Thin Advection-dominated Accretion Flow Research Articles

Kinematics and Collimation of the Two-sided Jets in NGC 4261: VLBI Study on Subparsec Scales

We report multifrequency VLBI studies of the subparsec scale structure of the two-sided jets in the nearby radio galaxy NGC 4261. Our analyses include new observations using the source frequency phase referencing technique with the Very Long Baseline Array at 44 and 88 GHz, as well as archival data at 15 and 43 GHz. Our results show an extended double-sided structure at 43/44 GHz and provide a clear image of the nuclear region at 88 GHz, showing a core size of ∼0.09 mas and a brightness temperature of ∼1.3 × 109 K. Proper motions are measured for the first time in the two-sided jets, with apparent speeds ranging from 0.31 ± 0.14 c to 0.59 ± 0.40 c in the approaching jet and 0.32 ± 0.14 c in the receding jet. The jet-to-counterjet brightness ratio allows us to constrain the viewing angle to between ∼54° and 84° and the intrinsic speed to between ∼0.30 c and 0.55 c. We confirm the parabolic shape of the upstream jet on both sides of the central engine, with a power-law index of 0.56 ± 0.07. Notably, the jet collimation is found to be already completed at subparsec scales, with a transition location of about 0.61 pc, which is significantly smaller than the Bondi radius of 99.2 pc. This behavior can be interpreted as the initial confinement of the jet by external pressure from either the geometrically thick, optically thin advection-dominated accretion flows or the disk wind launched from it. Alternatively, the shape transition may also be explained by the internal flow transition from a magnetically dominated to a particle-dominated regime.

Jet power extracted from ADAFs and the application to Fermi BL Lacertae objects

ABSTRACT We calculate the jet power of the Blandford–Znajek (BZ) model and the hybrid model based on the self-similar solution of advection-dominated accretion flows (ADAFs). We study the formation mechanism of the jets of BL Lacertae (BL Lacs) with known redshifts detected by the Fermi satellite after 10 yr of data (4FGL-DR2). The kinetic power of the jets of Fermi BL Lacs is estimated through radio luminosity. The main results are as follows. (1) We find that the jet kinetic power of about 72 per cent intermediate peak frequency BL Lacs (IBL) and 94 per cent high-frequency peak BL Lacs (HBL) can be explained by the hybrid jet model based on ADAFs surrounding Kerr black holes. However, the jet kinetic power of about 74 per cent low-frequency peak BL Lacs (LBL) cannot be explained by the BZ jet model or the hybrid model. (2) The LBL has a higher accretion rate than IBL and HBL. About 14 per cent IBL and 62 per cent HBL have pure optically thin ADAFs. However, 7 per cent LBL may have a hybrid structure consisting of an standard thin disc (SS) plus optically thin ADAFs. (3) After excluding the redshift dependence, there is a weak correlation between the jet kinetic power and the accretion disc luminosity for Fermi BL Lacs. (4) There is a significant correlation between inverse-Compton luminosity and synchrotron luminosity for Fermi BL Lacs. The slope of the relation between inverse-Compton luminosity and synchrotron luminosity for Fermi BL Lacs is consistent with the synchrotron self-Compton (SSC) process. The result may suggest that the high-energy components of Fermi BL Lacs are dominated by the SSC process.

Prospects for Ray-tracing Light Intensity and Polarization in Models of Accreting Compact Objects Using a GPU

The Event Horizon Telescope (EHT) has recently released high-resolution images of accretion flows onto two supermassive black holes. Our physical understanding of these images depends on the accuracy and precision of numerical models of plasma and radiation around compact objects. The goal of this work is to speed up radiative-transfer simulations used to create mock images of black holes for comparison with the EHT observations. A ray-tracing code for general relativistic and fully polarized radiative transfer through plasma in strong gravity is ported onto a graphics processing unit (GPU). We describe our GPU implementation and carry out speedup tests using models of optically thin advection-dominated accretion flow onto a black hole realized semianalytically and in 3D general relativistic magnetohydrodynamic simulations, low and very high image pixel resolutions, and two different sets of CPU+GPUs. We show that a GPU with high double precision computing capability can significantly reduce the image production computational time, with a speedup factor of up to approximately 1200. The significant speedup facilitates, e.g., dynamic model fitting to the EHT data, including polarimetric data. The method extension may enable studies of emission from plasma with nonthermal particle distribution functions for which accurate approximate synchrotron emissivities are not available. The significant speedup reduces the carbon footprint of the generation of the EHT image libraries by at least an order of magnitude.

