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We are currently in a"golden age" of astrophysics. Several fundamental discoveries have led to paradigm shifts in recent years. Among the most important are the establishment of a high precision cosmological "Standard Model," with dark energy, dark matter, and hot baryons as the dominating forms of the matter/energy density, and the confirmation of an inflationary phase in the early Universe, the discovery of extrasolar planets, the understanding that Gamma ray bursts mark the birth of black holes in distant galaxies, and the realization that supermassive black holes are an essential element of galaxy formation and evolution. Astrophysics has become an integral component of modern physics and leads the way to concepts beyond the standard models of elementary particle physics and relativity. X-ray astronomy is a crucial element of modern astrophysics and is particularly well suited to study matter under extreme conditions, black holes, and warm/hot baryons in the dark matter potential wells of the cosmic web. [ABSTRACT FROM AUTHOR]

The existence of a diffuse XRB was discovered more than 40 years ago [21]. The aim of the experiment was to measure the X-ray emission from the Moon, but the data showed a strong X-ray source about 30° away from the Moon (Sco X-1) and a diffuse emission approximately constant from all the directions observed during the flight. In 2002, Riccardo Giacconi received the Nobel prize for this discovery, among others. [ABSTRACT FROM AUTHOR]

X-ray observations of active galactic nuclei (AGN) are of wide astrophysical interest. The amount and efficiency of the energy release within the immediate neighbourhood of black holes belong to the most extreme physical processes observed to date. The most probable explanation for the huge amount of energy output (and for other observational parameters, such as the width of the optical emission lines and the strength of the radio emission) is given by the transformation of potential energy into radiation by accretion of matter onto a supermassive central black hole [43]. The velocities of the accreting matter reach values of about one third of the velocity of light, deduced by relativistically broadened line profiles (centred at about 6.4 keV), [49]. The emission from the matter around the black hole may vary on very short time scales of only a few hundred seconds. The corresponding changes in luminosity reach values of about 10{su10} solar luminosities. All this is further suggestive for the presence of supermassive black holes. Energy production processes and radiation mechanisms for the innermost regions of AGN are an important research field in X-ray astronomy and open a unique possibility to study matter under extreme gravity. Other astrophysical important aspects include the detection and study of binary black holes, expected to lead to strong gravitational wave emission and tidal disruption events of stars in the dense core around the central black holes. At larger distances from the black hole, the emission from optically thin plasma can be studied. Imprinted absorption and emission lines give information on the chemical composition of the gas and of infall and outflow velocities. The log N-log S distribution of AGN and resulting luminosity functions allow to study the density and luminosity evolution of AGN in dependence of redshift. The first compact obscured objects in the universe can be best studied with X-rays. [ABSTRACT FROM AUTHOR]

Gamma-ray bursts (GRBs) are short bursts of high-energy radiation from an unpredictable location in the sky. The γ-ray emission can rise to maximum intensity within fractions of a millisecond. During these short times GRBs are the brightest objects in the γ-ray sky. The energy spectra are nonthermal, with most of the power radiated in the 100-500 keV range, but photons up to 18 GeV or down to a few kiloelectronvolt have also been registered. The bursts have durations of typically 0.1-100 s, with a bimodal distribution separating "short" and "long" duration GRBs at ∼1-2s [57] with a relative occurrence of 1:4 [49]. In addition, short bursts are typically harder than the long bursts, supporting the conjecture that they form two classes of object (Fig. 24.1). [ABSTRACT FROM AUTHOR]

Clusters of galaxies are next to quasars, the most luminous X-ray sources in the Universe with radiation powers of the order of 10{su43}-10{su46} erg s{su-1}. The first detection of a cluster source was made with M87 in 1966 by Byram et al. [30], and 5 years later also the massive nearby clusters in the constellations Coma Berenices and Perseus were detected by Gursky et al. [71] and Fritz et al. [65]. With the use of the Uhuru satellite, the extended nature of the cluster X-ray sources could be established [85]. It turns out that the diffuse X-ray emission from clusters originates in a hot intracluster plasma with temperatures of several ten Million degrees, which radiates the bulk of its thermal radiation in the soft X-ray regime. As the hot plasma is tracing the shape of the cluster, the X-ray appearance provides us with information on the cluster structure. The soft X-ray band in which clusters radiate is fortunately also the wavelength regime for which X-ray telescopes with imaging optics provide us with a detailed picture of the X-ray sky. Therefore, galaxy clusters are among the most rewarding and informative objects for X-ray imaging studies. [ABSTRACT FROM AUTHOR]

In part II several different classes of X-ray sources have been introduced, which can be observed within the Milky Way. One of the largest uncertainties in their study is the distance of the more luminous sources, which are often known to not better than a factor of two. This results in an order of magnitude uncertainty in the X-ray luminosity and prevents the determination of the luminosity function of the various types of sources. In addition to these problems, the absorption within the plane of the Galaxy strongly suppresses soft X-ray emission. Therefore, supersoft X-ray sources (SSS) and the hot interstellar medium (ISM) can only be observed in the solar neighborhood. [ABSTRACT FROM AUTHOR]

