Weighing black holes in the universe

The determination of the mass of black holes in our universe is crucial to understand their physics nature but is a great challenge to scientists. In this paper I briefly review some methods that are currently used to estimate the mass of black holes, especially those in X-ray binary systems and in galactic nuclei. Our recent progress in improving the mass estimates of supermassive black holes in active galactic nuclei by involving some empirical relations is presented. Finally I point out the similarities and common physics in Galactic black hole X-ray binaries and active galactic nuclei, and demonstrate that the black hole mass estimation is very much helpful to understand the accretion physics around black holes.     

Tracing activity in distant supermassive black holes with X-rays

Supermassive black holes at the centres of galaxies have long been thought to be the engines of quasars, which emit more energy than any other sources in the Universe. In the local Universe, dormant supermassive black holes have been detected through the motions of stars and gas near the galactic centres. In the distant Universe, high energy X-ray observations are now revealing the accretion of matter onto supermassive black holes, even when the black holes are highly obscured by gas and dust. Great advances are being made in obtaining a cosmic census of supermassive black holes. The duration, times, and mass inflow rates to these black holes are being traced via multiwavelength follow-up observations with ground-based telescopes and a time history of the accretion is thereby being reconstructed.     

Extreme astrophysical sources

Extreme astrophysical sources, those where the largest energy transfers occur, give all way to gravity's fatal attraction to sustain their unrivalled power. With a special emphasis on space observations, we present an inventory of extreme astrophysical sources which follows the filiation supernovae, neutron stars and black holes, before ending with supermassive black holes. This ill-assorted population of cosmic sites has some traits in common, the black-hole relativistic-jet connection being commonplace, from solar-mass specimens up to supermassive ones.     

Astrophysical Black Holes as Natural Laboratories for Fundamental Physics and Strong-Field Gravity

Astrophysical tests of general relativity belong to two categories: 1) “internal”, i.e. consistency tests within the theory (for example, tests that astrophysical black holes are indeed described by the Kerr solution and its perturbations), or 2) “external”, i.e. tests of the many proposed extensions of the theory. I review some ways in which astrophysical black holes can be used as natural laboratories for both “internal” and “external” tests of general relativity. The examples provided here (ringdown tests of the black hole “no-hair” theorem, bosonic superradiant instabilities in rotating black holes and gravitational-wave tests of massive scalar-tensor theories) are shamelessly biased towards recent research by myself and my collaborators. Hopefully this colloquial introduction aimed mainly at astrophysicists will convince skeptics (if there are any) that space-based detectors will be crucial to study fundamental physics through gravitational-wave observations.     

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.     

Highlights in black hole astrophysics

I review the theoretical and observational data supporting the existence of black holes at various mass scales and then give a brief account of the recent progress in the understanding of the stellar tidal disruption process, which is expected to play a prominent role in fuelling large black holes in galactic nuclei.     

黑洞照亮宇宙——银河系中心黑洞及其物理意义

2020年度诺贝尔物理学奖颁发给为黑洞和超大质量致密天体做出突出贡献的三位科学家,他们分别从理论和观测上提供了令人信服的证明和证据。他们的工作打开了理解宇宙中大质量天体命运的窗口。人们普遍相信超大质量黑洞存在于每一个星系的中心,是这些黑洞照亮了再电离时期的宇宙,也是它们为揭开宇宙膨胀历史、暗能量宇宙演化性质、纳赫兹低频引力波等诸多谜团提供了十分强大的工具。预计未来5年内,反响映射和GRAVITY/VLTI联合观测将在以黑洞研究为支撑的领域取得重大进展。     

Higher-dimensional black hole solutions: approximate methods

I review progress in the last few years in constructing and analyzing many new classes of black holes that are possible in spacetimes of dimension larger than four.     

Rotating black holes in dilatonic Einstein-Gauss-Bonnet theory

We construct generalizations of the Kerr black holes by including higher-curvature corrections in the form of the Gauss-Bonnet density coupled to the dilaton. We show that the domain of existence of these Einstein-Gauss-Bonnet-dilaton (EGBD) black holes is bounded by the Kerr black holes, the critical EGBD black holes, and the singular extremal EGBD solutions. The angular momentum of the EGBD black holes can exceed the Kerr bound. The EGBD black holes satisfy a generalized Smarr relation. We also compare their innermost stable circular orbits with those of the Kerr black holes and show the existence of differences which might be observable in astrophysical systems.     

Observers in Kerr spacetimes: the ergoregion on the equatorial plane

We perform a detailed analysis of the properties of stationary observers located on the equatorial plane of the ergosphere in a Kerr spacetime, including light-surfaces. This study highlights crucial differences between black hole and the super-spinner sources. In the case of Kerr naked singularities, the results allow us to distinguish between “weak” and “strong ” singularities, corresponding to spin values close to or distant from the limiting case of extreme black holes, respectively. We derive important limiting angular frequencies for naked singularities. We especially study very weak singularities as resulting from the spin variation of black holes. We also explore the main properties of zero angular momentum observers for different classes of black hole and naked singularity spacetimes.     

Tidal Disruption Events

The majority of supermassive black holes in the Universe lie dormant and starved of fuel. These hidden beasts can be temporarily illuminated when an unlucky star passes close enough to be tidally disrupted and consumed by the black hole. Theorists first proposed in 1975 that tidal disruption events should be an inevitable consequence of supermassive black holes in galaxy nuclei and later argued that the resulting flare of radiation from the accretion of the stellar debris could be a unique signpost for the presence of a dormant black hole in the center of a normal galaxy. It was not until over two decades later that the first convincing tidal disruption event candidates emerged in the X-rays by the ROSAT All-Sky Survey. Since then, over a dozen total candidates have now emerged from searches across the electromagnetic spectrum, including the X-rays, the ultraviolet, and the optical. In the last couple of years, we have also witnessed a paradigm shift with the discovery of relativistic beamed emission associated with tidal disruption events. I review the census of observational candidates to date and discuss the exciting prospects for using large samples of tidal disruption events discovered with the next-generation of ground-based and space-based synoptic surveys to probe accretion disk and/or jet formation and black hole demographics.     

