Introduction: some historical remarks

The earliest detections of luminous X-ray sources (LX ≳ 1036 erg s−1) in globular clusters were made with the Uhuru and OSO-7 Observatories (Giacconi et al. 1972, 1974; Clark, Markert & Li, 1975; Canizares & Neighbours, 1975). About 10% of the luminous X-ray sources in our Galaxy are found in globular clusters. This implies that the probability (per unit mass) of finding a luminous X-ray source in a globular cluster is about two to three orders of magnitude higher than of finding one in the rest of our Galaxy (Gursky 1973; Katz 1975). Clearly, the conditions in globular clusters are very special in that they must be very efficient breeding grounds for X-ray binaries. For reviews that reflect the ideas in the late seventies and early eighties, see Lewin (1980), Lewin & Joss (1983), van den Heuvel (1983) and Verbunt & Hut (1987). At that time there was no evidence for a substantial population of binaries in globular clusters; e.g., Gunn and Griffin (1979) did not find a single binary in a spectroscopic search for radial velocity variations of 111 bright stars in M3.

Clark (1975) suggested that the luminous cluster sources are binaries formed by capture from the remnants of massive stars. Fabian, Pringle and Rees (1975) specified that they are formed via tidal capture of neutron stars in close encounters with main-sequence stars. Sutantyo (1975) suggested direct collisions between giants and neutron stars as a formation mechanism.

Introduction

The luminous super-soft X-ray sources (SSS) were recognized as an important new class of intrinsically bright X-ray sources by Trümper et al. (1991) (see also Greiner et al. 1991). In fact four of them had already been found in the Magellanic Clouds with the Einstein Observatory around 1980, but they had not been recognized as a separate new class (Long et al. 1981; Seward & Mitchell 1981). A careful analysis of the ROSAT data on the first LMC sources showed that while their X-ray luminosities can be as high as the Eddington limit (they range from ∼1036 to 1038 erg s−1), their X-ray spectra are extremely soft, typically peaking in the range 20–100 eV, corresponding to blackbody temperatures of ∼105 to ∼106 K. This is some two orders of magnitude lower than for a classical X-ray binary that contains an accreting neutron star or black hole. Some 40 SSS have been discovered with ROSAT, 16 in the Andromeda Nebula (M31), about a dozen in the Magellanic Clouds, 10 in our own Galaxy and one in NGC55. Since then, several dozens of SSS have been discovered with BeppoSAX, Chandra and XMM-Newton, mostly in external galaxies. The latter are listed in Section 11.10. A catalog of SSS is given in Greiner (2000a).

Introduction

Infrared (IR) luminous galaxies represent highly active stages in galaxy evolution that are not generally inferred in optically selected galaxy surveys (e.g., Rieke & Low 1972; Soifer, Neugebauer, & Houck 1987).

Introduction

Scope of this review

We focus on 18 black holes with measured masses that are located in X-ray binary systems. These black holes are the most visible representatives of an estimated ∼300 million stellar-mass black holes that are believed to exist in the Galaxy (van den Heuvel 1992; Brown & Bethe 1994; Timmes et al. 1996; Agol et al. 2002). Thus the mass of this particular form of dark matter, assuming ∼10 M⊙ per black hole, is ∼4% of the total baryonic mass (i.e., stars plus gas) of the Galaxy (Bahcall 1986; Bronfman et al. 1988). Collectively this vast population of black holes outweighs the galactic-center black hole, SgrA*, by a factor of ∼1000. These stellar-mass black holes are important to astronomy in numerous ways. For example, they are one endpoint of stellar evolution for massive stars, and the collapse of their progenitor stars enriches the Universe with heavy elements (Woosley et al. 2002). Also, the measured mass distribution for even the small sample of 18 black holes featured here is used to constrain models of black hole formation and binary evolution (Brown et al. 2000a; Fryer & Kalogera 2001; Nelemans & van den Heuvel 2001). Lastly, some black hole binaries appear to be linked to the hypernovae believed to power gamma-ray bursts (Israelian et al. 1999; Brown et al. 2000b; Orosz et al. 2001).

Introduction

Baade and Zwicky (1934) were the first to envision the formation of neutron stars as the end product of a supernova explosion. Their forward thinking was not vindicated for another three decades, with the discovery of the first radio pulsars by Bell and Hewish (Hewish et al. 1968). What Baade and Zwiscky could not have anticipated, however, was the menagerie of astrophysical objects that are now associated with neutron stars. Today, we observe them as magnetically braking pulsars, accreting pulsars in binary systems, isolated cooling blackbodies, sources of astrophysical jets, and emitters of high-luminosity bursts of X-rays. Here, we focus on two of the most extraordinary evolutionary paths of a neutron star, namely soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs).

Soft gamma repeaters were discovered as high-energy transient burst sources; some were later found also to be persistent X-ray pulsars, with periods of several seconds, that are spinning down rapidly. Anomalous X-ray pulsars are identified through their persistent pulsations and rapid spin down; some have also been found to emit SGR-like bursts. In spite of the differing methods of discovery, this convergence in the observed properties of the SGRs and AXPs has made it clear that they are, fundamentally, the same type of object. What distinguishes them from other neutron stars is the likely source of energy for their radiative emissions, magnetism.

