Fu-Guo Xie

Radiative efficiency of hot accretion flows

Over the past few years, our understanding of hot accretion flows has been improved significantly by two findings: (i) only a small fraction of the accretion flow available at the outer boundary can finally fall on to the black hole, while most of it is lost in the outflow; (ii) electrons can directly receive a large fraction of the viscously dissipated energy in the accretion flow (i. e. delta similar to 0.1-0.5). The radiative efficiency of the hot accretion flow when these two findings are taken into account has not yet been systematically studied, and this is the subject of our paper. We consider two regimes of the hot accretion model: advection-dominated accretion flows that lie in the regime of the low accretion rate, less than or similar to 10 alpha L-2(Edd)/c(2), and the luminous hot accretion flows (LHAFs) that lie above this accretion rate. For the LHAFs, we assume that the accretion flow has a two-phase structure above a certain accretion rate, and we adopt a simplification in our calculation of the dynamics. Our results indicate that the radiative efficiency of hot accretion flow increases with the accretion rate and that it is greatly enhanced by the direct viscous heating to electrons, compared to the previous case of delta << 1. When the accretion rate is high, the radiative efficiency of the hot accretion flow is comparable to that of a standard thin disc. We present fitting formulae of radiative efficiency as a function of accretion rate for various values of delta.

The Influences of Outflow on the Dynamics of Inflow

Both numerical simulations and observations indicate that in an advection-dominated accretion flow most of the accretion material supplied at the outer boundary will not reach the inner boundary. Rather, it is lost via outflow. Previously, the influence of outflow on the dynamics of inflow had been taken into account only by adopting a radius-dependent mass accretion rate = 0(r/rout)8 with s > 0. In this paper, based on a one-and-a-half-dimensional description of the accretion flow, we investigate this problem in more detail by considering the interchange of mass, radial and azimuthal momentum, and the energy between the outflow and inflow. The physical quantities of the outflow are parameterized based on our current understandings of the properties of outflow mainly from numerical simulations of accretion flows. Our results indicate that under reasonable assumptions about the properties of outflow, the main influence of outflow has been properly included by adopting = 0(r/rout)8.

Correlation between the photon index and X-ray luminosity of black hole X-ray binaries and active galactic nuclei: observations and interpretation

We investigate the observed correlation between the 2-10 keV X-ray luminosity (in unit of the Eddington luminosity; l(X) L-X/L-Edd) and the photon index (Gamma) of the X-ray spectrum for both black hole X-ray binaries (BHBs) and active galactic nuclei (AGNs). We construct a large sample, with 10(-9) less than or similar to l(X) less than or similar to 10(-1). We find that Gamma is positively and negatively correlated with l(X) when l(X) greater than or similar to 10(-3) and 10(-65) less than or similar to l(X) less than or similar to 10(-3), respectively, while Gamma is nearly a constant when l(X) less than or similar to 10(-6.5). We explain the above correlation in the framework of a coupled hot accretion flow-jet model. The radio emission always comes from the jet while the X-ray emission comes from the accretion flow and jet when l(X) is above and below 10(-6.5), respectively. More specifically, we assume that with the increase of mass accretion rate, the hot accretion flow develops into a clumpy and further a disc-corona two-phase structure because of thermal instability. We argue that such kind of two-phase accretion flow can explain the observed positive correlation.

Self‐similar solution of hot accretion flows with ordered magnetic field and outflow

Observations and numerical magnetohydrodynamic (MHD) simulations indicate the existence of outflows and ordered large-scale magnetic fields in the inner region of hot accretion flows. In this paper, we present the self-similar solutions for advection-dominated accretion flows (ADAFs) with outflows and ordered magnetic fields. Stimulated by numerical simulations, we assume that the magnetic field has a strong toroidal component and a vertical component in addition to a stochastic component. We obtain the self-similar solutions to the equations describing the magnetized ADAFs, taking into account the dynamical effects of the outflow. We compare the results with the canonical ADAFs and find that the dynamical properties of ADAFs such as radial velocity, angular velocity and temperature can be significantly changed in the presence of ordered magnetic fields and outflows. The stronger the magnetic field is, the lower the temperature of the accretion flow will be and the faster the flow rotates. The relevance to observations is briefly discussed.

