...$^a$¹

Corresponding author. E-mail: chris@astro.umd.edu

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... limit²

The Russian physicist Lev Landau independently calculated the upper mass limit of a white dwarf star, and his results were published in 1932 [3].

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... galaxies³

There have been recent suggestions of a possible population of ``intermediate mass'' black holes, with masses in the range of $10^3$-$10^5$$\rm\thinspace M_{\odot}$. (See the discussion in [13], for example.) The existence of such objects is still speculative, and detailed studies of their spectra, of the kind to be discussed in this review, do not yet exist. We shall therefore not consider these objects further in this review.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... years⁴

However, on time scales of 10-20 years, there are plans for both radio wavelength and X-ray interferometers which will be able to image nearby supermassive black holes on spatial scales comparable with that of the event horizon (see §5.3.4).

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... hole⁵

A dramatic exception to this statement occurs when two black holes of comparable mass merge. Large and complicated deviations from the Kerr metric can be readily obtained during such an event. However, such events are extremely rare and settle down to the Kerr metric on only a few light crossing times of the event horizon.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... morphology⁶

We will only describe the most coarse aspects of the classification scheme here -- the interested reader is directed to Chapter 1 of the book by Krolik [103] for further details.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...$y = 1$⁷

These particular expressions come from a solution of the Kompaneets equation, which under certain approximations describes the diffusion in phase space of photons undergoing Comptonization. Excellent discussions of the Kompaneets equation can be found in the review by Pozdnyakov et al. [138], the book by Rybicki and Lightman [137], and Chapter 24 of the book by Peebles [139].

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... depths⁸

The Thomson depth is the distance over which the electron scattering optical depth is unity.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... photon⁹

In fact, the K$\alpha$ emission line is a doublet (comprising of the K$\alpha_1$ and K$\alpha_2$ lines). Since the splitting between the K$\alpha_1$ and K$\alpha_2$ lines is only a few eV, it is not relevant for the current discussion of relativistically broadened emission lines. Thus, we will ignore the fine structure of the K$\alpha$ line for the remainder of this review.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... peaks¹⁰

It is customary in astronomy to call lower energies ``red'' and higher energies ``blue'', even when talking about lines in energy bands other than the optical

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...ASCA¹¹

From Asuka, pronounced ASCA, the name of a region in central Japan, and an era in Japanese history around 600 AD during which Asuka was the capital. It is also an ancient Japanese word meaning ``flying bird''.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... days¹²

During the early stages of the ASCA mission, this was an observation of unprecedented length. The typical ASCA observation rarely exceeded one day.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... (M 106)¹³

More precisely, this source is classified as a low-ionization nuclear emission region (LINER)

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.