XMM Science Objectives


X-ray astronomy primarily involves the study of plasmas with thermal temperatures in the range of 1E6 to 1E8 K. Such plasmas radiate the bulk of their energy in the X-ray regime, from ca. 0.1 to 10 keV. XMM works in the energy range from 0.1 to 15 keV. Apart from X-ray continuum emission, produced through processes such as e.g. thermal bremsstrahlung, a significant fraction of the total emissivity of hot thermal plasmas can arise from line emission. At the high temperatures mentioned above, abundant elements in cosmic gases, such as hydrogen and helium, are stripped of all their electrons. Only heavier elements can, depending on the temperature, retain their K or L shell electrons. Amongst the most prominent X-ray line emitting elements in the XMM passband are Iron, Oxygen, Magnesium, Sulfur, Silicon, Sodium, Calcium, Argon, Neon and Nickel. The study of transitions from these elements which are primarily in a hydrogenic or helium-like state (i.e. with either all or all but one electrons left in their outermost shell), represents an important diagnostic tool for an understanding of the physics of cosmic X-ray sources.

XMM's science objectives are described here in two parts:


XMM is designed specifically to investigate in detail the X-ray emission characteristics, i.e. the emission distributions, the spectra and the temporal variability, of cosmic sources down to a limiting flux of order 1E-16 erg/(s cm²). With its high throughput and moderate angular resolution, XMM is extremely sensitive to low surface brightness X-ray emission. Some astronomical sources (see below) are prominent X-ray emitters, but faint or even invisible in other parts of the electromagnetic spectrum. Therefore, high-quality X-ray observations of these objects are very important and cannot be replaced by data obtained through other observing techniques. Instead, X-ray observations supplement data from other wavebands, leading to a more complete picture of the universe. Other objects are bright not only in the X-ray, but also in other parts of the spectrum, e.g. the optical or UV. Due to internal reprocessing, some sources emit both X-rays and other photons, but the optical or UV emission sometimes lags the X-ray light. Therefore, to further broaden the scope of the investigations, the Optical Monitor (OM) onboard XMM offers the possibility to simultaneously study the optical/UV properties of the observed X-ray sources. The basic characteristics of XMM's X-ray telescopes are a 6" (FWHM) point-spread function, a 30' field of view, spectroscopic resolution (E/dE) in the range from a few ten to several hundred and a large effective area of 4650 cm². A more detailed description of XMM is provided in the XMM Users' Handbook.

X-ray observations can be conducted in different ways, depending on the scientific goals of the investigator. XMM has different science instruments, each of which can be operated in different modes so that the observations can be tuned to the scientific needs. The basic observing techniques are:

In addition, with its Optical Monitor (an optical/UV telescope mounted parallel to the three X-ray telescopes), XMM can perform another basic task:


XMM carries telescopes with CCD cameras in their focal surfaces which can image the X-ray sky with very high sensitivity and good angular resolution. This way, "pictures" of the sky as seen at X-ray energies are created. Similarly, images of the optical/UV emission from the same regions on the sky can be obtained contemporaneously with the OM.

X-ray spectroscopy

The same X-ray CCD cameras that are used for imaging can also register the energy of incoming X-ray photons. Therefore, radiation can also be analysed with respect to its spectral characteristics within the XMM passband (from 0.1 to 15 keV energy). The spectral resolution of the CCDs is only moderate (but as good as ASCA's!) and does not reveal the full complexity of many X-ray spectra. Therefore, XMM carries a different type of spectrometer, with much higher spectral resolution for very detailed studies in the 0.35 to 2.5 keV energy range, so-called "Reflection Grating Spectrometer"s (RGSs). The OM offers grisms for simultaneous low-resolution optical or UV spectroscopy.


The time of each photon's detection within the X-ray detector, in addition to the direction and energy (as in imaging mode), can be registered. This allows observers to perform studies of the homogeneity or variability of X-ray sources over time by counting how many events were registered over short time intervals. Operating the OM in its fast mode, the arrival times of individual optical or UV photons can be registered in the same fashion, thus allowing for comparative timing studies.

Optical identification of X-ray sources

It is a general goal of X-ray missions to detect and identify X-ray sources on the sky. XMM has the Optical Monitor (OM) onboard for contemporaneous X-ray and optical/UV observations. Both, the X-ray telescopes and the OM, are very sensitive and capable of detecting faint sources. However, since the pointing of satellites is not always perfectly accurate, it is sometimes difficult to determine unambiguously which X-ray emitting objects that might be visible on optical images of the sky have actually been observed. XMM has good X-ray imaging capabilities, with a width of the point-spread function's core of only 6". Together with good pointing reliability, this will make sure that the X-ray and optical images will be well-aligned, making it easy to identify sources in the field of view by comparing the images from the different instruments.