The compact structure of radio-loud broad absorption line quasars

We present the results of EVN+MERLIN very long baseline interferometry (VLBI) polarization observations of eight broad absorption line (BAL) quasars at 1.6 GHz, including four low-ionization BAL quasars (LoBALs) and four high-ionization BAL quasars (HiBALs) with either steep or flat spectra on Very Large Array (VLA) scales. Only one steep-spectrum source, J1122+3124, shows two-sided structure on the scale of 2 kpc. The other four steep-spectrum sources and three flat-spectrum sources display either an unresolved image or a core–jet structure on scales of less than 300 pc. In all cases, the marginally resolved core is the dominant radio component. Linear polarization in the cores has been detected in the range of a few to 10 per cent. Polarization, together with high brightness temperatures (from 2 × 10 9 to 5 × 10 10 K), suggests a synchrotron origin for the radio emission. There is no apparent difference in the radio morphologies or polarization between low-ionization and high-ionization BAL quasi-stellar objects (QSOs) or between flat- and steep-spectrum sources. We discuss the orientation of BAL QSOs with both flat and steep spectra, and consider a possible evolutionary scenario for BAL QSOs. In this scenario, BAL QSOs are probably a young population of radio sources that are compact steep spectrum or GHz peaked radio source analogues at the low end of radio power.

Monte Carlo simulations of global Compton cooling in inner regions of hot accretion flows

Hot accretion flows such as advection-dominated accretion flows are generally optically thin in the radial direction. Thus, photons generated at some radii can cool or heat electrons at other radii via Compton scattering. Such global Compton scattering has previously been shown to be important for the dynamics of accretion flows. Here, we extend previous treatments of this problem by using accurate global general relativistic Monte Carlo simulations. We focus on an inner region of the accretion flow (R < 600R(g)), for which we obtain a global self-consistent solution. As compared to the initial, not self-consistent solution, the final solution has both the cooling rate and the electron temperature significantly reduced at radii of greater than or similar to 10R(g). On the other hand, the radiation spectrum of the self-consistent solution has a shape similar to that of the initial iteration, except for the high-energy cut-off being at an energy lower by a factor of similar to 2 and the bolometric luminosity decreased by a factor of similar to 2. We also compare the global Compton scattering model with local models in spherical and slab geometries. We find that the slab model approximates the global model significantly better than the spherical one. Still, neither local model gives a good approximation to the radial profile of the cooling rate, and the differences can be up to two orders of magnitude. The local slab model underestimates the cooling rate at outer regions whereas it overestimates that rate at inner regions. We compare our modelling results to observed hard-state spectra of black hole binaries and find an overall good agreement provided any disc outflow is weak. We find that general relativistic effects in flows whose dynamics is modified by global Comptonization is crucial in approaching this agreement.

General relativistic model of hot accretion flows with global Compton cooling

We present a model of optically thin, two-temperature accretion flows using an exact Monte Carlo treatment of global Comptonization, with seed photons from synchrotron and bremsstrahlung emission, as well as with a fully general relativistic description of both the radiative and hydrodynamic processes. We consider accretion rates for which the luminosities of the flows are between similar to 10(-3) and 10(-2) of the Eddington luminosity. The black hole spin parameter strongly affects the flow structure within the innermost similar or equal to 10 gravitational radii. The resulting large difference between the Coulomb heating in models with a non-rotating and a rapidly rotating black hole is, however, outweighed by a strong contribution of compression work, much less dependent on spin. The consequent reduction of effects related to the value of the black spin is more significant at smaller accretion rates. For a non-rotating black hole, the compressive heating of electrons dominates over their Coulomb heating, and results in an approximately constant radiative efficiency of approximate to 0.4 per cent in the considered range of luminosities. For a rapidly rotating black hole, the Coulomb heating dominates, the radiative efficiency is similar or equal to 1 per cent and it slightly increases (but less significantly than estimated in some previous works) with increasing accretion rate. Our study neglects the direct heating of electrons, which effect can lead to larger differences between the radiative properties of models with a non-rotating and a rapidly rotating black hole than estimated here. Flows with the considered parameters produce rather hard spectra, with the photon spectral index Gamma similar to 1.6, and with high-energy cut-offs at several hundred keV. We find an agreement between our model, in which the synchrotron emission is the main source of seed photons, and observations of black hole binaries in their hard states and active galactic nuclei (AGNs) at low luminosities. In particular, our model predicts a hardening of the X-ray spectrum with increasing luminosity, as indeed observed below similar to 0.01L(E) or so in both black hole binaries and AGNs. Also, our model approximately reproduces the luminosity and the slope of the X-ray emission in Cen A.

