Introduction

When both the photometric transits and the radial velocity variations due to an extrasolar planet are observed, we are granted access to key quantities of the object that Doppler monitoring alone cannot provide. In particular, precise measurements of the planetary mass and radius allow us to calculate the average density and infer a composition.

Introduction

Regarded as an astrophysical mystery and a curiosity for decades, cosmic gammaray bursts are finally entering the mainstream of astronomy and astrophysics. In the past few years, we have learned that they lie at cosmological distances, and are probably caused, possibly among other things, by the collapses and subsequent explosions of massive stars. Energetically they are roughly analogous to supernovae, to which they may indeed be related in some cases; no new physics needs to be invented to explain their prodigious luminosities. Unlike supernovae, however, they are relatively rare, and their energy output is distributed quite differently over wavelength and time. They can probably be observed out to distances comparable to, or even farther than, those of the most distant quasars, which makes them useful to cosmologists as lighthouses to the early Universe. Finally, too, they hold the promise of revealing properties of early galaxies such as star formation rates and metallicities in ways that are unique. For all of these reasons, in addition to the facts that they signal the formation of black holes and drive ultra-relativistic winds, they have begun to attract the attention of people working in very diverse disciplines. The words “gamma-ray burst” have even begun to enter the vocabulary of the general public, which regards them with a certain morbid fascination.

It was not at all clear a decade ago that the study of gamma-ray bursts (GRBs) had such a promising future.

Introduction

Cataclysmic variables (CVs) are a distinct class of interacting binaries, transferring mass from a donor star to a degenerate accretor, a white dwarf (WD). In all observational determinations, and as is required by theory for stable mass transfer, the donor star is of lower mass than the accretor. For comprehensive overviews on the subject of CVs we refer to Hack & La Dous (1993) and Warner (1995).

The majority of CVs have orbital periods, Porb, between 75 min and 8 h (see Ritter & Kolb 2003) and consist of Roche lobe-filling main sequence donors and WDs. These are WD analogues of the low-mass X-ray binaries (LMXBs; see Chapter 1). In the period range 8 h–3 d the donors must have larger radii than dwarfs in order to fill their Roche lobes and are therefore evolved subgiants. A few CVs are found with Porb ∼ 200 d, which require giant donors for them to be lobe-filling. The absence of evolved CVs with periods ∼3 to ∼200 d is connected with the dynamical instability that results from an initial donor that had a mass larger than about 67% of that of the WD; such binaries will have experienced rapid mass transfer and shortened their periods during a common envelope phase (e.g., Iben & Livio 1993; see also Chapter 16). Beyond Porb ∼ 200 d, mass-transferring systems also exist.

The Space Telescope Science Institute Symposium on Planets to Cosmology: Essential Science in the Final Years of the Hubble Space Telescope took place during 3–6 May 2004.

These proceedings represent only a part of the invited talks that were presented at the symposium. We thank the contributing authors for preparing their manuscripts.

With some uncertainty concerning Hubble's next Servicing Mission still hanging, identifying the most crucial science to be performed by this superb telescope has become of paramount importance. With this goal in mind, the symposium examined a wide range of topics at the forefront of astronomy and astrophysics. The result is a magnificent collection of results, with a special emphasis on future research.

We thank Sharon Toolan of ST ScI for her help in preparing this volume for publication.

Extra-solar X-ray astronomy began with the historical paper in Physical Review Letters by Giacconi, Gursky, Paolini, and Rossi (1962). Now, more than four decades later, X-ray astronomy is central to many aspects of astronomy. In 2002, Riccardo Giacconi was awarded the Nobel Prize in Physics “for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources”. In the decade since the publication of X-ray Binaries – the predecessor of the present book – the study of compact stellar X-ray sources has received enormous impetus from observations with the BeppoSAX, Rossi X-ray Timing Explorer (RXTE), Chandra, and XMM-Newton X-ray observatories. In addition, many exciting new results on these X-ray sources have also been produced in the radio, infrared, optical and ultraviolet bands. Highlights include the discovery in low-mass X-ray binaries of millisecond X-ray pulsations, confirming the connection with the millisecond radio pulsars. Millisecond and sub-millisecond quasi-periodic oscillations (QPO) were discovered that are thought to provide a direct view of regions of strong-field gravity near neutron stars and black holes. The discovery of X-ray, optical and radio afterglows of gamma-ray bursts (GRB) firmly established their long-suspected cosmological distances. Super-luminal motion of radio jets was discovered in accreting black-hole binaries. Dozens of ultra-luminous X-ray sources (ULX) have been detected in many galaxies. Their origin is still not clear; some may be accreting intermediate-mass (i.e., of order 103 M⊙) black holes (IMBH).