The above observing techniques can be used to study many different types of X-ray sources on the sky with XMM. We outline below, in very general terms, some of the areas in which XMM can help us make progress in the observation - and thereby our understanding - of celestial objects.

Cosmic X-ray background radiation

The question how much of the observed "diffuse" X-ray background radiation comes from discrete sources and which fraction of it is truely diffuse is the matter of a long-lasting debate. The technique to investigate the nature of the extragalactic X-ray background is to obtain extremely sensitive observations of an "empty" field, i.e. a "deep field" pointing. In this example we show a simulation of a 200 ks XMM EPIC observation, performed by D. Lumb.

With extremely long XMM observations a limiting flux of order 10E-16 erg/(s cm²) can be reached (with a PSF of 6"), compared to ca. 10E-15 erg/(s cm²) (over a 35" beam) with ROSAT. Therefore, a much lower confusion limit will be reached on the search for discrete sources in the early Universe. This will render it possible to extend current log(N)-log(S) plots (of the number of sources, N, found in a certain flux interval [S to S+dS]) for population studies over cosmological time scales to much fainter flux limits than before.

Typical XMM pointings will be quite long (of order several ks to several 10 ks). This, together with XMM's large photon collecting area, will make average X-ray pointings very sensitive observations. Thus, significant results on the different sources contributing to the X-ray background will also come from studies of serendipitous sources in XMM pointings. Like this, good source count statistics can be achieved over large areas on the sky. In this context it is also important to note that XMM will observe up to 15 keV, where the optical depth for incoming X-ray radiation is lower than, for example, in the ROSAT band.

Elliptical galaxies and clusters of galaxies

The distribution of the gravitationally heated hot gas in elliptical galaxies and clusters, under the assumption that it is approximately in hydrostatic equilibrium, reflects the distribution of mass, i.e. the shape of the gravitational potential of these systems; the gas temperature is a measure of the depth of the potential well in which it is confined. The typical temperatures, namely just below 1 keV (indicating equilibrium temperatures of order 10E7 K) for single ellipticals, kT = 1...2 keV (1 to 2x10E7 K) for poor clusters and values in the 2 to 10 keV range for rich clusters (2x10E7 to 10E8 K) result in a maximum emissivity in the XMM band. Typical X-ray luminosities range from ca. 10E41 erg/s for individual elliptical galaxies to 10E45 erg/s for rich clusters. Thus, rich clusters belong - together with AGNs and QSOs - to the most luminous X-ray sources in the Universe.

XMM observations can be used to study several key properties of the hot intracluster medium. Spatially resolved spectroscopy will allow the determination of the radial variations of the gas density, temperature and metallicity. The knowledge of metallicities is important in the context of the chemical evolution of galaxies and gas in clusters. Central cooling flows, with decreasing gas temperatures towards the centres of the clusters, might lead to the accretion of relatively cool gas in the nuclear regions of elliptical galaxies and clusters. Such material potentially feeds the central engine and/or relates to present-day star formation in these systems. X-ray observations can also be used to map the distribution of hot gas and thereby the distribution of matter in elliptical galaxies and clusters. Up to 30% of the total mass of galaxy clusters has been identified as X-ray emitting intracluster gas. This is a significant fraction of the formerly "missing" mass and XMM offers a possibility to trace even fainter emission than previous satellites (and thus more gas and thereby mass).

In some cases, e.g. the Perseus cluster, X-ray observations can also be used to study the interaction of a jet emanating from a radio core in a massive elliptical galaxy with the ambient hot gas.

XMM's sensitivity will allow us to observe clusters out to cosmologically relevant redshifts (z>1), rendering possible investigations on the cosmological evolution of galaxy clusters and their central galaxies. As an example, we display here a simulation of EPIC spectroscopy of the galaxy cluster Abell 2199, calculated by H. Siddiqui with the XMM Science Simulator (SciSim) software.

