The high energy X-ray universe

Abstract

Since its beginning in the early 1960s, the field of X-ray astronomy has exploded, experiencing a ten-billion-fold increase in sensitivity, which brought it on par with the most advanced facilities at all wavelengths. I will briefly describe the revolutionary first discoveries prior to the launch of the Chandra and XMM-Newton X-ray observatories, present some of the current achievements, and offer some thoughts about the future of this field.

High-energy astrophysics has been successful because high-energy processes play a dominant role in the creation and dynamic and chemical evolution of structures in the universe. From the formation and death of stars to the growth of clusters of galaxies, we find that high-energy phenomena accelerate particles to relativistic energies and/or heat plasmas to tens of millions of degrees. As a consequence, high-energy photons are emitted whose properties closely reflect the underlying physical processes.

The lowest-energy photons of this high energy spectrum that can reach Earth from cosmological distances unimpeded by galactic absorption, are in the X-ray range of wavelengths. Because in most processes the photon number per unit energy decreases with increasing energy, this range will yield the largest number of transmitted photons. In addition, it is possible to construct grazing incidence telescopes which provide images over a finite field with angular resolutions comparable to that obtained in the optical domain by ground-based telescopes (0.5”) and a sensitivity sufficient to study the most distant objects in the universe.

For these reasons, I have concentrated my remarks on only one aspect of high-energy astronomy, namely X-ray astronomy, and even more strictly to high-resolution X-ray research with imaging X-ray telescopes. This is a field which has exploded over the last 50 years, experiencing a ten-billion-fold increase in sensitivity, bringing it on par with the most advanced facilities at all wavelengths.

After a few remarks on the revolution in our vision of the universe already brought about by X-ray observations before the advent of the Chandra and X-ray Multi-Mirror Mission-Newton (XMM-Newton) observatories, I will describe some of the current achievements in the field and conclude with some thoughts about the future.

1962–1999

Starting with the discovery of Sco X-1 in 1962 (1), the following 8 years were characterized by the efforts of finding new sources and solving the riddle of the nature of these extraordinary stars emitting orders of magnitude more X-rays than normal stars. The effort was successful with respect to supernovas like the Crab Nebula and its pulsar (2), but not for Sco X-1-like sources.

The launch of Uhuru in 1970 (3) brought about the discovery of X-ray binary systems containing a neutron star (Cen X-3) or a stellar mass black hole (Cyg X-1). Apart from showing that a binary system can remain bound through the supernova explosion that created the compact objects, these findings established directly the existence of stellar mass black holes. For neutron stars, one could use such systems as an astrophysical laboratory in which to measure the mass, moment of inertia, and equation of state (4).

The energy source for the very high X-ray energy fluxes (10³⁶–10³⁸ erg/cm² sec) was found to be the gravitational in-fall of gas from the companion onto the compact objects, a process a hundred times more efficient per nucleon than fusion (5). This became the generally accepted explanation not only for stellar sources but also for the emission of active galactic nuclei (AGNs), all of which appear to contain a supermassive black hole (>10⁶ solar masses).

The other important discovery from Uhuru was the existence of high-temperature plasmas (10⁶–10⁷ K) in clusters of galaxies which contained five times more mass than the luminous mass (6). We now believe that most of the baryonic mass of the universe resides in high-temperature plasmas.

Uhuru also confirmed the 1962 finding of an isotropic X-ray background with a flux greater than the sum of the then-known X-ray sources. It was in an effort to understand the nature of this background that in 1963 I made the first proposal to National Aeronautics and Space Administration (NASA) for a Chandra-sized X-ray telescope, together with Herbert Gursky (7).

The interest in X-ray astronomy changed greatly with the launch in 1978 of Einstein, the first observatory carrying an imaging X-ray telescope, when the observations came to include all classes of celestial objects. At this point in time, X-ray astronomy became relevant to all of astronomy (8). There followed a number of telescope missions, most notably the Röntgen Satellite (ROSAT) satellite (9) in 1990, which performed an all-sky survey and provided many of the targets for the subsequent Chandra and XMM-Newton observatories.

