh-energy gamma-ray emission
School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）；Key laboratory of Modern Astronomy and Astrophysics(Nanjing University),Ministry of Education（南京大学现代天文与天体物理教育部重点实验室）；Tianfu Cosmic Ray Research Center（天府宇宙线研究中心）
Latest observational data provides evidence that the emissions from Sgr A* originate from an accretion disc within ten gravitational radii of the dynamical centre of Milky Way. We investigate the physical processes responsible for the variable observed emissions from the compact radio source Sgr A*. We study the evolution of the variable emission region and analyse light curves and time-resolved spectra of emissions originated at the surface of the accretion disk, close to the event horizon, near the marginally stable orbit of a Kerr black hole.
Upcoming Event Horizon Telescope (EHT) observations will provide a unique opportunity to reveal the innermost region of the radiative inefficient accretion flow (RIAF) around the Galactic black hole, Sgr A*. Depending on the flow dynamics and accumulated magnetic flux, the innermost region of an RIAF could have a quasi-spherical or disk-like geometry. Here we present a phenomenological model to investigate the characteristics of the black hole shadow images with different flow geometries, together with the effect of black hole spin and flow dynamics. The resulting image consists in general of …
We study the dynamical interaction of the G2 cloud with the accretion flow around Sgr A* by means of three-dimensional, hydrodynamic simulations. We show the effects of the rotating accretion flow on the evolution of G2 by projecting the cloud density onto the plane of the sky, and extracting position-velocity diagrams. We study a number of possible orientations of the cloud orbit with respect to the disk. We find that once the center of mass of the cloud has crossed the pericenter, the differences between models becomes significant. Models with the cloud counter-rotating with respect to the d…
How accretion proceeds around the massive black hole in the Galactic center and other highly sub-Eddington accretors remains poorly understood. The orbit of the S2 star in the Galactic center passes through the accretion disk of the massive black hole and any observational signature from such interaction may be used as an accretion probe. Because of its early stellar type, S2 is expected to possess a fairly powerful wind. We show here that the ram pressure of the accretion disk shocks the stellar wind fairly close to the star. The shocked fluid reaches a temperature of ~ 1 keV and cools effici…
We study effects of heating by dark matter (DM) annihilation on black hole gas accretion. We observe that, for reasonable assumptions about DM densities in spikes around supermassive black holes, as well as DM masses and annihilation cross-sections within the standard WIMP model, heating by DM annihilation may have an appreciable effect on the accretion onto Sgr A$^*$ in the Galactic center. Motivated by this observation we study the effects of such heating on Bondi accretion, i.e. spherically symmetric, steady-state Newtonian accretion onto a black hole. We consider different adiabatic indice…
Soft gamma repeaters and anomalous X-ray pulsars are believed to be magnetars, i.e. neutron stars powered by extreme magnetic fields, B~10^(14)-10^(15) Gauss. The recent discovery of a soft gamma repeater with low magnetic field (< 7.5x10^(12) Gauss), SGR 0418+5729, which shows bursts similar to those of SGRs, implies that a high surface dipolar magnetic field might not be necessary for magnetar-like activity. We show that the quiescent and bursting properties of SGR 0418+5729 find natural explanations in the context of low-magnetic field Quark-Nova (detonative transition from a neutron star t…
We present the first spectral energy distributions produced self-consistently by 2.5D general relativistic magneto-hydrodynamical (GRMHD) numerical simulations, where radiative cooling is included in the dynamical calculation. As a case study, we focus on the accretion flow around the supermassive black hole in the Galactic Centre, Sagittarius A* (Sgr A*), which has the best constrained physical parameters. We compare the simulated spectra to the observational data of Sgr A* and explore the parameter space of our model to determine the effect of changing the initial magnetic field configuratio…
Sgr A* is probably the supermassive black hole being investigated most extensively due to its proximity. Several theoretical models for its steady state emission have been proposed in the past two decades. Both the radiative-inefficient accretion flow and the jet model have been shown to well explain the observed spectral energy distribution. Faraday rotation measure (RM) has been unambiguously measured at submillimeter wavelength, but has only been tested against the accretion flow model. Here we first calculate the RM based on the jet model and find that the predicted value is two orders of …