Normal and starburst galaxies

External galaxies host several kinds of objects that are bright enough in the X-ray regime to be observable for modern X-ray observatories such as XMM. Among these, Seyfert-1 nuclei are the potentially brightest sources (see below). Other bright sources are high-mass X-ray binaries (HMXRBs; also below). HMXRBs are the most likely candidates for the brightest unresolved sources detected outside the nuclei of external galaxies so far. XMM offers a sensitivity that will allow for spectroscopic studies of individual bright X-ray binaries in galaxies out to the distance of the Virgo cluster (ca. 15 Mpc). Another prominent contributor to the X-ray emission of galaxies is the hot phase of their ISM. Starburst galaxies in particular have relatively high X-ray luminosities (of order 10E41 erg/s), of which a substantial fraction comes from diffuse hot gas. Detailed spectroscopy of this gas will allow us to assess the abundance of various elements in different areas within these galaxies and thereby get a handle on their chemical composition and evolution. XMM, with its spatial resolution of 6", allows the observer a much better discrimination between compact sources and diffuse emission in galaxies than had been possible before. The above implies that XMM will lead us away from integral spectra of entire galaxies in the past to dedicated X-ray studies of various individual X-ray sources that they comprise.

Investigations of the hot ionised medium in galaxies are not restricted to their disks - soft X-ray emission has been detected from the halos of up to now about 20 nearby edge-on galaxies. Most of these are starburst galaxies (with very high star formation rates), but also galaxies not classified as starbursts can have enough power to heat their halos to X-ray temperatures (e.g. NGC 891). The hot gas produced in large numbers of type II supernovae can in some cases be shown to reach escape velocity. Therefore, starburst superwind outflows are a likely source for metal injection into intergalactic space. Since many galaxies in the past (at z = 0.6 to 0.7) show extremely blue colours indicative of massive star formation, the rate at which metals were expelled into the intergalactic medium might have been much higher in the past than it is now. This could contribute significantly to the detected metal content of absorption line systems on the sightlines towards distant quasars. XMM will help us to put much tighter constraints on various ascpects of this scenario, including e.g. the metallicity of gas leaving galaxies.

Active galactic nuclei and quasars

As in many other parts of the electromagnetic spectrum, most AGNs and QSOs are also bright in the X-ray regime. In the framework of the unified scheme of AGNs, it is believed that hard X-ray emission (typically with a power law spectral slope) from Seyfert 1 systems arises from the nuclei. Additional radiation might come from the broad-line regions (BLRs) encompassing the cores. In the same theory, the emission of Seyfert 2 nuclei is thought to be composed of heavily absorbed continuum radiation from the nuclei and, superimposed on this, scattered nuclear continuum flux from material close to the rotation axis of the massive central object that is re-emitted at lower energies. The difference between the two types of Seyfert nuclei in this picture is that Seyfert 1 cores are directly visible (in viewing geometries close to face-on), while Seyfert 2's are heavily obscured by the molecular tori around their central black holes (i.e. viewed at higher inclination angles). BL Lacs are believed to be face-on Seyfert 1 systems with their beams directed exactly towards us, which gives rise to the observed hard and highly variable X-ray radiation. BL Lac spectra lack line emission, probably because little reflected radiation is observed in these viewing geometries.

Current X-ray observations suffer from weaknesses in spectral resolution, limited width of energy passband, lack of simultaneous optical/UV data, insufficient timing resolution or combinations of these. XMM will allow for spectroscopy of X-ray sources with a spectral resolution of ca. 3.5 eV at 1 keV energy (RGS, -1. order) and even ca. 1.5 eV in the -2. grating order. This, in conjunction with XMM's large effective area and its well-sampled line spread function (LSF), will enable users to obtain high signal-to-noise spectra in the energy range from 0.35 to 2.5 keV (together with moderate resolution imaging spectroscopy over the 0.1 to 15 keV band) for detailed investigations of the diagnostic lines in the spectra of AGNs. The fact that the XMM bandpass reaches to an energy of 15 keV is important, because one can penetrate very deep into the central area of an AGN at high energies due to the low absorption cross section of high-energy X-rays in interstellar matter.

For research on AGNs and QSOs it is of particular importance that sources can be observed simultaneously with the Optical Monitor (OM), which allows for studies of the time lag between the emission of (primary) X-ray and (secondary) optical/UV radiation, a quantity that will provide crucial information on the internal radiation transport, i.e. the re-processing of the emission coming from the nuclear area.