Chandra

The Chandra Observatory was launched on July 23, 1999 (Fig. 1). The 1.2-m telescope has a collecting area of ≈1000 cm², even though the polished area of the mirror surfaces is ≈10 m², due to the grazing incidence requirements of the double-reflection optics. The observatory was placed into a highly elliptical orbit with initial altitude ranging from 10,000 to 139,000 km. The spacecraft spends most of its time outside the Van Allen belts, thus gaining sensitivity due to the low background. Chandra has operated continuously for 10 years, and to my knowledge both spectrometers and imagers have suffered only minor degradations (10). I will use three examples to indicate the power of Chandra and its applicability to a wide range of astrophysical phenomena: the observation of the molecular cloud ρ Ophiuchi, that of the supernova remnant in the Crab Nebula, and that of the central region of Cen A.

Fig. 1.

The Chandra Observatory launched on July 23, 1999.

The first is a 27-h observation of young stellar objects (about 100,000 years old) in ρ Ophiuchi, the molecular cloud some 500 light years from Earth (11). Fig. 2 is only the first frame of a time-elapsed movie obtained during the observation, showing the occurrence of several X-ray flares. In the surveyed region of 17 × 17 arc min, some 100 sources are observed above the 10²⁸ erg/sec detection limit (assuming that all of the sources are in the same cloud). Although these objects had been well studied in the radio and infrared domains, earlier X-ray observations did not have the wide-energy band sensitivity and the unprecedented angular resolution of Chandra to study the flaring activity, the elemental abundance, and the magnetic field configurations of these deeply embedded objects. Perhaps one of the most interesting results is the study of magnetic loop lengths, based on magnetic loop reconnection mode, where one obtains both solar-type loops (10¹⁰–10¹¹ cm) as well as larger ones connecting the star to its inner accretion disk. It is clear that such studies are essential to help us formulate plausible models of stellar formation that can account for the dissipation of angular momentum and magnetic fields during the formation process.

Fig. 2.

Young stellar objects in ρ Ophiuchi.

The second is the observation of the Crab Nebula Supernova (1054 ad), which remains one of the most fascinating objects in the sky. It was observed early in the mission as a calibration source(!) and some unique spatial and spectral features were observed. This included an X-ray inner ring with an X-ray torus (possibly a hollow tube) as well as knots along the inner ring and along the inward extension of the X-ray jet (12). Fig. 3 shows the first exposure of a much more ambitious coordinated program of Hubble Space Telescope (Hubble or HST) and Chandra observations carried out between August 2000 and April 2001 at 11- and 22-day intervals, respectively (13). The visible light data were obtained at 5500A (F547M) with the Wide Field Planetary Camera 2 and the X-ray data with the Chandra AXAF CCD Imaging Spectrometer (ACIS) instrument in a 150 × 150 arc sec field. Very Large Array (VLA) radio results were obtained between February and April 2001 (14). The phenomenology is extremely complex, and comparison between the images at different wavelengths shows striking similarities and differences. The most obvious features are the “wisps” which appear to be circular rings in the equatorial plane of the pulsar moving outward at typical speeds of 0.5 c. These wisps are observed in all three bands. A feature observed both in X-ray and optical are the polar and possibly counter-polar jets, not observed in radio. A rather unusual X-ray feature is an inner X-ray ring which appears composed of about two dozen knots, much brighter in the X-ray than in optical, and which may be associated with the shock that turns the pulsar cold relativistic plasma into synchrotron-emitting plasma.

Fig. 3.

Crab Nebula Supernova (Chandra and Hubble Space Telescope).

The physical processes which give rise to these emissions are not yet fully understood; it is clear that much still remains to be done to unravel the pulsar and nebular physics, and that X-ray observations will play a crucial role in this endeavor. Whereas a self-consistent model appears to satisfactorily explain both X-ray and optical emission as due to a pulsar wind of electrons (and positrons) with a high Lorentz factor, γ_wind > 10⁴, it is difficult to explain the fact that the bulk of the radio-emitting electrons have γ < 10⁴. An interesting suggestion is that because the lifetime of the radio-emitting electrons is greater than the age of the supernova, they could have been injected in the past by a different process.