It has long been suggested that supermassive black holes in nonactive galaxies can be tracked down by occasional tidal disruptions of stars approaching the black holes. A tidal disruption event would reveal itself by a luminous flare of electromagnetic radiation. This chapter describes the X-ray detection of the highest amplitudes of variability ever recorded among galaxies and their interpretation in terms of the long-sought tidal disruption events. [ABSTRACT FROM AUTHOR]

Looking at the sky on a clear and dark night, the first and most impressive view is the myriad of stars twinkling on the firmament and emitting light from inconceivable distances. The space between them seems to be empty. Apart from other, apparently moving objects, such as comets and planets, it was thought for a long time that the universe was infinite and static, and Newton considered the stars to represent an absolute frame of reference. Progress in astronomy is often driven by technology. The increasing use of telescopes in the seventeenth and eighteenth centuries revealed structures hitherto unseen. Messier, hunting for comets, discovered similarly looking patches on the sky that were not moving and decided therefore to make a compilation (published in several catalogues of star clusters and nebulae until 1781), to avoid further confusion. Among them Andromeda (Messier 31, or M 31 for short) and Orion (M 42) are the most famous ones.{su1} The physical nature of the nebulae became only clear with the advent of spectroscopy in the nineteenth century, its main proponents being Fraunhofer, who discovered absorption lines in the spectrum of the sun, and Bunsen and Kirchhoff who associated spectral lines in the laboratory with light emitted by specific atoms. By then it had become evident that the space between the stars, the so-called interstellar medium (ISM), is by no means empty. [ABSTRACT FROM AUTHOR]

Supernova remnants (SNRs) hold an important place in the history of X-ray astronomy, being associated with many observational "firsts." The first radio/optical object associated with an X-ray source was the Crab Nebula. Cassiopeia A (Cas A) was the first X-ray source for which an X-ray spectrum showed evidence for line emission. SNRs were among the first extended X-ray sources to be imaged, and with their complex, filamentary structure, they are also among the most photogenic X-ray sources. [ABSTRACT FROM AUTHOR]

The center of our Galaxy lies, within the constellation Sagittarius at a distance of ∼8 kpc, behind a large column density of gas and dust. This prevents observations over a wide band ranging from optical to soft X-ray wavelengths. The exploration of this region had to await the instrumental developments of, first, radio, later also of submillimeter and infrared, and most recently, X-ray- and γ-ray astronomy. The Galactic Center is one of the scientifically most interesting regions for astrophysics. It is the closest galactic nucleus, a factor of 100 closer than the next external nucleus, M31. Thus, the relative proximity of our Galactic Center, combined with high angular resolution instruments now available, provides the opportunity to differentiate and study a wealth of highly unusual objects, possibly unique, but probably standard occupants of a normal galactic core. [ABSTRACT FROM AUTHOR]

The idea of neutron stars can be traced back to the early 1930s, when Subrahmanyan Chandrasekhar discovered that there is no way for a collapsed stellar core with a mass more than 1.4 times the solar mass, M{in⊙}, to hold itself up against gravity once its nuclear fuel is exhausted. This implies that a star left with M > 1.4 M{in⊙} (the Chandrasekhar limit) would keep collapsing and eventually disappear from view. [ABSTRACT FROM AUTHOR]

Binary systems that are composed of a black hole and a normal star are called black-hole binaries. In this chapter, the observational evidence for black holes and the specific properties of the black-hole binaries are discussed. [ABSTRACT FROM AUTHOR]

In this chapter neutron stars (NS) are discussed, which are powered through accretion from a companion star in a binary system. The companion is either a "normal star" or a White Dwarf. X-ray binaries where the mass receiving compact object is a White Dwarf or Black Hole, as well as isolated NS, which are powered by a different kind of energy supply (spin energy, magnetic field energy, thermal energy content or accretion from the interstellar medium), are discussed in other chapters of this book. [ABSTRACT FROM AUTHOR]