We present general relativistic magnetohydrodynamic (GRMHD) numerical simulations of the accretion flow around the supermassive black hole in the Galactic centre, Sagittarius A* (Sgr A*). The simulations include for the first time radiative cooling processes (synchrotron, bremsstrahlung, and inverse Compton) self-consistently in the dynamics, allowing us to test the common simplification of ignoring all cooling losses in the modeling of Sgr A*. We confirm that for Sgr A*, neglecting the cooling losses is a reasonable approximation if the Galactic centre is accreting below ~10^{-8} Msun/yr i.e.…
SgrA$^*$ is the supermassive black hole candidate at the center of the Galaxy and an ideal laboratory to test general relativity. Following previous work by other authors, we use the Polish doughnut model to describe an optically thin and constant angular momentum ion torus in hydrodynamical equilibrium and model the accretion structure around SgrA$^*$. The radiation mechanisms are bremsstrahlung, synchrotron emission, and inverse Compton scattering. We compute the spectrum as seen by a distant observer in Kerr and non-Kerr spacetimes and we study how an accurate measurement can constrain poss…
We propose a 2-temperature radial dynamical model of plasma flow near Sgr A* and fit the bremsstrahlung emission to extensive quiescent X-Ray Chandra data. The model extends from several arcseconds to black hole (BH) gravitational radius, describing the outer accretion flow together with the infalling region. The model incorporates electron heat conduction, relativistic heat capacity of particles and feeding by stellar winds. Stellar winds from each star are considered separately as sources of mass, momentum and energy. Self-consistent search for the stagnation and sonic points is performed. M…
A pair of giant gamma-ray bubbles have been revealed by the {\it Fermi} LAT. In this paper we investigate their formation mechanism. Observations have indicated that the activity of the supermassive black hole located at the Galactic center, Sgr A*, was much stronger than the present time. Specifically, one possibility is that while Sgr A* was also in the hot accretion regime, the accretion rate should be $10^3-10^4$ times higher during the past $\sim 10^7$ yr. On the other hand, recent MHD numerical simulations of hot accretion flows have unambiguously shown the existence of strong winds and …
Low-angular-momentum accretion shocks can power weak-to-moderate X-ray flares from SgrA*
International Center for Theoretical Sciences, Tata Institute of Fundamental Research（塔塔基础研究所国际理论科学中心）；Center for Theoretical Physics, Polish Academy of Sciences（波兰科学院理论物理中心）；Department of Theoretical Physics and Astrophysics, Masaryk University（马萨里克大学理论与天体物理学系）；McGill University（麦吉尔大学）；Trottier Space Institute（特罗蒂尔太空研究所）
We examine the low angular momentum flow model for Sgr A* using two-dimensional hydrodynamical calculations based on the parameters of the specific angular momentum and total energy estimated in the recent analysis of stellar wind of nearby stars around Sgr A*. The accretion flow with the plausible parameters is non-stationary and an irregularly oscillating shock is formed in the inner region of a few tens to a hundred and sixty Schwarzschild radii. Due to the oscillating shock, the luminosity and the mass-outflow rate are modulated by several per cent to a factor of 5 and a factor of 2-7, res…
We analyze 134 ks Chandra ACIS-I observations of the Galactic Centre (GC) performed in July 2011. The X-ray image with the field of view $17' \times 17'$ contains the hot plasma surrounding the Sgr~A*. The obtained surface brightness map allow us to fit Bondi hot accretion flow to the innermost hot plasma around the GC. We have fitted spectra from region up to $5"$ from Sgr~A* using a thermal bremsstrahlung model and four Gaussian profiles responsible for K$_α$ emission lines of Fe, S, Ar, and Ca. The X-ray surface brightness profile up to $3"$ from Sgr~A* found in our data image, was successf…
We propose a two-temperature radial inflow-outflow model near Sgr A* with self-consistent feeding and conduction. Stellar winds from individual stars are considered to find the rates of mass injection and energy injection. These source terms help to partially eliminate the boundary conditions on the inflow. Electron thermal conduction is crucial for inhibiting the accretion. Energy diffuses out from several gravitational radii, unbinding more gas at several arcseconds and limiting the accretion rate to <1% of Bondi rate. We successfully fit the X-Ray surface brightness profile found from the e…
