A new generation of X-ray observatories is under way to study the hottest parts of the observable universe. Among them the European Space Agency’s X-ray Multi-Mirror Mission (XMM – now named XMM-Newton) will play a key role.
A universe made of silent stars and galaxies peacefully drifting through the vast emptiness of space and time – the awe-inspiring view through an optical astronomical telescope has, over recent years, been enlarged by impressions of a much more violent and dynamic universe: that of extremely hot plasmas and high-energy particles.
Ever since the first X-ray telescopes succeeded in observing astrophysical objects above the blanket of the atmosphere, they have found ubiquitous plasmas at temperatures above a million degrees, often accompanied by particles (electrons and ions) in the mega-electronvolt range and higher. In most cases the origin of the large amount of energy, oftenreleased in explosive events, is unknown. The considerable diagnostic power of these processes deserves detailed study and has been the motive for several space missions for a quarter of a century.
The Einstein and EXOSAT satellites were among the pioneers in the 1970s and 1980s, revealing a great variety of cosmic X-ray sources. The ROSAT satellite, together with a few further missions launched in the 1990s, provided the first comprehensive view of X-ray phenomena in the universe, detecting more than 150 000 sources across the sky. So far we know of X-ray emission from stellar atmospheres (such as the Sun), star-forming regions, accreting neutron stars and black hole environments, supernova remnants, the interstellar gas, external galaxies and gas in galaxy clusters. Quite unexpectedly, even small brown dwarf stars, planets and the envelopes of icy comets have recently been added to the list of prolific X-ray emitters.
What has been missing in X-ray astronomy is a class of major observatories, equivalent to the Hubble Space Telescope. Such missions, conceived in the 1980s with great foresight, are now in orbit for the first time. NASA’s Chandra X-ray observatory was successfully launched in July 1999 and has already sent back a series of breathtaking pictures and spectra. The European Space Agency’s (ESA) X-ray Multi-Mirror Mission (XMM) followed on 10 December 1999 with a picture-perfect launch on an Ariane 5 rocket.
A new view of the X-ray universe
Both missions will reach new frontiers. For the first time, high-resolution spectroscopy is routinely performed in the X-ray range. Chandra’s fine mirror optics increase the image sharpness by a factor of 10 over the best previous missions, with an angular resolution (0.5 arcseconds), thus competing with large ground-based optical telescopes.
XMM’s strength, in contrast, is its vast collecting area and therefore its sensitivity to both imaging and high-resolution spectroscopy. This is possible thanks to new technologies in mirror fabrication. In contrast with optical light, X-rays are focused through hollow hyperbolic/parabolic mirror shells by grazing reflection at the entrance of the telescope. Instead of using one or a few such mirrors (four in Chandra’s case), XMM carries three telescopes, each consisting of 58 concentrically nested mirrors that provide more collecting area than all previous X-ray satellites added together.
The three telescopes observe in parallel so that three independent imaging charge-coupled device (CCD) European photon imaging cameras (EPIC) and two reflection grating spectrometers (RGS) can be fed simultaneously. These detectors are located at the far end of the telescope, approximately 7.5 m from the mirrors.
A further unique advantage of XMM is its complementary optical telescope that, despite its small diameter of 30 cm, competes with largeground-based telescopes in its brightness limits. It addresses important scientific questions, because many high-energy sources emit not only X-rays but also ultraviolet and optical photons after reprocessing within the source.
Altogether, XMM is a huge observatory measuring more than 10 m in length and 4 m in diameter, and weighing nearly 4 tons. It constitutes the largest scientific satellite that ESA has ever built and is one of the key projects of Europe’s astrophysics science programme.