Heating or Cooling: Study of Advective Heat Transport in the Inflow and the Outflow of Optically Thin Advection-dominated Accretion Flows

Advection is believed to be the dominant cooling mechanism in optically thin advection-dominated accretion flows (ADAFs). When outflow is considered, however, the first impression is that advection should be of opposite sign in the inflow and the outflow, due to the opposite direction of radial motion. Then how is the energy balance achieved simultaneously? We investigate the problem in this paper, analyzing the profiles of different components of advection with self-similar solutions of ADAFs in spherical coordinates (r θ ϕ). We find that for n < 3γ/2 − 1, where n is the density index in ρ ∝ r −n and γ is the heat capacity ratio, the radial advection is a heating mechanism in the inflow and a cooling mechanism in the outflow. It becomes 0 for n = 3γ/2 − 1, and turns to a cooling mechanism in the inflow and a heating mechanism in the outflow for n > 3γ/2 − 1. The energy conservation is only achieved when the latitudinal (θ direction) advection is considered, which takes an appropriate value to maintain energy balance, so that the overall effect of advection, no matter the parameter choices, is always a cooling mechanism that cancels out the viscous heating everywhere. For the extreme case of n = 3/2, latitudinal motion stops, viscous heating is balanced solely by radial advection, and no outflow is developed.

VERTICAL STRUCTURE OF ADVECTION-DOMINATED ACCRETION FLOWS

We solve the set of hydrodynamic (HD) equations for optically thin Advection Dominated Accretion Flows (ADAFs) by assuming radially self-similar in spherical coordinate system $ (r, \theta, \phi) $. The disk is considered to be steady state and axi-symmetric. We define the boundary conditions at the pole and the equator of the disk and to avoid singularity at the rotation axis, the disk is taken to be symmetric with respect to this axis. Moreover, only the $ \tau_{r \phi} $ component of viscous stress tensor is assumed and we have set $ v_{\theta} = 0 $. The main purpose of this study is to investigate the variation of dynamical quantities of the flow in the vertical direction by finding an analytical solution. As a consequence, we found that the advection parameter, $ f^{adv} $, varies along the $ \theta $ direction and reaches to its maximum near the rotation axis. Our results also show that, in terms of no-outflow solution, thermal equilibrium still exists and consequently advection cooling can balance viscous heating.

GENERAL RELATIVISTIC MAGNETOHYDRODYNAMIC SIMULATIONS OF THE HARD STATE AS A MAGNETICALLY DOMINATED ACCRETION FLOW

(Abridged) We present one of the first physically-motivated two-dimensional general relativistic magnetohydrodynamic (GRMHD) numerical simulations of a radiatively-cooled black-hole accretion disk. The fiducial simulation combines a total-energy-conserving formulation with a radiative cooling function, which includes bremsstrahlung, synchrotron, and Compton effects. By comparison with other simulations we show that in optically thin advection-dominated accretion flows, radiative cooling can significantly affect the structure, without necessarily leading to an optically thick, geometrically thin accretion disk. We further compare the results of our radiatively-cooled simulation to the predictions of a previously developed analytic model for such flows. For the very low stress parameter and accretion rate found in our simulated disk, we closely match a state called the "transition" solution between an outer advection-dominated accretion flow and what would be a magnetically-dominated accretion flow (MDAF) in the interior. The qualitative and quantitative agreement between the numerical and analytic models is quite good, with only a few well-understood exceptions. According to the analytic model then, at significantly higher stress or accretion, we would expect a full MDAF to form. The collection of simulations in this work also provide important data for interpreting other numerical results in the literature, as they span the most common treatments of thermodynamics, including simulations evolving: 1) the internal energy only; 2) the internal energy plus an explicit cooling function; 3) the total energy without cooling; and 4) total energy including cooling. We find that the total energy formulation is a necessary prerequisite for proper treatment of radiative cooling in MRI accretion flows.