Introduction

Section 2.6.4 established the fundamental concept underlying ideal MHD, namely that magnetic flux is frozen into the plasma. This concept means that the magnetic topology of an ideal MHD plasma cannot change because a change in magnetic topology would require a change of magnetic flux within the frame of the plasma. Chapter 10 showed that ideal MHD plasmas are susceptible to two distinct types of instabilities, pressure-driven and current-driven. Pressure-driven modes draw on free energy associated with heavy fluids stacked on top of light fluids in an effective gravitational field whereas current-driven instabilities draw on free magnetic energy and involve the plasma attempting to increase its inductance in a flux-conserving manner. Both of these instabilities occur on the Alfvén time scale defined as some characteristic distance divided by vA.

It is possible for an MHD equilibrium to be stable to all ideal MHD modes and yet not be in a lowest energy state. Because ideal MHD does not allow the topology to change, a plasma that is not initially in the lowest energy state will not be able to access this lowest energy state if the lowest energy state is topologically different from the initial state. However, the lowest energy state could be accessed by non-ideal modes, i.e., modes that violate the frozen-in flux condition, and so the available free energy could drive an instability involving these non-ideal modes.

This text is based on a course I have taught for many years to first-year graduate and senior-level undergraduate students at Caltech. One outcome of this experience has been the realization that although students typically decide to study plasma physics as a means towards some specific goal, they often conclude that the study of this subject has an attraction and charm of its own; in a sense the journey becomes as enjoyable as the destination. This conclusion is shared by me and I feel that a delightful aspect of plasma physics is the frequent transferability of ideas between extremely different applications so, for example, a concept developed in the context of astrophysics might suddenly become relevant to fusion or vice versa.

Applications of plasma physics are many and varied. Examples include controlled thermonuclear fusion, ionospheric physics, magnetospheric physics, solar physics, astrophysics, plasma propulsion, semiconductor processing, antimatter confinement, and metals processing. Furthermore, because plasma physics is extremely rich in both concepts and regimes, it has often served as an incubator for new ideas in applied mathematics. Concepts first developed in one of the areas listed above frequently migrate rather quickly to one or more of the other areas so it is very worthwhile to keep abreast of developments in areas of plasma physics outside of one's immediate field of interest.

Introduction

The last decade has witnessed amazing progress in our empirical and theoretical understanding of galaxy formation and evolution.

Overview

We begin this chapter by developing the concept of conservation of particles in phase-space and then use this concept as the basis for establishing the three main models of plasma dynamics, namely Vlasov theory, two-fluid theory, and magnetohydrodynamics (MHD). The Vlasov model is the most detailed and characterizes plasma dynamics by following the temporal evolution of electron and ion velocity distribution functions. The two-fluid model is intermediate in complexity and approximates plasma as a system of mutually interacting, finite-pressure electron and ion fluids. The MHD model is the least detailed and approximates plasma as a single, finite-pressure, electrically conducting fluid. The question of which of these models to use when analyzing a given situation is essentially a matter of selecting the best tool for the task and furthermore, just as a mechanic might alternate between using a screwdriver and a pair of pliers for a specific task, it is often advantageous to alternate between these models when analyzing a specific problem. As we develop these three models, we will also take the opportunity to explore some immediate and important fundamental consequences of these models, most notably the strong dependence of a collisionless plasma on its past history (Vlasov model) and the freezing of magnetic flux into the arbitrarily moving frame of a perfectly conducting plasma (MHD).

Introduction

In the 1995 X-ray Binaries book edited by Lewin, van Paradijs and van den Heuvel, the chapter on Normal galaxies and their X-ray binary populations (Fabbiano 1995) began with the claim that “X-ray binaries are an important component of the X-ray emission of galaxies. Therefore the knowledge gathered from the study of Galactic X-ray sources can be used to interpret X-ray observations of external galaxies. Conversely, observations of external galaxies can provide us with uniform samples of X-ray binaries, in a variety of different environments.” This statement was based mostly on the Einstein Observatory survey of normal galaxies (e.g., Fabbiano 1989; Fabbiano, Kim & Trinchieri 1992). Those results have been borne out by later work, yet at the time the claim took a certain leap of faith. Now, nearly a decade later, the sensitive sub-arcsecond spectrally resolved images of galaxies from Chandra (Weisskopf et al. 2000), complemented by the XMM-Newton (Jansen et al. 2001) data for the nearest galaxies (angular resolution of XMM-Newton is ∼15″), have made strikingly true what was then largely just wishful anticipation.

While a substantial body of ROSAT and ASCA observations exists, which was not included in the 1995 chapter, the revolutionary quality of the Chandra (and to a more limited degree of XMM-Newton) data is such that the present review will be based on these most recent results.

Introduction

Non-neutral plasmas had one less species than a conventional plasma (one species instead of two); dusty plasmas have one more species (three species instead of two). Not surprisingly, the addition of another species provides new freedoms, which give rise to new behaviors.

The third species in a dusty plasma, electrically charged dust grains, typically have a charge to mass ratio quite different from that of electrons or ions. Several methods for charging dust grains are possible. Electron bombardment is the usual means for charging laboratory dusty plasmas, but photoionization or radioactive decay could also be operative mechanisms and may be important for certain space and astrophysical situations. Photoionization would make dust grains positive because photoionization causes electrons to leave dust grains. Radioactive decay of dust grains would make the dust grains develop a polarity opposite to that of the particle emitted in the decay process, e.g., alpha particle emission by dust grains would make the dust grains negative.

We shall consider here only the typical laboratory dusty plasma situation where the plasma is weakly ionized and dust grain charging is due to electron bombardment. Negative charging occurs because the electrons, being much lighter than the ions and usually much hotter, have a much larger thermal velocity than the ions. As a result, when a dust grain is inserted into the plasma it is initially subject to a greater flux of impacting electrons than impacting ions, thereby causing the dust grain to become negatively charged.