Novel X-ray instrumentation
All XMM detectors are based on CCDs. These not only allow for fine imaging with the EPIC cameras but also provide sufficient intrinsic energy resolution in the X-ray range to resolve broad spectral features and to model plasma temperatures and chemical abundances across the field of view. The latter is approximately 30 arcminutes in diameter (about the diameter of the full moon), which is ideal for the efficient mapping of large astrophysical structures (e.g. star-forming regions or supernova remnants). One of the cameras (the so-called EPIC pn) has been optimized for extremely rapid read-out through new technology down to a time resolution of 30 ms.
The converging X-rays from two of the three mirror systems pass through a reflection grating assembly – a system of numerous thin-grooved plates that disperse half of the incoming light into ahigh-resolution spectrum. The latter is then recorded by a CCD strip within the RGS detector system. The CCDs are cooled to -80 °C for optimum operation.
Dispersive systems such as the RGS are ideal for isolated point sources (e.g. stars and quasars), although some of the convolution between source geometry and spectral energy distribution can be disentangled through the modelling of extended sources as well. The energy resolution (E/DE) of the RGS detectors reaches 800 – sufficient to separate all important atomic emission lines in the range of sensitivity (0.35-2.5 keV), in particular the lines of the iron L-shell. The EPIC cameras will complement the spectroscopy up to 15 keV, albeit with much lower resolution.
The combination of the X-ray and optical/ultraviolet detectors with various filters makes XMM an efficient multiwavelength observatory that simultaneously obtains X-ray images with moderate energy resolution, high-resolution X-ray spectra of selected objects within the field of view, and a variety of optical and ultraviolet photometric and spectroscopic measurements, all with fine time resolution.
Astrophysical X-ray spectroscopy is a relatively new field that nevertheless provides the key for the understanding of many hot cosmic sources. Optically thin plasmas, with temperatures exceeding a million degrees, radiate much of their energy in spectral lines due to atomic transitions in trace elements such as carbon, nitrogen, oxygen, neon, magnesium, silicon, sulphur and, in particular, iron.
Plasma probes
These emission lines are excellent diagnostic probes of the plasma conditions. Their strengths and ratios can be used to deduce temperatures, chemical abundances and emission characteristics. “Forbidden” lines of highly ionized helium-like ions contain information about electron densities and are therefore used to deduce plasma pressure. The RGS spectral resolution is also sufficient to allow the observation of plasma motion or turbulence through wavelength shifts or line broadening, when the velocities exceed 100-200 km/s – quite common in astrophysical plasmas. Absorption features and continua provide further information on the state of the plasma.
XMM has been designed to address a range of astrophysical problems, including:
* the origin of cometary X-rays, possibly related to fluorescence or scattering of solar light;
* coronal heating in magnetic stellar atmospheres and of magnetic fields on low-mass brown dwarfs;
* the importance of magnetic accretion to spin-down of young stars and to the ionization of their immediate environments;
* the determination of the metallicity (composition), distribution and energy budget of the hot interstellar gas with implications for galactic evolution;
* the spatial and spectral study of supernova remnants to understand the chemical evolution in their progenitors, the expansion of their shells into the interstellar space and therefore the chemical enrichment of the galaxy;
* the measurement of accretion phenomena on neutron stars and towards black holes in binary systems to infer the geometry of these systems or the role of magnetic fields;
* the observation of direct and reprocessed radiation from accretion disks around massive nuclei of active galaxies;
* the measurement of the radial variations of the density, temperature and metallicity of the gas in galaxy clusters, to deduce their mass composition and to study elemental enrichment due to supernova ejecta.
Using XMM’s capabilities
Unravelling the mysteries of cosmic sources often requires us to disentangle a variety of parallel processes; the luxury of well defined physical states familiar to laboratory physicists is rarely available. In a good example of this complexity, the heating of cosmic plasmas is poorly understood, even if large source samples are at hand. Almost half of the observed X-ray sources are stars that are surrounded by magnetically enclosed, structured and highly dynamic atmospheres (coronae) heated to several million degrees. The heating mechanism is unknown, but it may be related to explosive energy releases – so-called flares – that transform free magnetic energy into heat.