Stellar black holes, neutron stars, pulsars, binary stars

Accreting systems in general are potentially bright X-ray sources. This is not restricted to massive black holes in the centres of galaxies, but applies to stellar accretion systems as well, as e.g. cataclysmic variable stars (where mass accretion from a low-mass companion is occurring onto a white dwarf) and X-ray binaries. In X-ray binaries, the accreting object is either a neutron star or a black hole and the mass flow comes from a companion star. The X-ray luminosities can be extreme, up to of order 10E39 erg/s. This number applies to high-mass X-ray binaries (HMXRBs), i.e. black hole candidates, which thus can produce a substantial fraction (up to some 10%) of the total observable X-ray flux of an entire galaxy. The spectra of X-ray binaries have approximately power law shapes with a high energy cutoff around 10-20 keV (which distinguishes them from AGN power law spectra) and 6.4 keV Fe K alpha line emission. The Fe line emission is a diagnostic to investigate the matter encompassing a binary star (typical column densities, i.e. numbers of atoms/ions along the line of sight to an emitting source, lie in the range from 10E22 to 10E23/cm²). Fe 6.4 keV fluorescence emission with a high equivalent width but low line width is expected from relatively cool matter and Fe at relatively low ionisation stages, while 6.7 keV emission arises from radiative recombination of H-like Fe followed by cascade processes in relatively hot matter. XMM, with its good energy resolution and simultaneous high sensitivity, will be very well-suited to conduct such studies into great detail, thus revealing the internal geometry of binary systems.

Another potentially interesting area of future XMM research lies in X-ray timing analyses of binary stars. X-ray emission is observed e.g. from pulsars in binaries. Pulse periods range from the millisecond regime up to ca. 15 minutes. One extraordinary feature of X-ray pulsars, e.g., is that they are "spinning up", i.e. decreasing the length of their pulsation period over time, while radio pulsars are invariably spinning down. The pulse profiles of pulsars contain information not only on the geometrical configuration of these systems, but also on the accretion column near the magnetic poles of the neutron stars. The EPIC p-n camera offers extremely high time resolution for studies of rapidly variable sources. Since the changes in pulse profiles are accompanied by changes in the energy spectra, it is argued that anisotropic radiation transfer (Thomson scattering) must play an important role in pulsars. Again, much information will also be contained in the Fe emission lines, because the 6.4 keV radiation is caused by fluorescent re-emission of continuum emission absorbed by relatively cool matter surrounding the central source. Thus, X-ray observations can help in various ways to properly constrain radiation transfer calculations.

New, improved X-ray observations will also contribute to more accurate mass determinations of neutron stars, which can be compared with radio measurements to see whether X-ray data can corroborate the "canonical" value of 1.4 solar masses.

Supernova remnants

Among the most prominent Galactic X-ray sources are supernova remnants (SNRs). X-rays are emitted from the hot gas itself and also when the expanding gaseous shell of the SNR hits and shock-ionises the ambient medium. X-ray observations, in particular imaging spectroscopy with good spatial and spectral resolution and high-resolution spectroscopy, will provide details of the ongoing line emission processes (thermal vs. nonthermal emission), thus tracing the interaction of expanding shell and ambient medium. Earlier ROSAT observations have, for example, for the first time been used to investigate so-called "bullets" of gas preceding the general shell on their way into the surrounding medium. It is most likely that the expansion of this material has been locally faster than in its surrounding due to the existence of cavities in the ambient gas.

With temperatures in the range of a few tenths of a keV to a few keV, SNRs emit the maximum of their thermal radiation in the XMM bandpass. A wealth of lines from elements, such as Fe, O, Mg, S, Si, Na, Ca, Ni, Ne and Ar, can be expected, primarily in the energy range from ca. 0.5 to 2 keV, but also at higher energies (Fe K alpha at 6.4 keV). Detailed studies of these lines, revealing the temperature and ionisation structure of the SNRs, are first-class diagnostics of the nuclear fusion processes going on in the SN precursor stars, of the metal enrichment of interstellar matter via SNRs and of the importance of shock heating.

SNRs are also believed to contribute measureably to the total soft X-ray spectra of external galaxies. Extragalactic supernovae most likely to be seen in the X-ray regime (types II and V) have massive progenitors. However, X-ray observations of extragalactic supernovae and their remnants are still sparse. This is caused by a lack of both angular resolution and sensitivity of previous missions, two areas in which XMM will make significant progress.

The hot phase of the Galactic ISM

Early soft X-ray all-sky surveys, in particular that conducted by the University of Wisconsin in the 1970'ies, indicated that the intensity of diffuse soft X-ray radiation received from high Galactic latitudes is higher than within the disk. This led to the discovery of a soft X-ray halo in our Galaxy. Most of the halo emission is irradiated at an energy of about 0.25 - 0.3 keV, similar to the temperatures of halos in external galaxies. Galactic studies, despite our unfavourable location and viewing geometry from within the disk (and therefore also within the X-ray emitting medium), offer the chance to study the properties of the hot gas in great detail, like e.g. its volume filling factor, total energy content and its distribution with respect to the other phases of the ISM.

