Exploring the Universe

The following Sections are included:

• Preface

• University of Rome “La Sapienza” Honorary Degree in Physics Awarded to Riccardo Giacconi

• Table of Contents

The years 1960 to 1970 saw the emergence of x-ray astronomy. By 1960 plans were being made for cosmic x-ray observations by a number of groups around the world, based on extrapolations from solar and cosmic ray observations. Still the discovery of significant fluxes of x-rays was a complete surprise in 1962. Over the next several years, follow-up observations were made and many new groups entered the field. Notable discoveries included the identification of the Crab Nebula as an x-ray source and the identification of Sco X-1 with a 13^th magnitude stellar object that had the characteristics of an old nova. By the end of the decade the Uhuru x-ray satellite was flying and planning was underway for the HEAO spacecraft, including the Einstein Observatory, and many smaller instruments.

The following Sections are included:

• Introduction
  • A Sensible World, Not a World On Paper

• The Origin of The Uhuru and Einstein Projects

• UHURU

• THE EINSTEIN X-RAY OBSERVATORY

• THAT WAS THEN, THIS IS NOW

• References

The history of high resolution X-ray optics for astronomical observations will be reviewed. This may be divided into roughly three eras, the early conceptual and invention phase, an intermediate period in which the technology for X-ray optics was defined by the traditional methods and experience in the optical industry, and the present era in which both adequate metrology and quantitative fabrication techniques have resulted in the superb mirrors of ROSAT and AXAF. The physics of these optics and their design and fabrication will be discussed. The progress in the field will be illustrated by examples of historical and recent X-ray mirrors. Some speculations on future missions and progress will be made.

The focusing telescope which preceded X-ray astronomy has become the principal instrument of an X-ray observatory and will be even more predominant in the future. We can expect the focusing telescope to evolve into several instruments with specific adaptations. The launch of XMM will initiate the era of high throughput telescopes which will be the keystone of the next generation X-ray observatories of the US and Europe. Focusing telescopes are also being developed for the 10 to 100 keV band. In addition the lobster eye geometry is receiving considerable attention on account of its very wide field of view capability which would permit new classes of instruments for broad sky monitoring and all sky surveys.

The following Sections are included:

• The Early Years – Literary, Nuclear, and Personal Links

• Ariel-5 – My Chance for Independence

• Moving Apart – But Staying Close

• References

Spectroscopy is traditionally characterized by the sacrifice of quantum efficiency for high spectral resolution . Since X-ray astronomy is a photon-limited discipline, the choice between high resolution for very few sources versus much lower resolution for many more has not always been an easy one. The development of new thermal detectors with both high resolving power and high efficiency offers the opportunity to avoid such compromises in the future.

In this article we review the measurements and understanding of the X-ray background (XRB), discovered by Giacconi and collaborators 35 years ago. We start from the early history and the debate whether the XRB is due to a single, homogeneous physical process or to the summed emission of discrete sources, which was finally settled by COBE and ROSAT. We then describe in detail the progress from ROSAT deep surveys and optical identifications of the faint X-ray source population. In particular we discuss the role of active galactic nuclei (AGNs) as dominant contributors for the XRB, and argue that so far there is no need to postulate a hypothesized new population of X-ray sources. The recent advances in the understanding of X-ray spectra of AGN is reviewed and a population synthesis model, based on the unified AGN schemes, is presented. This model is so far the most promising to explain all observational constraints. Future sensitive X-ray surveys in the harder X-ray band will be able to unambiguously test this picture.

We discuss the role of the luminosity function in the interpretation of source counts of extragalactic X-ray sources. Given that the luminosity function derived from a given complete sample is a linear combination of the contributions of each of the sample sources, we illustrate various effects on the basis of source counts contributed by a single source. In particular, we show the effect on the source counts of density evolution, luminosity evolution, spectral index, and absorption.

The astrophysics of extragalactic radio sources intersects with many of Riccardo Giacconi's central interests — X-ray observations of the hot intergalactic gas in galaxies and clusters of galaxies, studies of active galaxies with the Hubble Space telescope, the cosmological evolution of active galaxies, and so on. A brief history of the contribution of studies of extragalactic radio sources to astrophysical cosmology is presented. New HST and infrared observations of a complete sample of 3CR radio galaxies are described. These are shown to be among the most massive galaxies at redshifts z > 1 and provide important clues to the formation of these massive galaxies and the nature of active galactic nuclei. The evolutionary history of these objects with cosmic epoch can now be related to the star-formation history of galaxies as a whole, thanks to observations with the Hubble Space Telescope.