The cyclic variation of the outburst recurrence time in GRS 1747–312

We present an analysis of the long-term evolution of outbursts in the neutron star soft X-ray transient GRS 1747–312. Observations taken from ASM/ RXTE, in the 1.5–12 keV passband, are utilized. We reveal a cyclic behavior in the residuals of the outburst recurrence time with respect to the mean value of T C = 136 ± 2 days. The profile of this cycle is approximately sinusoidal; the remaining cycle-to-cycle fluctuations possess a considerably smaller amplitude. We find that, although the peak flux of the outbursts displays a significant scatter at a given phase of the cycle, the most luminous outbursts occur after the longest T C. The fluence displays a large scatter for the individual outbursts and tends to decrease with time. We argue that although the cycle-length of ∼5.4 yr is compatible with that of the presumed magnetic activity of the late-type donor, it cannot be explained by variations of the mass outflow from the donor to the disk. In our interpretation, the stellar activity is translated to variations of T C via interaction of the magnetic field of the spots on the donor with the magnetic field of the disk. This gives rise to a variable efficiency of the removal of the angular momentum from the quiescent disk during the activity cycle of the donor. This mechanism can be strengthened by accompanying variations of the radius of the optically thin advection-dominated accretion flow in quiescence. We show that the peak mass accretion rate onto the neutron star in the individual outbursts of GRS 1747–312 is considerably more stable than in two other similar systems with frequent outbursts, Aql X-1 and 4U 1608–52; this allows the cyclic modulation of T C to show itself in GRS 1747–312.

New Analytical Formulae for Optically Thin Accretion Flows

Abstract In a previous paper, we described new analytic formulae for optically thick supercritical accretion flows (Watarai 2006, hereafter paper 1). Here, we present analytic formulae for optically thin one-temperature accretion flows including the advection-dominated regime, using the “semi-iterative” method, described in paper 1. Our analytic formulae have two real solutions. The first solution corresponds to advection-dominated accretion flow (ADAF), and the second solution corresponds to radiation-dominated accretion flow, described by Shapiro, Lightman, and Eardley (the so-called SLE model). Both solutions are given by a cubic equation for the advection parameter, $f$, which is the ratio of the advection cooling rate, $Q_{\rm adv}$, to the viscous heating rate, $Q_{\rm vis}$, i.e., $f=Q_{\rm adv}/Q_{\rm vis}$. Most previous studies assumed that $f$ is constant ($f \sim 1$ for the ADAF). However, it is clear that $f$ should be a function of the physical parameters of the radiative-cooling dominated regime. We found that the ratio $f$ can be written as a function of the radius, mass-accretion rate, and viscous parameter, $\alpha$. Using this formula, we can estimate the transition radius from the inner optically thin ADAF to the outer optically thick standard disk, which can be measured using observations of the quiescent state in black hole X-ray binaries.

Blob Ejection from Advection-dominated Accretion Flow: Observational Consequences

There is increasing evidence for the presence of an optically thin advection-dominated accretion flow (ADAF) in low luminosity active galactic nuclei and radio-loud quasars. The present paper is devoted to explore the fate of a blob ejected from an ADAF, and to discuss its observational consequences. It is inevitable for the ejected blob to drastically expand into its surroundings. Consequently, it is expected that a group of relativistic electrons should be accelerated, which may lead to nonthermal flares, since a strong shock will be formed by the interaction between the blob and its surroundings. Then the blob cools down efficiently, leading to the appearance of recombination lines about $10^5$s after its ejection from an ADAF. We apply this model to NGC 4258 for some observational prediction, and to PKS 2149--306 for the explanation of observational evidence. Future simultaneous observations of recombination X-ray lines and continuum emission are highly desired to test the present model.