Analytical effective one-body formalism for extreme-mass-ratio inspirals with eccentric orbits

Extreme-mass-ratio inspirals(EMRIs) are among the most important sources for future spaceborne gravitational wave detectors. In this kind of system, compact objects usually orbit around central supermassive black holes on complicated trajectories. Usually, these trajectories are approximated as the geodesics of Kerr space-times, and orbital evolution is simulated with the help of the adiabatic approximation. However, this approach omits the influence of the compact object on its background. In this paper, using the effective one-body formalism, we analytically calculate the trajectory of a nonspinning compact object around a massive Kerr black hole in an equatorial eccentric orbit(omitting the orbital inclination) and express the fundamental orbital frequencies in explicit forms. Our formalism includes the first-order corrections for the mass ratio in the conservative orbital motion. Furthermore, we insert the mass-ratio-related terms into the first post-Newtonian energy fluxes. By calculating the gravitational waves using the Teukolsky equations, we quantitatively reveal the influence of the mass of the compact object on the data analysis. We find that the shrinking of geodesic motion by taking small objects as test particles may not be appropriate for the detection of EMRIs.     

Abbott-Deser-Tekin Charge of Dilaton Black Holes with Squashed Horizons

We consider the conserved charge of static black holes with squashed horizons in the Einstein-Maxwell-dilaton theory via both the Abbott-Deser-Tekin(ADT)method and its off-shell generalization.We Grst make use of the original ADT method to compute the mass of the dilaton squashed black holes in terms of three different reference spacetimes,which are the asymptotic geometry,the flat background and the spacetime of the KaluzaKlein monopole with boundary matched to the original metric,respectively.Each mass satisfies the Grst law of black hole thermodynamics,although the mass computed on the basis of the boundary matching the KaluzaKlein monopole is different from that of the other two reference spacetimes.Then the mass of the black holes is evaluated through the off-shell generalized ADT method.     

Black Hole Complementary Principle and Noncommutative Membrane

In the spirit of black hole complementary principle, we have found the noncommutative membrane of Scharzchild black holes. In this paper we extend our results to Kerr black hole and see the same story. Also we make a conjecture that spacetimes are noncommutative on the stretched membrane of the more general Kerr-Newman black hole.     

Black Hole Complementary Principle and Noncomm utative Membrane

In the spirit of black hole complementary principle, we have found the noncommutative membrane of Scharzchild black holes. In this paper we extend our results to Kerr black hole and see the same story. Also we make a conjecture that spacetimes are noncommutative on the stretched membrane of the more general Kerr-Newman black hole.     

Possible effects of a cosmological constant on black hole evolution

We explore possible effects of vacuum energy on the evolution of black holes. If the universe contains a cosmological constant, and if black holes can absorb energy from the vacuum, then black hole evaporation could be greatly suppressed. For the magnitude of the cosmological constant suggested by current observations, black holes larger than 4×10²⁴ g would accrete energy rather than evaporate. In this scenario, all stellar and supermassive black holes would grow with time until they reach a maximum mass scale of 6×10⁵⁵ g, comparable to the mass contained within the present day cosmological horizon.     

Supermassive black holes in local galaxies

Over the past decade we have learned that probably all ellipticals and bulges of galaxies contain central supermassive black holes (SMBH). SMBH masses correlate well with the luminosities, and in turn the stellar masses of the bulges harboring them, with about 0.15% of the bulge mass being found in the SMBH. Pure disk galaxies, on the other hand, do not, in general, seem to contain SMBHs. Here we review the best cases for SMBH detection in galaxies, discuss methods and associated uncertainties, summarize correlations between SMBH masses and host galaxy properties, and finally address possible future developments. To cite this article: R. Bender, R.P. Saglia, C. R. Physique 8 (2007).     

Light-cone distribution amplitudes of the ground state bottom baryons in HQET

We prove the existence of hidden symmetries in the general relativity theory defined by exact solutions with generic off-diagonal metrics, nonholonomic (non-integrable) constraints, and deformations of the frame and linear connection structure. A special role in characterization of such spacetimes is played by the corresponding nonholonomic generalizations of Stackel–Killing and Killing–Yano tensors. There are constructed new classes of black hole solutions and we study hidden symmetries for ellipsoidal and/or solitonic deformations of “prime” Kerr–Sen black holes into “target” off-diagonal metrics. In general, the classical conserved quantities (integrable and not-integrable) do not transfer to the quantized systems and produce quantum gravitational anomalies. We prove that such anomalies can be eliminated via corresponding nonholonomic deformations of fundamental geometric objects (connections and corresponding Riemannian and Ricci tensors) and by frame transforms.     

Physics of accretion flows around compact objects

Several physical and astrophysical problems related to accretion onto black holes and neutron stars are briefly reviewed. I discuss the observed differences between these two types of compact objects in quiescent Soft X-ray Transients. Then I review the status of various non-standard objects suggested as an alternative to black holes. Finally, I present new results and a suggestion about the nature of the jet activity in Active Galactic Nuclei. To cite this article: J.-P. Lasota, C. R. Physique 8 (2007).