银河系中心超大质量黑洞

通过对位于银河系中心的非热、致密射电源人马座A*（Sagittarius A*）的高分辨率甚长基线干涉(VLBI)观测,文章作者及其合作者成功地得到人马座A*的固有辐射区域的直径仅为1个天文单位,支持其是超大质量黑洞的物理解释.文章在较详细地介绍此研究的同时,也简要提及了从黑洞概念的最早提出至今的200多年里人们在黑洞物理认知上的一些重大进展.可以预期,未来亚毫米波VLBI观测将有望揭示银河系中心超大质量黑洞的阴影结构.     

Evolution of the dark matter distribution at the galactic center

Annihilation radiation from neutralino dark matter at the Galactic center (GC) would be greatly enhanced if the dark matter were strongly clustered around the supermassive black hole (SBH). The existence of a dark matter "spike" is made plausible by the observed, steeply rising stellar density near the GC SBH. Here the time-dependent equations describing gravitational interaction of the dark matter with the stars are solved. Scattering of dark matter particles by stars would substantially lower the dark matter density near the GC SBH over 10 Gyr, due both to kinetic heating and to capture of dark matter particles by the SBH. This evolution implies a decrease by several orders of magnitude in the observable flux of annihilation products compared with models that associate a steep, dark matter spike with the SBH.     

Bardeen black hole surrounded by perfect fluid dark matter

We derive an exact solution for a spherically symmetric Bardeen black hole surrounded by perfect fluid dark matter (PFDM). By treating the magnetic charge g and dark matter parameter \begin{document}$\alpha$\end{document} as thermodynamic variables, we find that the first law of thermodynamics and the corresponding Smarr formula are satisfied. The thermodynamic stability of the black hole is also studied. The results show that there exists a critical radius \begin{document}$r_{+}^{C}$\end{document} where the heat capacity diverges, suggesting that the black hole is thermodynamically stable in the range \begin{document}$0<r_{+}<r_{+}^{C}$\end{document} . In addition, the critical radius \begin{document}$r_{+}^{C}$\end{document} increases with the magnetic charge g and decreases with the dark matter parameter \begin{document}$\alpha$\end{document} . Applying the Newman-Janis algorithm, we generalize the spherically symmetric solution to the corresponding rotating black hole. With the metric at hand, the horizons and ergospheres are studied. It turns out that for a fixed dark matter parameter \begin{document}$\alpha$\end{document} , in a certain range, with the increase of the rotation parameter a and magnetic charge g, the Cauchy horizon radius increases while the event horizon radius decreases. Finally, we investigate the energy extraction by the Penrose process in a rotating Bardeen black hole surrounded by PFDM.     

Gravitational collapse of dark energy field configurations and supermassive black hole formation

Dark energy is the dominant component of the total energy density of our Universe. The primary interaction of dark energy with the rest of the Universe is gravitational. It is therefore important to understand the gravitational dynamics of dark energy. Since dark energy is a low-energy phenomenon from the perspective of particle physics and field theory, a fundamental approach based on fields in curved space should be sufficient to understand the current dynamics of dark energy. Here, we take a field theory approach to dark energy. We discuss the evolution equations for a generic dark energy field in curved space-time and then discuss the gravitational collapse for dark energy field configurations. We describe the 3 + 1 BSSN formalism to study the gravitational collapse of fields for any general potential for the fields and apply this formalism to models of dark energy motivated by particle physics considerations. We solve the resulting equations for the time evolution of field configurations and the dynamics of space-time. Our results show that gravitational collapse of dark energy field configurations occurs and must be considered in any complete picture of our Universe. We also demonstrate the black hole formation as a result of the gravitational collapse of the dark energy field configurations. The black holes produced by the collapse of dark energy fields are in the supermassive black hole category with the masses of these black holes being comparable to the masses of black holes at the centers of galaxies.     

银河系中心超大质量黑洞的探索历程

经过逾半个世纪的探索,天文学家确认在我们银河系的中心存在一个4百万倍太阳质量的致密天体,很可能是爱因斯坦广义相对论所预言的黑洞。文章简要回顾了探索这个大质量致密天体过程中的若干里程碑。     

HESS observations of the galactic center region and their possible dark matter interpretation

The detection of gamma rays from the source HESS J1745-290 in the Galactic Center (GC) region with the High Energy Spectroscopic System (HESS) array of Cherenkov telescopes in 2004 is presented. After subtraction of the diffuse gamma-ray emission from the GC ridge, the source is compatible with a point source with spatial extent less than 1.2;{'}(stat) (95% C.L.). The measured energy spectrum above 160 GeV is compatible with a power law with photon index of 2.25+/-0.04(stat)+/-0.10(syst) and no significant flux variation is detected. It is finally found that the bulk of the very high energy emission must have non-dark-matter origin.     