The gas cloud G2 falling toward Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, is supposed to provide valuable information on the physics of accretion flows and the environment of the black hole. We observed Sgr A* with four European stations of the Global Millimeter Very Long Baseline Interferometry Array (GMVA) at 86 GHz on 1 October 2013 when parts of G2 had already passed the pericenter. We searched for possible transient asymmetric structure -- such as jets or winds from hot accretion flows -- around Sgr A* caused by accretion of material from G2. The…
Gas clouds are present in the Galactic centre, where they orbit around the supermassive black hole. Collisions between these clumps reduce their angular momentum, and as a result some of the clumps are set on a plunging trajectory. Constraints can be imposed on the nature of past accretion events based on the currently observed X-ray reflection from the molecular clouds. We discuss accretion of clouds in the context of enhanced activity of Sgr A* during the past few hundred years. We put forward a scenario according to which gas clouds bring material close to the horizon of the black hole on <…
We study the dynamical evolution of the putative gas clouds G1 and G2 recently discovered in the Galactic center. Following earlier studies suggesting that these two clouds are part of a larger gas streamer, we combine their orbits into a single trajectory. Since the gas clouds experience a drag force from background gas, this trajectory is not exactly Keplerian. By assuming the G1 and G2 clouds trace this trajectory, we fit for the drag force they experience and thus extract information about the accretion flow at a distance of thousands of Schwarzschild radii from the black hole. This range …
Non-linear interaction between the electromagnetic fields (EMF) occurs when vacuum polarization in quantum electrodynamics (QED) happens. The field of non-linear electrodynamics, which may result from this interaction, could have important effects on black hole physics. This paper considers the asymptotically flat black hole solution in Einstein-nonlinear electrodynamics (NLE) fields. We study the effect of the NLE parameters on the black hole deflection angle using the Gauss-Bonnet theorem in weak field limits, shadow cast using the null geodesics method, and thin accretion disk using the Nov…
We report 235 GHz linear and circular polarization (LP and CP) detections of Sgr A* at levels of $\sim10\%$ and $\sim-1\%$, respectively, using ALMA. We describe the first full-Stokes modeling of an observed submillimeter flare with an adiabatically-expanding synchrotron hotspot using a polarized radiative transfer prescription. Augmented with a simple full-Stokes model for the quiescent emission, we jointly characterize properties of both the quiescent and variable components by simultaneously fitting all four Stokes parameter light curves. The hotspot has magnetic field strength $71$ G, radi…
We carry out a series of simulations of G2-type clouds interacting with the black hole at the galactic center, to determine why no large changes in the luminosity of Sgr A* were seen, and to determine the nature of G2. We measure the accretion rate from the gas cloud onto Sgr A* for a range of simulation parameters, such as cloud structure, background structure, background density, grid resolution, and accretion radius. For a broad range of parameters, the amount of cloud material accreted is small relative to the amount of background material accreted. The total accretion rate is not signific…
Some intermediate polars show outbursts that are much shorter than those observed in normal dwarf novae, and whose origin has remained unelucidated for a long time. We examine here the case of V1223 Sgr, an intermediate polar that showed a short outburst in 1984, and compare the outburst characteristics with the predictions of the magnetospheric gating model. We use the archival data from the AAVSO from which we extract the outburst profiles. We use our code for computing the time-dependent evolution of an accretion disc truncated by the white dwarf magnetic field, using a simple description o…
Recent advances in general relativistic magnetohydrodynamic simulations have expanded and improved our understanding of the dynamics of black-hole accretion disks. However, current simulations do not capture the thermodynamics of electrons in the low density accreting plasma. This poses a significant challenge in predicting accretion flow images and spectra from first principles. Because of this, simplified emission models have often been used, with widely different configurations (e.g., disk- versus jet-dominated emission), and were able to account for the observed spectral properties of accr…