A personal cross section through 25 yrs of progress in the field.

We summarize the pioneering work in the study of galaxies and clusters of galaxies made possible by the advent of imaging X-ray astronomy that was realized with the launch of the Einstein Observatory. We emphasize the uniqueness of the contributions from X-ray astronomy and focus on the discovery of hot coronae and dark halos around early-type galaxies, the determination of mass distributions and baryon fractions in clusters, and cluster structure. We discuss the cosmological implications of these investigations.

The following Sections are included:

• Introduction

• Why do we need catalogues of clusters of galaxies

• Where we are going

• More

• Final remarks

• REFERENCES

I review recent observational progress made in the search for and study of distant galaxy clusters, with particular emphasis on the evolution of the abundance of X-ray clusters out to z ~ 1. The very existence of massive collapsed objects at high redshift challenges theories of structure formation. New deep X-ray surveys are now revealing that the bulk of the population of galaxy clusters is not evolving since at least z ≃ 0.8, with possible mild evolution limited to only the most luminous, presumably most massive systems. Thus far, a well defined sample of high redshift (z ≳ 1) clusters has been difficult to assemble and represents one of the most challenging observational tasks for the next decade. The next generation of X-ray instruments in conjuction with the new imaging and spectroscopic capabilities offered by 8-meter class telescopes will open up a new window on the physical understanding of the evolutionary processes involved.

The global gas flows in elliptical galaxies are initiated by stellar mass loss and their diagnostics rely on X-ray observations. The flows are controlled by a number of factors, including supernova heating, the depth and shape of the potential well as determined by the amount and distribution of bright and dark matter, AGN fueling and its feedback effects, interaction with the intracluster medium, and star formation. As a result no steady-state solution can satisfactorily describe the complex, evolutionary behavior of the gas flows, which can experience supersonic wind, subsonic outflow, and inflow phases, and transitions between one such flow regime to another. Having identified heating by Type Ia SN's as one of the key factors controlling the flows, constraints on its evolution with cosmological time are derived by considering the total amount of iron contained in whole clusters of galaxies, while the iron abundance in individual galaxy flows can set constraints on the present rate of SNIa's in ellipticals. The central issue of the problem remains the fate of the gas. It is argued that in one way or another, via SN-driven winds, ram pressure stripping, or AGN violent ejection, most of the gas is ultimately expelled from galaxies thus joining the intracluster medium.

I review the beginnings of the field of stellar X-ray astronomy, concentrating on the period immediately preceeding, and immediately following, the launch of the Einstein Observatory. The wealth of data was such that, within the first two years following launch, major discoveries were made by scientists from both the Einstein Observatory PI groups and Einstein Observatory Guest Observers which established stellar X-ray astronomy as a new astronomical discipline: Discovery of early and late-type stars, as well as young protostars, as soft X-ray sources; discovery of the “dividing line” separating X-ray emitting and X-ray quiet giant and supergiant stars; and establishment of the “solar-stellar connection” as a paradigm for understanding X-ray emission from late-type stars.

The discovery of X-ray emission from normal stars throughout most of the HR diagram was among the momentous findings of the Einstein Observatory. The mechanism responsible for X-ray activity in late type stars is believed to be essentially similar to the one operating in the Sun, i.e. dissipation of magnetic energy in an optically thin corona. Coronal magnetic fields should thus induce structuring of the coronal plasma in loop-like structures, as observed in the Sun. I shall discuss on the structure of stellar coronae, essentially along the lines of a program initiated by the late G.S. Vaiana after the early observations of the solar corona by Skylab, and continued in Palermo. I shall show how spectrally resolved X-ray observations can evidence the existence of stellar coronal loops, both in the static case and during flares.

The discovery of binary X-ray sources with UHURU introduced many new concepts to astronomy. It provided the canonical model which explained X-ray emission from a large class of galactic X-ray sources: it confirmed the existence of collapsed objects as the source of intense X-ray emission; showed that such collapsed objects existed in binary systems, with mass accretion as the energy source for the X-ray emission; and provided compelling evidence for the existence of black holes. This model also provided the basis for explaining the power source of AGNs and QSOs. The process of discovery and interpretation also established X-ray astronomy as an essential sub-discipline of astronomy, beginning its incorporation into the mainstream of astronomy.