Dynamics of the Transition from a Thin Accretion Disk to an Advection‐dominated Accretion Flow

We consider an optically thin advection-dominated accretion flow (ADAF) that is connected at a finite transition radius to an outer optically thick, geometrically thin disk. We include turbulent energy transport and examine ADAF models that satisfy the following boundary conditions at the transition radius: (1) the temperature of the gas is much lower than the virial temperature, (2) the rotation is super-Keplerian, and (3) the net radial flux of energy is outward. We numerically solve the height-integrated viscous hydrodynamic equations with these boundary conditions. We find that the Bernoulli parameter is positive for a wide range of radius, indicating that outflows may be possible from ADAFs. Turbulent energy transport enhances the Bernoulli parameter. We compare our numerical global solutions with two published analytical solutions. We find that the solution of Honma represents the transition region well, while the self-similar solution of Narayan & Yi works better away from the transition. However, neither analytical solution is able to represent the density or angular momentum profile in the inner region of the ADAF, where the flow makes a sonic transition.

A Cosmic Battery

We show that the Poynting-Robertson drag effect in an optically thin advection-dominated accretion flow around active gravitating objects generates strong azimuthal electric currents that give rise to astrophysically significant magnetic fields. Although the mechanism is most effective in accreting compact objects, it also seems very promising as a way to account for the generation of stellar dipolar fields during the late protostellar collapse phase, when the star approaches the main sequence.

Advection‐dominated Accretion Model of Sagittarius A*: Evidence for a Black Hole at the Galactic Center

Sagittarius A*, which is located at the Galactic center, is a puzzling source. It has a mass of M = (2.5 ± 0.4) × 106 M☉, which makes it an excellent black hole candidate. Observations of stellar winds and other gas flows in its vicinity suggest a mass accretion rate of ≳ few × 10−6 M☉ yr-1. However, such an accretion rate would imply a luminosity greater than 1040 ergs-1 if the radiative efficiency is the usual 10%, whereas observations indicate a bolometric luminosity less than 1037 ergs-1. The spectrum of Sgr A* is unusual, with emission extending over many decades of wavelength. We present a model of Sgr A* that is based on a two-temperature optically thin advection-dominated accretion flow. The model is consistent with the estimated M and and fits the observed fluxes in the centimeter/millimeter and X-ray bands, as well as upper limits in the submillimeter and infrared bands; the fit is less good in the radio spectrum below 86 GHz and in γ-rays above 100 MeV. The very low luminosity of Sgr A* is explained naturally in the model by means of advection. Most of the viscously dissipated energy is advected into the central mass by the accreting gas, and therefore the radiative efficiency is extremely low, ~5 × 10-6. A critical element of the model is the presence of an event horizon at the center that swallows the advected energy. The success of the model could thus be viewed as confirmation that Sgr A* is a black hole.

Spectrum of Optically Thin Advection‐dominated Accretion Flow around a Black Hole: Application to Sagittarius A*

The global structure of optically thin advection-dominated accretion flows composed of two-temperature plasma around black holes is calculated. We adopt the full set of basic equations, including the advective energy transport in the energy equation for the electrons. The spectra emitted by the optically thin accretion flows are also investigated. The radiation mechanisms that are taken into account are bremsstrahlung, synchrotron emission, and Comptonization. The calculation of the spectra and that of the structure of the accretion flows are made to be completely consistent by calculating the radiative cooling rate at each radius. As a result of the advection domination for the ions, the heat transport from the ions to the electrons becomes practically zero, and the radiative cooling balances with the advective heating in the energy equation of the electrons. Following up on the successful work of Narayan et al., we applied our model to the spectrum of Sgr A*. We find that the spectrum of Sgr A* is explained by the optically thin advection-dominated accretion flow around a black hole of mass MBH = 106 M☉. The parameter dependence of the spectrum and the structure of the accretion flows is also discussed.

Advection‐dominated Accretion: Global Transonic Solutions

We obtained global transonic solutions representing optically thin advection-dominated accretion flow by solving the full set of differential equations describing such systems. We found that far from the sonic point self-similar solutions are an excellent approximation of the global flow structure if the accretion rate is well below the maximum value above which no optically thin solutions exist.