The third observation is that of the center of Centaurus A (Cen A, NGC 5128), which is the nearest active galaxy at 3.5 Mpc. Cen A is believed to contain a supermassive black hole and has been studied for decades at all wavelengths over a wide range of spatial scales. The bright inner radio lobes extend several arc min from the nucleus and the bright outer lobes by several degrees. Optically, Cen A appears as an elliptical galaxy with a dark dust lane through its center. In X-rays its morphology has been studied since 1979 with Einstein and ROSAT. Fig. 4 shows the image obtained by Chandra with a resolution of 0.5 arc sec (15). We can resolve its complex X-ray structure into several components: the bright central nucleus which appears extended by a few tenths of an arc sec; the jet which appears to be composed of extended knots and diffused emission and a counter-jet; and absorption due to the dust is observed in the inner region of the jet (30 arc sec from the nucleus). The extended disk of 20-pc radius observed with Hubble (2.5 arc sec diameter) is not observed. The X-ray structure of the jet differs significantly from the radio structure and raises new questions about the acceleration mechanism of particles in the jet.

Fig. 4.

Center of Cen A (NGC 5128).

Some 200 point-like sources, which appear concentrated around the galactic center, have also been detected. Most of them appear to be X-ray binaries; some are identified with known globular clusters. The number of these objects varies from galaxy to galaxy, and may reflect differences in star formation history or in the mechanism of creation of the X-ray binaries. Given its proximity, the study of Cen A will remain of great interest in advancing our understanding of the region surrounding the central black hole and of the mechanism of acceleration of particles in the jet.

I hope these examples make clear how the high angular resolution achieved in X-ray observations with Chandra gives us a powerful tool in extending our studies from the nearest stars to the nuclei of active galaxies.

Deep Surveys

The high angular resolution of Chandra, coupled with its sensitivity, has also been essential to provide a great step forward in the study of the nature of the 2–8 keV X-ray background, and to begin to study the history of formation of supermassive black holes in the early universe. Fig. 5 shows an X-ray picture of a 14 × 14 arc min region in the Southern Hemisphere, which has become known as Chandra Deep Field South (CDFS). The field was selected because of its low column density of hydrogen and the absence of bright objects. Early in the Chandra mission, I was privileged to obtain a million seconds of observing time in this field. With the completion of two 1-Ms exposures, one in the CDFS by our group (16) and one in the North (CDFN) by Brandt's group at Penn State (17), the original X-ray glow discovered in 1962 is almost completely resolved in individual sources by confusion-free Chandra images. The high angular resolution results in a very low background for point sources over the field of view, and we are therefore in a signal-limited rather than a signal-to-noise-limited situation. Some of the weakest sources in the surveys comprised a total of 11 photons collected in 10 days, roughly a photon per day. Fig. 6 shows the Log of n > S as a function of Log S, where N is the number of sources/degree² and S is the minimum detectable flux. The flux limits reached with increased exposures and more sophisticated analysis techniques are 3 × 10⁻¹⁷ erg cm⁻² s⁻¹ in the 0.5–2 keV range and 2 × 10⁻¹⁶ erg·cm⁻²·sec⁻¹ in the 2–8 keV range, thus about 10⁻¹⁰ times the Sco X-1 flux.

Fig. 5.

Chandra Deep Field South (CDFS).

Fig. 6.

Log of N(>S) as a function of Log S.

The cumulative spectral properties of the 346 sources detected in the CDFS result in an average photon index of 1.375 ± 0.015 in the 1–10 keV range for a galactic column density of N_h = 8 × 10 ¹⁹ cm⁻², well in agreement with the measured total background index. We observe that the spectrum of the X-ray sources becomes harder as we go to fainter sources, and this resolves the apparent paradox that the spectrum of the local AGNs seemed too soft to explain the X-ray background. The CDFS X-ray picture provides in fact the largest density of AGNs (n = 1/min²) from any survey. Is all of the background explained by the observed sources in the surveys? The answer appears to be yes in the 0.5–2 keV region and possibly no in the 2–10 keV region (18). At lower energies the main difficulty is the knowledge of the integrated background flux, and the possibly missing 10% can be explained by the contribution of sources just below the threshold of detection. At higher energies, due to poor statistics, as much as 40% of the background could still be missing, leaving therefore opportunity for future searches to discover the missing sources (19).

