Accreting Black Holes

This chapter provides a general overview of the theory and observations of black holes in the Universe and on their interpretation. We briefly review the black hole classes, accretion disk models, spectral state classification, the AGN classification, and the leading techniques for measuring black hole spins. We also introduce quasi-periodic oscillations, the shadow of black holes, and the observations and the theoretical models of jets.Comment: 41 pages, 18 figures. To appear in "Tutorial Guide to X-ray and Gamma-ray Astronomy: Data Reduction and Analysis" (Ed. C. Bambi, Springer Singapore, 2020). v3: fixed some typos and updated some parts. arXiv admin note: substantial text overlap with arXiv:1711.1025

The Blandford-Znajek mechanism and emission from isolated accreting black holes

In the presence of a magnetic field, rotational energy can be extracted from black holes via the Blandford-Znajek mechanism. We use self-similar advection dominated accretion (ADAF) models to estimate the efficiency of this mechanism for black holes accreting from geometrically thick disks, in the light of recent magnetohydrodynamic disk simulations, and show that the power from electromagnetic energy extraction exceeds the accretion luminosity for ADAFs at sufficiently low accretion rates. We consider the detectability of isolated stellar mass black holes accreting from the ISM, and show that for any rapidly rotating holes the efficiency of energy extraction could reach 0.01. The estimated total luminosity would be consistent with the tentative identification of some EGRET sources as accreting isolated black holes, if that energy is radiated primarily as gamma rays. We discuss the importance of emission from the Blandford-Znajek mechanism for the spectra of other advection dominated accretion flows, especially those in low luminosity galactic nuclei.Comment: ApJL, in pres

Powerful jets from accreting black holes: evidence from the optical and infrared

A common consequence of accretion onto black holes is the formation of powerful, relativistic jets that escape the system. In the case of supermassive black holes at the centres of galaxies this has been known for decades, but for stellar-mass black holes residing within galaxies like our own, it has taken recent advances to arrive at this conclusion. Here, a review is given of the evidence that supports the existence of jets from accreting stellar-mass black holes, from observations made at optical and infrared wavelengths. In particular it is found that on occasion, jets can dominate the emission of these systems at these wavelengths. In addition, the interactions between the jets and the surrounding matter produce optical and infrared emission on large scales via thermal and non-thermal processes. The evidence, implications and applications in the context of jet physics are discussed. It is shown that many properties of the jets can be constrained from these studies, including the total kinetic power they contain. The main conclusion is that like the supermassive black holes, the jet kinetic power of accreting stellar-mass black holes is sometimes comparable to their bolometric radiative luminosity. Future studies can test ubiquities in jet properties between objects, and attempt to unify the properties of jets from all observable accreting black holes, i.e. of all masses.Comment: 26 pages, 4 figures, 1 table. Invited chapter for the edited book "Black Holes and Galaxy Formation", Nova Science Publishers, Inc., at pres

Using radio emission to detect isolated and quiescent accreting black holes

We discuss the implications of new relations between black holes' masses, X-ray luminosities and radio luminosities, as well as the properties of the next generation of radio telescopes, for the goal of finding isolated accreting black holes. Because accreting black holes have radio-to-X-ray flux ratios that increase with decreasing luminosity in Eddington units, and because deep surveys over large fields of view should be possible with planned instrumentation such as LOFAR, radio surveys should be significantly more efficient than X-ray surveys for finding these objects.Comment: 5 pages, 1 table, accepted to MNRAS Letter

Advection-Dominated Accretion and Black Hole Event Horizons

The defining characteristic of a black hole is that it possesses an event horizon through which matter and energy can fall in but from which nothing escapes. Soft X-ray transients (SXTs), a class of X-ray binaries, appear to confirm this fundamental property of black holes. SXTs that are thought to contain accreting black holes display a large variation of luminosity between their bright and faint states, while SXTs with accreting neutron stars have a smaller variation. This difference is predicted if the former stars have horizons and the latter have normal surfaces.Comment: 11 pages, including 2 tables and 2 figures. To appear in The Astrophysical Journal Letter