Introduction

Many accreting neutron stars erupt in spectacular thermonuclear conflagrations every few hours to days. These events, known as Type I X-ray bursts, or simply X-ray bursts, are the subject of our review. Since the last review of X-ray burst phenomenology was written (Lewin, van Paradijs & Taam 1993; hereafter LVT), powerful new X-ray observatories, the Rossi X-ray Timing Explorer (RXTE), the Italian–Dutch BeppoSAX mission, XMM-Newton and Chandra have enabled the discovery of entirely new phenomena associated with thermonuclear burning on neutron stars. Some of these new findings include: (i) the discovery of millisecond (300–600 Hz) oscillations during bursts, so-called “burst oscillations”; (ii) a new regime of nuclear burning on neutron stars which manifests itself through the generation of hours-long flares about once a decade, now referred to as “superbursts”; (iii) discoveries of bursts from low accretion rate neutron stars; and (iv) new evidence for discrete spectral features from bursting neutron stars.

It is perhaps surprising that nuclear physics plays such a prominent role in the phenomenology of an accreting neutron star, as the gravitational energy released per accreted baryon (of mass mp), GMmp/R ≈ 200 MeV, is so much larger than the nuclear energy released by fusion (≈5 MeV when a solar mix goes to heavy elements). Indeed, if the accreted fuel was burned at the rate of accretion, any evidence of nuclear physics would be swamped by the light from released gravitational energy.

Introduction

HST and quasar host galaxy studies have grown up together over the past 30 years. Indeed, “the imaging of low-redshift quasars at high angular resolution (∼0″.1) is one of the principal scientific goals for which the Hubble Space Telescope was designed” (Bahcall, Kirhakos, & Schneider 1994). The nice demonstration by Kristian (1973) that nearby quasars are, in fact, surrounded by “fuzz” in deep 200-inch photographs provided timely input for the design of HST and its instruments, the specifications for which were outlined by the Large Space Telescope Science Working Group in 1974 (HST website). While HST has changed the way we look at quasar hosts, the ultimate goal of our studies has not changed over the decades. Then, as now, we strive to understand the roles played by quasars in galaxy evolution.

Introduction

This chapter deals with X-ray emission from isolated neutron stars for which the energy for the observed X-rays is thought to originate from the rotation of the neutron star, or from an internal heat reservoir following formation. Rotation power can manifest itself as pulsed emission, or as nebular radiation produced by a relativistic wind of particles emitted by the neutron star. Residual heat of formation is observed as soft X-ray emission from young neutron stars. Such thermal radiation, however, can also be produced as a result of reheating from internal or external sources. Rotation-powered pulsed and nebular X-ray emission, as well as thermal emission, can often be observed in a single object simultaneously; this is both fascinating and annoying, as one invariably contaminates the study of the other. There are also a handful of neutron stars for which the origin of the observed X-ray emission is unclear but may be related to the above processes; we will discuss those as well.

Rotation-powered neutron stars are generally referred to as “radio pulsars” since it is at radio wavelengths that the vast majority of the catalogued population (currently numbering ∼1400) is observed. However, the radio emission is energetically unimportant, and we now know of several rotation-powered neutron stars that are not detected as radio sources in spite of deep searches (e.g. Crawford et al. 1998; McLaughlin et al. 2001). We therefore use the more physically motivated term “rotation-powered.”

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).