ROSAT for the first time provided a detailed view of the Galactic X-ray halo. However, many questions remained unanswered. XMM offers us a chance to revisit particularly interesting objects with much improved sensitivity and resolution in both the spatial and the spectral regime. Such data will provide better constraints for radiation transport calculations, which will shed new light on the composition of the ISM and its total energy balance, including the sources of ionisation. The metallicity and energy balance of hot gas in the Galactic halo are key parameters in investigations of the chemical evolution of our Galaxy via metal enrichment of the Galactic ISM by stellar winds and supernovae and the distribution of metals via disk-halo interactions (so-called Galactic "fountains" or "chimneys", depending on outflow velocity).

Cool gas

Not only the hot phase of the ISM can be studied, but also the properties of cooler gas (with temperatures far below 1E6 K). This material absorbs X-rays quite efficiently. The absorption characteristics of the gas can be used to study, e.g. its metal content. XMM will be able to measure absorption edges due to intervening gas on the sightlines to background X-ray emitters, thus helping us to understand the physics and chemistry of the foreground material. The depth of an absorption edge is a direct measure of the abundance of the element causing the edge. The total amount of gas along the sightline can also be determined by fitting an equivalent HI column density to the low energy part of observed X-ray spectra, which suffer from absorption losses on the way to the observer, primarily at energies below 0.4 keV. This is one kind of investigation in which sources are not studied in emission, but absorption.

Stellar coronae

While it had been known for quite a while that massive stars, primarily of spectral types O and B, are X-ray sources, it has been found only by recent satellite missions that stars of almost all spectral types emit X-rays. The bulk of the X-ray emission is thought to come from their million degree coronae, which are thin plasmas approximately in collisional equilibrium. However, stellar X-ray luminosities are low, making them invisible outside our own Galaxy. Stellar X-ray luminosities are of order 10E26 to 10E31 erg/s for late-type stars (spectral types F to M) and ca. 10E29 to 10E34 erg/s for the above-mentioned early-type stars (O and B).

With its extreme sensitivity, XMM will make many more stars (probably several thousand) accessible to X-ray observations than any other previous mission. Important questions to be addressed by such observations are, e.g. the heating mechanism of the coronae (which is as yet unknown), and various aspects of the interaction of the hot coronal plasma with the stellar magnetic field. XMM will enable both more detailed studies of individual stars as well as better statistical studies of much larger samples than available at present.

A SciSim simulation of an RGS spectrum of the nearby star Capella gives an impression of the wealth of lines and the spectral resolution and sensitivity to be expected.


ROSAT and SAX observations for the first time revealed that comets are sources of X-ray emission. In fact, it is currently not understood which mechanism dominates the unexpectedly high observed fluxes observed, e.g. from comets Hyakutake and Hale-Bopp. In the case of Hale-Bopp, evidence has been found that the X-ray emission arises from an interaction of Solar X-rays with cometary dust. In other cases, however, scattering of Solar X-rays and fluorescent emission in the coma cannot account for the observed flux densities (falling short of the observed values by about factors of 10³ or more) and the source geometries. Some X-ray emission was found to be offset from the nuclei of the comets, along a line normal to the Sun-nucleus line, which is inconsistent with models of pure reflection of Solar radiation. Therefore, although scattering of Solar X-rays by dust appears to play an important role in one observed event in Hale-Bopp, no general solution as to the origin of X-rays from comets is known yet.

XMM observations will help in the future to observationally confirm which of the several currently controversial theoretical models describes the X-ray emission of comets correctly. Given the extremely limited amount of solid information available in the literature at the moment, one can expect XMM to perform pioneering work in this field. One should bear in mind, however, that XMM cannot track the fast movement of comets once they are nearby. Instead, one will need to let the source pass through XMM's field of view.


The above indicates that, within the rapidly evolving field of X-ray astronomy, XMM will be a major milestone. Its expected versatility in astronomical research is based on its very high sensitivity, good optics and innovative detectors. Compared to the current generation of X-ray satellites, XMM will offer improved capabilities in all three general observing techniques (imaging, spectroscopy and photometry), with the additional advantages of a wide energy passband and simultaneous optical/UV observations.

[last update: 24 Feb 1998 by M. Dahlem]
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