Jet-dominated quiescent states in black hole X-ray binaries: the case of V404 Cyg

The dynamical structure and radiative properties of the quiescent state (X-ray luminosity less than or similar to 10(34) erg s(-1)) of black hole X-ray transients (BHXTs) remain unclear, mainly because of low luminosity and poor data quantity. We demonstrate that the simultaneous multi-wavelength (including radio, optical, ultraviolet and X-ray bands) spectrum of V404 Cyg in its bright quiescent state can be well described by the radiation from the companion star and more importantly, the compact jet. Neither the outer thin disc nor the inner hot accretion flow is important in the total spectrum. Together with recent findings, i.e. the power-law X-ray spectrum and the non-variable X-ray spectral shape (or constant photon index) in contrast to the dramatic change in the X-ray luminosity, we argue the quiescent state spectrum of BHXTs is actually jet-dominated. Additional observational properties consistent with this jet model are also discussed as supporting evidence.

Gamma-ray emission from proton-proton interactions in hot accretion flows

We present a model of gamma-ray emission through neutral pion production and decay in two-temperature accretion flows around supermassive black holes. We refine previous studies of such a hadronic gamma-ray emission by taking into account (1) relativistic effects in the photon transfer and (2) absorption of gamma-ray photons in the radiation field of the flow. We use a fully general relativistic description of both the radiative and hydrodynamic processes, which allows us to study the dependence on the black hole spin. The spin value strongly affects the gamma-ray emissivity within similar to 10 gravitational radii. The central regions of flows with the total luminosities L less than or similar to 10(- 3) of the Eddington luminosity (L-Edd) are mostly transparent to photons with energies below 10 GeV, permitting investigation of the effects of space-time metric. For such L, an observational upper limit on the gamma-ray (0.1-10 GeV) to X-ray (2-10 keV) luminosity ratio of L0.1-10 GeV/L2-10 keV < 0.1 can rule out rapid rotation of the black hole; on the other hand, a measurement of L0.1-10 GeV/L2-10 keV similar to 0.1 cannot be regarded as the evidence of rapid rotation, as such a ratio can also result from a flat radial profile of gamma-ray emissivity (which would occur for non-thermal acceleration of protons in the whole body of the flow). At L greater than or similar to 10(- 2)L(Edd), the gamma-ray emission from the innermost region is strongly absorbed and the observed gamma-rays do not carry information on the value of a. We note that if the X-ray emission observed in Centaurus A comes from an accretion flow, the hadronic gamma-ray emission from the flow should contribute significantly to the MeV/GeV emission observed from the core of this object, unless it contains a slowly rotating black hole and protons in the flow are thermal.

X-ray spectra of hot accretion flows

We study radiative properties of hot accretion flows in a general relativistic model with an exact treatment of global Comptonization, developed in our recent works. We note a strong dependence of electron temperature on the strength of magnetic field and we clarify that the underlying mechanism involves the change of the flow structure, with more strongly magnetized flows approaching the slab geometry more closely. We find that the model with thermal synchrotron radiation being the main source of seed photons agrees with the spectral index versus Eddington ratio relation observed in black hole transients below 1 per cent of the Eddington luminosity, L-Edd, and models with a weak direct heating of electrons (small delta) are more consistent with observations. Models with large delta predict slightly too soft spectra, furthermore, they strongly overpredict electron temperatures at similar to 0.01 L-Edd. The low-luminosity spectra, at less than or similar to 0.001 L-Edd, deviate from a power-law shape in the soft X-ray range, and we note that the first-scattering bump often resembles a thermal like component, with the temperature of a few hundred eV, superimposed on a power-law spectrum. The model with thermal Comptonization of thermal synchrotron radiation does not agree with well-studied AGNs observed below similar to 0.01 L-Edd, for which there is a substantial evidence for the lack of an inner cold disc. This indicates that the model of hot flows powering AGNs should be revised, possibly by taking into account an additional (but internal to the flow) source of seed photons.