We can now turn to the study of the nature of the sources. The high angular resolution of Chandra makes it possible to make unambiguous identifications as shown in Fig. 7 (20). The isointensity X-ray contours are plotted on HST optical pictures of the same area. Identifications are made with a precision of 0.5 arc sec, consistent with the precision of both X-ray and optical locations. The corresponding magnitudes of the optical counterparts reach completeness up to I = 26.4 and V = 27.6. Thus, the study of optical counterparts, in particular spectroscopy, requires the use of the European Southern Observatory Very Large Telescope (ESO-VLT) or Keck, which is the largest available ground-based telescope. This is required to obtain spectroscopic measurements of the redshifts of the candidate objects, although photometric redshifts can be obtained from multifilter surveys. In Fig. 8, I show a compilation by W. Zheng of the CDFS sources, separated by categories, of the spectroscopic (137) and photometric (205) redshifts, for galaxies and type I (broad optical lines with unabsorbed X-ray emission) and type II (narrow optical lines with at least partially obscured spectra) AGNs (21). This corresponds to 99% of all of the detected sources in the field. The results show the sources getting harder at high redshifts in the X-ray and in the U-K index, which may be due to a K correction. The number of type II AGNs declines significantly at z > 2 and that of galaxies at z > 1, which indicates the limits of the survey for obscured AGNs and for fainter galaxy sources. From number counts and X-ray background models we expect, however, a dominant contribution from galaxies at fainter fluxes.

Fig. 7.

HST identifications of CDFS sources.

Fig. 8.

Compilation of CDFS sources separated by categories.

We have studied the X-ray variability of sources in CDFS (22). We find that 50% of the sources with sufficient statistics exhibit significant variations with timescales from a day to a year. In fact, the general conclusion is that we find variability whenever we have sufficient statistics; this is in agreement with what is found in CDFN, and is strong confirmation of the nature of the sources. The prevalence of short-term variability (<2 days) implies a scale of 2 × 10⁻³ pc, corresponding to the inner part of the accretion disk of a supermassive black hole. The amount of variability seems to evolve with redshift, with X-ray sources more variable in the past.

Although it is not surprising to find that the X-ray background is composed of discrete sources and in particular by AGNs, as had been already shown by Einstein and ROSAT, it is interesting to note that most of the pre-Chandra models for the background failed to predict the observed distributions, which leaves important issues to be solved about the evolution of supermassive black hole populations at early epochs. When and how did the first supermassive black hole form? How is their activity related to star formation, and how are both related to mergers and large-scale environment? What is the history of nuclear activity over a galaxy life time? The answers to these questions require wide and deep surveys which we hope will be carried out in the future (23).

Cluster Research

The advent of the Chandra and XMM-Newton observatories has brought about a qualitative improvement in the study of the formation and evolution of clusters under the influence of gravity as well as their internal dynamics determined by the astrophysical processes occurring within their potential wells. We now observe X-ray clusters at very early epochs (z = 1.4), without any indication of a turnover in the cluster population. Apart from studying them for their intrinsic interest, the study of their formation and evolution greatly contributes to cosmology. I will discuss a few examples that illustrate these points.

The Perseus Cluster.

The Perseus Cluster (A 426) is the brightest X-ray cluster in the sky and has been well studied. Its emission is due to thermal bremsstrahlung and line radiation from the hot intracluster medium and is sharply peaked on NGC 1275, a cD galaxy at its center containing a supermassive black hole. Fig. 9 shows a picture of the Perseus Cluster in which we see the combined image in X-rays obtained with Chandra (24), in radio by the VLA (25), and in the optical band by the Hubble Telescope. Jets from the nucleus of NGC 1275 have inflated bubbles to the immediate north and south of the galaxy displacing the Intra Cluster Medium (ICM). Indications of ancient bubbles devoid of radio photons can also be observed. Fabian and his collaborators have accumulated 900 ks of exposure on this object in the pursuit of the missing cooling flows. The radiative cooling time in the inner tens of kpc is 250 Myr, and this could lead to a cooling flow of 100 solar masses per year. But this is true only if there is no heat input. Energy from the bubbles appears to be the source of the balancing heat, although the mechanisms of energy transport and dissipation are not well understood.

Fig. 9.

A combined image of the Perseus Cluster.