Spin and mass of the nearest supermassive black hole

Quasi-periodic oscillations (QPOs) of the hot plasma spots or clumps orbiting an accreting black hole contain information on the black hole mass and spin. The promising observational signatures for the measurement of black hole mass and spin are the latitudinal oscillation frequency of the bright spots in the accretion flow and the frequency of black hole event horizon rotation. Both of these frequencies are independent of the accretion model and defined completely by the properties of the black hole gravitational field. Interpretation of the known QPO data by dint of a signal modulation from the hot spots in the accreting plasma reveals the Kerr metric rotation parameter, \(a=0.65\pm 0.05\) , and mass, \(M=(4.2\pm 0.2)10^6M_\odot \) , of the supermassive black hole in the Galactic center. At the same time, the observed 11.5 min QPO period is identified with a period of the black hole event horizon rotation, and, respectively, the 19 min period is identified with a latitudinal oscillation period of hot spots in the accretion flow. The described approach is applicable to black holes with a low accretion rate, when accreting plasma is transparent up to the event horizon region.     

The eccentricity enhancement effect of intermediate-mass-ratio-inspirals: dark matter and black hole mass

It was found that dark matter(DM) in an intermediate-mass-ratio-inspiral(IMRI) system has a significant enhancement effect on the orbital eccentricity of a stellar massive compact object, such as a black hole(BH),which may be tested by space-based gravitational wave(GW) detectors, including LISA, Taiji, and Tianqin in future observations. In this paper, we study the enhancement effect of the eccentricity for an IMRI under different DM density profiles and center BH masses. Our results are as follows:(1) in terms of the general DM spike distribution,the enhancement of the eccentricity is basically consistent with the power-law profile, which indicates that it is reasonable to adopt the power-law profile;(2) in the presence of a DM spike, the different masses of the center BH will affect the eccentricity, which provides a new way for us to detect the BH’s mass;and(3) considering the change in the eccentricity in the presence and absence of a DM spike, we find that it is possible to distinguish DM models by measuring the eccentricity at a scale of approximately 105 GM/c2.     

Microlensing of Kepler stars as a method of detecting primordial black hole dark matter

If the dark matter consists of primordial black holes (PBHs), we show that gravitational lensing of stars being monitored by NASA's Kepler search for extrasolar planets can cause significant numbers of detectable microlensing events. A search through the roughly 150,000 light curves would result in large numbers of detectable events for PBHs in the mass range 5×10(-10) M(⊙) to 10(-4) M(⊙). Nondetection of these events would close almost 2 orders of magnitude of the mass window for PBH dark matter. The microlensing rate is higher than previously noticed due to a combination of the exceptional photometric precision of the Kepler mission and the increase in cross section due to the large angular sizes of the relatively nearby Kepler field stars. We also present a new formalism for calculating optical depth and microlensing rates in the presence of large finite-source effects.     

Constraints on parameters of dark matter and black hole in the Galactic Center

A concept of dark matter (DM) is introduced. As for other anomalies, we describe two ways to solve DM problem, namely a conservative way when we have to find substances with DM properties or we have to change a fundamental gravity law. We discuss constraints on DMconcentration near the Galactic Center from apocenter shift data.     

Redshift of a compact binary star in the neighborhood of a supermassive black hole

We present a method that allows to calculate the redshift of a compact binary star which moves in a strong external gravitational field, e.g. the field of a supermassive black hole. If the binary is close enough to the field center, its motion can be relativistic, while the relative motion of the stars in the binary is still non-relativistic. The latter can be computed in a co-moving reference frame. We calculate the redshift in a co-moving frame in Fermi coordinates as a series in the size of the binary \(\varrho \). The obtained expression is written in a covariant form up to linear terms in \(\varrho \). Using the equations of motion that have been obtained in a previous paper, we present the method of calculation of the redshift of a sample binary star moving in the field of a black hole. We illustrate this method by numerical model for the case of spherically-symmetric (Schwarzschild) black hole. The method can be applied to the analysis of the timing data of pulsars in the vicinity of the Galactic Center, and to the reconstruction of the binary star motion by its redshift.     

Cosmological evolution of supermassive black holes in galactic centers unveiled by hard X-ray observations

We review the current understanding of the cosmological evolution of supermassive black holes in galactic centers elucidated by X-ray surveys of active galactic nuclei (AGNs). Hard X-ray observations at energies above 2 keV are the most efficient and complete tools to find “obscured” AGNs, which are dominant populations among all AGNs. Combinations of surveys with various flux limits and survey area have enabled us to determine the space number density and obscuration properties of AGNs as a function of luminosity and redshift. The results have essentially solved the origin of the X-ray background in the energy band below ∼10 keV. The downsizing (or anti-hierarchical) evolution that more luminous AGNs have the space-density peak at higher redshifts has been discovered, challenging theories of galaxy and black hole formation. Finally, we summarize unresolved issues on AGN evolution and prospects for future X-ray missions.     

Absorption of massive scalar field by a charged black hole

Absorption and reflection of charged, massive scalar field by the Reisner-Nordstrom black hole are investigated through a numerical computation. The absorption is suppressed when (Schwarzschild radius) < (Compton wave length) and the amplification of the wave occurs when the level crossing condition is satisfied.     