A popular theory holds that unstable magnetic fields reconnect and accelerate electrons and ions. The particles travel along the magnetic field lines towards the denser but much cooler gases near the stellar surface. Collisions with the ambient cool gas provide signatures of non-thermal X-rays, perhaps marginally detectable by the sensitive EPIC cameras at 15 keV. Subsequent prompt heating of the cool layers produces a first light flash in the ultraviolet regime – the domain of the XMM optical monitor – thus providing an important measure of the energy and of the involved surface area.
As the high-energy particles lose their energy in the target through thermalization, a rapid temperature increase to tens of millions of degrees makes the plasma radiate X-rays, and new diagnostic emission lines show up in the RGS. The time profile, sensitively recorded by the EPIC cameras, depends on the incoming energy, radiative losses and conductive losses. The pressure build-up drives the plasma along the magnetic field lines away from the star at approximately the speed of sound. Spectral lines become Doppler-shifted in the early phase, or broadened through turbulence – signatures that may become measurable in the RGS spectra.
Fractionation of elements (elements with a low first ionization potential appear to be preferentially lifted into the atmosphere) can now be determined through ratios of lines from the different elements available from X-ray spectroscopy.
As the pressure in the closed magnetic fields builds up, forbidden atomic lines become suppressed: several of the easily observable lines in the high-resolution spectra thus provide ideal barometers. Rapid heating may drive the plasma out of ionization equilibrium for some time, and cooling plasma may eventually absorb some of the X-ray emission. Again, spectroscopy provides unequivocal information about such processes.
Powerful tool
Given that only simplistic models are to hand and many details are not yet understood, the combined analysis of all datasets will provide a powerful tool to refine models and to help us to understand cosmic plasmas. Solving the mysteries in the above example alone would help greatly in the understanding of explosive plasma heating mechanisms in many astrophysical sources. It would also help to reveal the source of solar and stellar winds (of importance to the understanding of solar-terrestrial physics), which are pivotal in star formation and stellar evolution. In addition it would help us to understand the physics of mass accretion from accretion discs to forming stars, explain parts of the diffuse X-ray background seen in our, and external, galaxies, and perhaps even contribute to a modelling of the origin of low-energy cosmic rays. In-depth observations of many other types of object will further contribute to our understanding of the high-energy universe.
XMM science observations are organized mainly in two sections:
* Guaranteed Time has been allocated for the instrument Co-Investigator research institutes. This provides a unique opportunity for these institutes to define large, fundamental astrophysical projects within international expert teams. It also guarantees an appropriate scientific return for the instrument-contributing institutes and countries.
* The Guest Observer Programme for the first two years is the outcome of projects submitted by astronomers worldwide and selected in the autumn of 1999 by expert panels. It contains typically shorter projects to be worked out later by the proposing teams.
XMM’s launch on board the fourth Ariane 5 was a great success for both the Ariane programme and for XMM. After a flawless countdown, a precise launch carried XMM into a high orbit with a period of 48 h. The orbit injection was so accurate that enough fuel was left on board to extend XMM’s lifetime by up to 20 years.
An extensive commissioning phase culminated in the release of the first pictures and spectra on 9 February as a foretaste of more science to come. On the same day XMM was renamed XMM-Newton, in honour of the great physicist and inventor of spectroscopy, a science that has now made its way into X-ray astrophysics.
* The Swiss Paul Scherrer Institute (PSI) is one of the Instrument Co-Investigator institutes for the reflection grating spectrometer, together with the Mullard Space Science Laboratory (MSSL), UK, and Columbia University, under the leadership of the Space Research Organization of the Netherlands. Dr Güdel is also observing principal investigator of several observing programmes in the Guaranteed and Guest Observer sections, to be undertaken with these instruments, and devoted to magnetized stellar plasmas and hot stellar winds, including star-formation regions.