The very deep exposures have revealed an extremely complex phenomenology in the X-rays. Both cool gas and shocks surrounding the inner bubble have been observed. Ripples in the surrounding gas have been interpreted as sound waves generated by the activity of the central supermassive black hole. The ICM is not highly turbulent and the viscosity appears high enough to dissipate the energy carried out by the sound waves.

Data from the 900-ks exposure were sufficient to obtain detailed maps in three energy bands in the 0.3–7 keV range. The ripples are very evident out to 60–80 kpc. Temperature, density, pressure, and entropy maps were obtained. Detailed analysis of the shock fronts shows that the temperature does not change across the shock front, and this requires an efficient thermal conductive mechanism. Pressure variation in the N-S channel suggests a sequence of bubbles revealing the activity of the central radio source for the past 10⁸ years.

These studies demonstrate the complexity and wide range of phenomena produced by the interaction of the supermassive black hole at the center and the ICM. The role played by such interactions in the evolution of the ICM is not yet clear. Cluster cores are revealed as extremely complex regions which depend on the energy outburst from the central supermassive black hole and the structure of the magnetic fields which may result from past activity.

The Bullet Cluster (1E 0657-558).

It has been known for more than 70 years that the gravitational potentials of clusters of galaxies are too deep to be due to the baryon mass of the galaxies observable in the optical domain. The discovery almost 40 years ago of the intracluster plasma revealed that most of the baryonic mass was in fact in this high-temperature component previously undetected. Yet the 5-fold increase of the mass was still insufficient to account for the observed gravitational potential. This could be explained either with the existence of dark matter or with possible alterations of the law of gravity. It is only when we find an ongoing galaxy cluster merger with another cluster that we have an instance of separation of the dark matter and the intracluster plasma, and that we can discriminate between these two hypotheses. During the merger the galaxies behave as collisionless particles, whereas the X-ray-emitting plasma behaves like a fluid and experiences ram pressure. Therefore, the galaxies spatially decouple from the plasma in the course of the collision. A very interesting example exists in 1E 0657–588, a cluster at z = 0.296 discovered by Tucker, Tananbaum, and Remillard with Einstein, that was noted for its very high temperature (26).

Fig. 10 shows in the lower right corner an X-ray picture of the system obtained by Chandra in 2002 (27). The smaller cluster is moving away from the main cluster at a speed of 4,700 km/sec and the cores of the clusters passed through each other 100 Myr ago. In the absence of dark matter the gravitational potential should trace the dominant visible matter component, the X-ray plasma. If, on the other hand, the mass is dominated by dark matter, which is believed to have a small interaction cross-section, the potential will trace that component which will coincide spatially with the collisionless galaxies.

Fig. 10.

The Bullet Cluster.

The contours in the picture in the upper left of the figure show the gravitational potential derived by Clowe and his colleagues (28) from the weak and strong lensing observations carried out with a variety of optical telescopes, including the ESO-VLT, the 2.2-m European Southern Observatory/Max-Planck Gesellschaft Telescope, the 6.5-m Magellan, and the Hubble Advanced Camera for Surveys. Superposition of the mass contours on the X-ray map in the lower right shows a clear separation of the X-ray plasma from the gravitating (dark) matter. It is clear that the dark matter hypothesis is strongly favored. There is ongoing work to reconstruct the dynamical history of the cluster and to set an upper limit on the interaction cross-section of the dark matter. It is interesting to note that without X-ray observations such conclusions could not be reached, and that the Chandra sensitivity and angular resolution were essential in carrying them out.

Clusters of Galaxies and Cosmology

We have seen how Chandra images of hot plasmas in clusters of galaxies provide us with a powerful new tool to study the evolution of their energy content and of the ratio of baryons to dark matter. We have also been aware for the last 20 years of the potential use of X-ray cluster observations to probe the mass and energy content of the universe (29).