The X-ray spectrum of the Galactic X-ray binary V4641 Sgr in outburst has been found to exhibit a remarkably broad emission feature above 4 keV, with inferred equivalent widths up to 2 keV. Such a feature was first detected during the X-ray flaring activity associated with the giant outburst that the source experienced in 1999 September. The extraordinarily large equivalent width line was then ascribed to reflection/reprocessing of fluorescent Fe emission within an extended optically thick outflow enshrouding the binary system as a result of a short-lived, super-Eddington accretion episode. Ma…
The recent discovery of the G2 cloud of dense, ionized gas on a trajectory toward Sgr A*, the black hole at the dynamical center of the Galaxy, offers a unique opportunity to observe an accretion event onto a massive black hole as well as to probe its immediate environment. Simulations and models predict increased X-ray and radio variability resulting from increased accretion driven by drag on an atmosphere of hot, X-ray emitting gas surrounding Sgr A*. Here, we present X-ray and radio light curves of the emission resulting from the potential encounter of the G2 cloud with a relativistic jet f…
Dense, multi-phase accretion disk atmosphere in the low-luminosity state of black hole transientV4641 Sgr
University of Oxford（牛津大学）；University of Cape Town（开普敦大学）；University of Warwick（华威大学）；Eberhard-Karls Universität Tübingen（蒂宾根大学）；Instituto de Astrofísica de Canarias（加那利群岛天体物理研究所）；Universidad de La Laguna（拉帕尔马大学）；Ehime University（爱媛大学）
We examine 1D two-temperature accretion flows around a supermassive black hole, adopting the specific angular momentum λ, the total specific energy εand the input accretion rate \dot M_{input} = 4.0x10^{-6} solar mass/yr estimated in the recent analysis of stellar wind of nearby stars around Sgr A*. The two-temperature flow is almost adiabatic even if we take account of the heating of electrons by ions, the bremsstrahlung cooling and the synchrotron cooling, as long as the ratio βof the magnetic energy density to the thermal energy density is taken to be as β< 1. The different temperatures of …
The accretion of matter onto intermediate polar White Dwarfs (IPWDs) seems to provide attractive conditions for acceleration of particles to high energies in a strongly magnetized turbulent region at the accretion disk inner radius. We consider possible acceleration of electrons and hadrons in such region and investigate their high energy radiation processes. It is concluded that accelerated electrons loose energy mainly on synchrotron process producing non-thermal X-ray emission. On the other hand, accelerated hadrons are convected onto the WD surface and interact with dense matter. As a resu…
Decomposing an arbitrary electron energy distribution into sums of Maxwellian and power law components is an efficient method to calculate synchrotron emission and absorption. We use this method to study the effect of non-thermal electrons on submm images and spectra of the Galactic center black hole, Sgr A*. We assume a spatially uniform functional form for the electron distribution function and use a semi-analytic radiatively inefficient accretion flow and a 2D general relativistic MHD snapshot as example models of the underlying accretion flow structure. We develop simple analytic models wh…
The Event Horizon Telescope is a global very-long baseline interferometer capable of probing potential deviations from the Kerr metric, which is believed to provide the unique description of astrophysical black holes. Here we report an updated constraint on the quadrupolar deviation of Sagittarius A* within the context of a radiatively inefficient accretion flow model in a quasi-Kerr background. We also simulate near-future constraints obtainable by the forthcoming eight-station array and show that in this model already a one-day observation can measure the spin magnitude to within $0.005$, th…
In accretion-based models for Sgr A* the X-ray, infrared, and millimeter emission arise in a hot, geometrically thick accretion flow close to the black hole. The spectrum and size of the source depend on the black hole mass accretion rate $\dot{M}$. Since Gillessen et al. have recently discovered a cloud moving toward Sgr A* that will arrive in summer 2013, $\dot{M}$ may increase from its present value $\dot{M}_0$. We therefore reconsider the "best-bet" accretion model of Moscibrodzka et al., which is based on a general relativistic MHD flow model and fully relativistic radiative transfer, for…
Stellar-wind Fueled Accretion onto Sagittarius A* in the Presence of a Nuclear Star Cluster
Northwestern University（西北大学）；Institute for Advanced Study（高等研究院）；Canadian Institute for Theoretical Astrophysics（加拿大理论天体物理研究所）；University of Maryland（马里兰大学）