After a few remarks about the early history of the subject we present a short review of the present observational situation regarding the X-ray emission from isolated neutron stars. In total 32 objects have been detected with ROSAT, the majority of which are radio pulsars showing non -thermal (magnetospheric) emission. For three radio pulsars and three point sources in SNRs thermal emission has been seen which probably comes from the photospheric of the cooling neutron star. A third class comprising the objects represents neutron stars according matter from the interstellar medium.

The advantages of grazing incidence X-ray telescopy, pioneered by Riccardo Giaconni and his associates, are mentioned, and the prospects for studies of the extra-galactic X-ray background and grain-scattered halos of eclipsing X-ray binaries by AXAF are described.

The following sections are included:

• Introduction

• Equivalent width of a K_α line in the background spectrum

• The giant molecular clouds in central 100 parsec of the Galaxy

• Scattering of X-Ray lines on neutral gas

• Reference

The relativistic precession model for quasi periodic oscillations, QPOs, in low mass X-ray binaries is reviewed. The behaviour of three simultaneous types of QPOs is well matched in terms of the fundamental frequencies for geodesic motion in the gravitational field of the accreting compact object, for reasonable star masses and spin frequencies. The model ascribes the higher frequency kHz QPOs, the lower frequency kHz QPOs and the horizontal branch oscillations to the Keplerian, periastron precession and nodal precession frequencies of matter inhomogeneities orbiting close to the inner edge of the accretion disk. The remarkable correlation between the centroid frequency of QPOs in both neutron star and black hole candidate low mass X-ray binaries is very well fit by the model. Some testable predictions are described. QPOs from low mass X-ray binaries might provide an unprecedented laboratory to test general relativity in the strong field regime.

Riccardo Giacconi, probably more than any other single individual, established x-ray astronomy as an essential sub-discipline of astronomy. Its incorporation into the mainstream of astronomy was substantially completed with the Einstein Observatory which, with its imaging capabilities and its Guest Observer program, invited non-x-ray astronomers to use the facility. It was therefore perhaps fitting that when optical astronomy moved into space, with the Hubble Space Telescope, it called on Riccardo to oversee the transition. He brought with him lessons about building and operating space observatories, experience working with NASA on large science projects, a business-like approach to attacking tasks, and his unique vision and abilities. Among the guiding principles he brought to HST were: involvement of a strong scientific research staff in all aspects of the program; establishment of a vital, active research environment; attention to “science system engineering” and applying a rational scientific approach to problems; creation of an atmosphere of “ruthless intellectual honesty;” and maintenance of the highest regard for process. These formed the basis both for attacking the problems of HST, and for building an Institute to do so.

The HST has become one of the most important instruments to investigate the physical nature of the optical counterparts to radio-jets. High-spatial and spectral resolution observations of a variety of sources has been carried-out. HST observations of the complete 3CR sample of radio sources has led to the discovery of several new optical counterparts to radio jets, more than doubling the sample of known objects. HST observations have revealed, for the first time at optical wavelengths, proper motions in the M87 jet, with doppler-boosted velocities, as high as 2.5c. Also relevant to the understanding of the physical properties of these jets are the numerous HST observations of the central regions of AGN. These include LINERS, Seyferts 1 and 2, as well as powerful radio sources. The HST results are in broad agreement with the unified theories for AGN. However, these results have also made important modifications to the basic theory, such as the need for additional sources of ionization (e.g., shocks) to explain the observed filamentary structure in the Narrow-Line Region.

The following Sections are included:

• Introduction

• The remarkable properties of 1138–262

• Is 1138–262 at the center of a cluster?

• Is 1138–262 a proto-cD galaxy?

• References

This paper considers how the first subgalactic structures produced the UV radiation that ionized the intergalactic medium before z = 5. These ‘pre-galaxies’ may eventually be detectable.

We recall the generalization of the Feynman-Metropolis-Teller approximation for a compressed atom using a relativistic Fermi-Thomas model. These results within a Wigner-Seitz approximation lead to a new equation of state for white dwarfs and to a new value of their critical mass, smaller than the one obtained by Chandrasekhar. The possible observations of these effects in binary neutron stars are outlined.