Thermodynamic effects of Bardeen black hole surrounded by perfect fluid dark matter under general uncertainty principle

The thermodynamics of Bardeen black hole surrounded by perfect fluid dark matter is investigated. We calculate the analytical expresses of corresponding thermodynamic variables, e.g., the Hawking temperature, entropy of the black hole. In addition, we derive the heat capacity to analyze the thermal stability of the black hole. We also compute the rate of emission in terms of photons through tunneling. By numerical method, an obvious phase transition behavior is found. Furthermore, according to the general uncertainty principle, we study the quantum corrections to these thermodynamic quantities and obtain the quantum-corrected entropy containing the logarithmic term. Lastly, we investigate the effects of the magnetic charge g, the dark matter parameter k and the generalized uncertainty principle parameter α on the thermodynamics of Bardeen black hole surrounded by perfect fluid dark matter under general uncertainty principle.     

Dark matter production at the LHC from black hole remnants

We study dark matter production at CERN LHC from black hole remnants (BHR). We find that the typical mass of these BHR at the LHC is ∼5–10 TeV which is heavier than other dark matter candidates, such as axion, axino, neutralino, etc. We propose the detection of this dark matter via single jet production in the process pp → jet + BHR (dark matter) at CERN LHC. We find that for zero impact parameter partonic collisions, the monojet cross section is not negligible in comparison to the standard model background and is much higher than the other dark matter scenarios studied so far. We also find that dσ/dp _T of jet production in this process increases as p _T increases, whereas in all other dark matter scenarios the dσ/dp _T decreases at CERN LHC. This may provide a useful signature for dark matter detection at the LHC. However, we find that when the impact parameter dependent effect of inelasticity is included, the monojet cross section from the above process becomes much smaller than the standard model background and may not be detectable at the LHC.     

Spin and mass of the supermassive black hole in the Galactic Center

A new method for exact determination of the masses and spins of black holes from the observations of quasi-periodic oscillations is discussed. The detected signal from the hot clumps in the accretion plasma must contain modulations with two characteristic frequencies: the frequency of rotation of the black hole event horizon and the frequency of the latitudinal precession of the clump’s orbit. Application of the method of two characteristic frequencies for interpretation of the observed quasi-periodic oscillations from the supermassive black hole in the Galactic center in the X-rays and in the near IR region yields the most exact, for the present, values of the mass and the spin (Kerr parameter) of the Sgr A* black hole: M = (4.2 ± 0.2) × 10⁶ M _⊙ and a = 0.65 ± 0.05. The observed quasi-periodic oscillations with a period of about 11.5 min are identified as the black hole event horizon rotation period and those with a period of about 19 min are identified as the latitudinal oscillation period of the hot spot orbits in the accretion disk.     

The accretion of dark energy onto a black hole

The stationary, spherically symmetric accretion of dark energy onto a Schwarzschild black hole is considered in terms of relativistic hydrodynamics. The approximation of an ideal fluid is used to model the dark energy. General expressions are derived for the accretion rate of an ideal fluid with an arbitrary equation of state p = p(ρ) onto a black hole. The black hole mass was found to decrease for the accretion of phantom energy. The accretion process is studied in detail for two dark energy models that admit an analytical solution: a model with a linear equation of state, p = α(ρ ? ρ₀), and a Chaplygin gas. For one of the special cases of a linear equation of state, an analytical expression is derived for the accretion rate of dark energy onto a moving and rotating black hole. The masses of all black holes are shown to approach zero in cosmological models with phantom energy in which the Big Rip scenario is realized.     

Joule–Thomson expansion of RN-AdS black hole immersed in perfect fluid dark matter

In this paper, we study the Joule–Thomson expansion for RN-Ad S black holes immersed in perfect fluid dark matter. As perfect fluid dark matter is one of the dark matter candidates, we are interested in how it influences the thermodynamic properties of black holes. Firstly, the negative cosmological constant could be interpreted as thermodynamic pressure and its conjugate quantity as the thermodynamic volume, which give us more physical insights into the black hole. Moreover, we derive the thermodynamic definitions and study the critical behaviour of the black hole. Secondly,the explicit expression of Joule–Thomson coefficient is obtained from the basic formulas of the pressure, the volume, the entropy and the temperature. Then, we obtain the inversion curves in terms of charge Q and parameter λ. Furthermore, we analyse the isenthalpic curve in T–P graph with the cooling–heating region determined by the inversion curve. At last, we derive the ratio of minimum inversion temperature to critical temperature and compare the result with that in the RN-Ad S case.     

Absorption and defocusing of electromagnetic radiation by a Schwarzschild black hole

A comparison of electromagnetic radiation absorbed by a black hole and that emitted at infinity is made for selected circular orbits of a charged particle in a Schwarzschild geometry. Radiation intensity and beaming processes for geodesic as well as ultra-relativistic unbound orbits are compared and contrasted with the special relativistic results of classical electrodynamics.