Cataclysmic variables (CVs) are mass-transfer binaries, in which the accretor is a white dwarf. The secondary can be a normal star with a hydrogen-rich envelope or a second white dwarf. Energy released in the gravitational field of the accreting white dwarf powers the phenomena observed in cataclysmic variables, while subsequent nuclear processing of the accreted matter leads to various types of nova events and supersoft X-ray sources (SSS). Much of the physics of CVs and the related objects is basically similar to that of X-ray binaries, in which the accretor is a neutron star or a black hole. When Sco X-1 was discovered in 1961, CVs were already known [11, 73, 93], and after its optical identification with a faint blue star, Shklovsky suggested that in Sco X-1 "we observe a binary system similar to WZ Sagittae in which one of the components is a neutron star" [79]. The nature of the bright galactic X-ray sources was finally settled by the exciting results obtained with Uhuru (1970-1973) as reminisced by Giacconi [19]. At least three CVs, 4U1228-29 (EX Hya) [94], 4U1809+50 (AM Her), and 4U 1849-31 (V1223 Sgr), were found among the fainter sources in the 4U-catalog [14]. The optical identification of the faint X-ray sources from Uhuru and other early missions was a tedious process, however, because of the comparatively low angular resolution and sensitivity of the early X-ray missions and the visual faintness of the CVs. A breakthrough was the identification of 4U 1809+50 with the long-known 12-15 mag variable star AM Her and Tapia's [84] discovery that AM Her was circularly polarized. The object was immediately interpreted as containing a strongly magnetic accreting white dwarf and became the prototype of the subclass of polars, a term coined by Krzeminski [41]. [ABSTRACT FROM AUTHOR]

Classical nova explosions are the third most violent explosions that can occur in a galaxy, exceeded only by a supernova explosion and a γ-ray burst. With total liberated energies of more than 10{su45} erg s{su-1} novae are less energetic than supernovae but nova outbursts are far more frequent in a galaxy than supernovae. According to the now commonly accepted standard model, nova explosions happen in cataclysmic binary systems that are close binaries with one member, a white dwarf, and the other member, a main sequence or slightly evolved star that fills its Roche lobe. Mass transfer from the secondary to the white dwarf through the inner Lagrangian point leads to the formation of an accretion disk. Ultimately, because of viscous processes, the material ends up on the surface of the white dwarf. The accreted layer of hydrogen-rich material on top of the white dwarf will grow in thickness and temperature and density will increase. If a certain critical pressure is reached at the bottom of the accreted envelope (which must be, at least partially, degenerate), explosive thermonuclear burning of hydrogen via the CNO cycle will start. The evolution of this "thermonuclear runaway" (TNR) depends upon the mass and luminosity of the white dwarf, the mass accretion rate, and the chemical composition of the accreted layer (cf. e.g., [33]). The temperature at the hydrogen burning zone will grow to values exceeding 10{su8} K. Luminosities are of the order of the Eddington luminosity L{inEdd}, peak luminosities in the early phases after the onset of the TNR can even be in excess of L{inEdd} by factors up to ten. Convection turns on and transports the energy generated to the surface of the accreted envelope. Radioactive decay of β{su+}-active nuclei eventually provides the energy to eject the nova shell. Ejection velocities are between several hundred and several thousand kilometer per second. According to the TNR model the entire character of the outburst, light curve, ejection velocities, and timescale of the outburst evolution depend on the amount of CNO nuclei present in the accreted envelope (Starrfield & Sparks [34]). One of the predictions of the TNR model was that novae should have strong overabundances of CNO nuclei, which was later verified to a full extent. The CO nuclei are supposed to be mixed from the white dwarf into the accreted envelope. Abundance studies of novae revealed the existence of a new class of white dwarfs; about 25% of all novae show strong overabundances of O, Ne, and Mg that come from a white dwarf with an ONeMg core. Novae are empirically classified by the decay time t{in3}, the time it takes for the visual brightness to decline from maximum brightness by three magnitudes, which is a measure of the violence of the outburst. Fast novae have t{in3} < 100{sud}, slow novae t{in3} ≥ 100{sud}. According to the number of outbursts observed, one distinguishes between classical and recurrent novae. For recurrent novae two or more outburst were recorded, while for classical novae only one outburst was observed. In the following, we shall concentrate on classical novae that are much more frequent than recurrent novae. However, it should be mentioned that on longer timescales all novae are believed to be recurrent. [ABSTRACT FROM AUTHOR]

Thermal soft X-ray emission is detected from many hot hydrogen-rich white dwarfs (spectral type DA) with an effective temperature in excess of 20 000 K. Most of the objects with T{ineff} <40000 K have virtually pure hydrogen atmospheres while the majority of the hotter ones emit X-ray fluxes lower than predicted by hydrogen model atmospheres and, therefore, must contain heavier elements as absorbers. From ROSAT and EUVE observations it was concluded that trace metals sustained by radiative levitation increase the atmospheric X-ray opacity. [ABSTRACT FROM AUTHOR]

The history of X-ray astronomy is closely related to solar system research: the first attempts ever to detect X-rays from a celestial object concentrated onto the Sun [21], and the (unsuccessful) attempt to detect X-rays from the Moon, in 1962, is generally considered as the birth of X-ray astronomy [22]. In the recent decade, our knowledge about the X-ray properties of solar system objects was considerably enhanced. We know today that not only the Sun and Moon radiate in X-rays, but that our solar system is full of X-ray sources, including all planets from Venus to Saturn, and even comets. [ABSTRACT FROM AUTHOR]