Fig. 11 shows Chandra pictures of three clusters (Abell 2009, MS 2137.3–2353, and MS 1137.5+6624) at distances from us of 1, 3.5, and 6.7 billion years. They are from a sample of 26 clusters of galaxies which have been studied by Steve Allen and his colleagues (30) to determine the ratio of the hot gas and the dark matter in the cluster (a unique capability provided by X-ray observations). This ratio depends on the assumed distance to the cluster and, if one assumes a constant ratio of baryonic to dark matter; one can determine a distance scale that best fits the data. Their result shows that the expansion of the universe was first decelerating and then began to accelerate at about 6 billion years’ age in accordance with the observations of Wilkinson Microwave Anisotropy Probe (WMAP) and Hubble.

Fig. 11.

Three Chandra clusters at different redshifts.

Recently, a very extensive series of Chandra studies has been completed on a sample of 86 clusters from ROSAT surveys (31). Fig. 12 from the work of Vikhlinin and his colleagues shows the sensitivity of the cluster mass function to the cosmological model. The left panel shows the measured mass function and the predicted model for a cosmology in close concordance Cold Dark Matter with a cosmological constant (ACDM). In the right panel, both data and model are computed with Ω_Λ = 0. The predicted number of counts is in strong disagreement with the data. The authors show how the X-ray data when used in conjunction with optical and microwave data substantially reduce the uncertainties on the cosmological parameters. A much larger sample of clusters (>2,000) could provide measurement of a linear perturbation factor with a precision of 1% for each 0.1-z bin to z = 2. The X-ray cluster deep surveys will be particularly useful when used in conjunction with data from future X-ray facilities, such as the International X-Ray Observatory (IXO), and with optical and radio observations from James Webb Space Telescope (JWST), European Extremely Large Telescope (E-ELT), and Atacama Large Millimeter Array (ALMA). Such data will be essential in imposing constraints on the dark-energy equation of state and for testing non-GR models of cosmic acceleration.

Fig. 12.

Sensitivity of the cluster mass function to the cosmological model.

Future Surveys

In such a brief talk I cannot adequately cover the hopes and expectations for future facility development in high-energy astrophysics. The ongoing National Academy of Sciences (NAS) Decadal survey is currently trying to chart a course for astronomy over the next 10–20 years. The next major X-ray facility appears to be a collaborative endeavor of ESA, Japan, and NASA, named IXO, which may come to fruition late in this period. Plans for a much larger facility [Generation-X (Gen-X)] appear even more remote, particularly considering the current NASA overcommitment in optical and perhaps in gravitational wave astronomy.

I will just mention a much more modest project which I have particularly at heart, and which could be carried out in the near future. Together with a number of colleagues at many institutions, I have been urging a new high-sensitivity, high-resolution X-ray survey of the sky that could be accomplished by use of current technology: a wide-field X-ray telescope (WFXT). Such a telescope would play the same role in X-ray astronomy as the Sloan Digital Sky Survey (SDSS) has played in optical astronomy. Whereas the sensitivity gap between the SDSS and the Hubble deep surveys is only a factor of 2.5 × 10², in X-rays the sensitivity gap between the ROSAT survey and the Chandra deep surveys is a factor between 10⁴ and 10⁵. WFXT could fill this gap, and with a resolution between 5 and 10 arc sec it could directly identify AGNs and clusters.

Fig. 13 shows how the WFXT mission could expand our horizon in cluster (and AGN) research. It will open up a largely unexplored period in cosmic history back to the epoch when the first proto clusters form and the first virialized structures start glowing in X-rays. Red dots show ROSAT clusters out to z = 1.3. Yellow dots show XMM-Newton and Chandra clusters spectroscopically confirmed to date. For WFXT, we show for clarity with blue dots only the clusters that are expected at z > 1.

Fig. 13.

WFXT mission detection of distant clusters.

Fig. 14 shows the grasp of a WFXT mission compared with all current or planned X-ray observatories. The essential feature of WFXT is an optics design that provides high-resolution imaging over the entire field (32).

Fig. 14.

Grasp of WFXT mission compared with current and planned X-ray observatories.

Very high quality measurements of cluster mass functions and spatial correlation functions over a wide range of masses, spatial scales, and redshifts will be important in expanding the cosmological discovery space. X-ray observations will be particularly useful in the study of the formation of early large structures and their interaction with the evolving supermassive black holes. They will also provide a powerful tool in searching for departures from the concordance ACDM cosmological model. Finding such departures would have far-reaching implications for our understanding of the fundamental physics which govern the universe.