Introduction
Introduction

Before the late 1970s, only four galaxies had been detected emitting X-rays: the Milky Way, M31, and the Magellanic Clouds. Buildingon earlier work in the field, ESA’s X-ray observatory, Exosat, was launched in May 1983. It was active until April 1986, by which time it had made 1780 X-ray observations.

In 1982, an ‘X-ray Multi-Mirror’ astronomy mission was proposed. In 1984, a group of European scientists developed the ‘Horizon 2000’ long-term plan for ESA’s scientific program. The idea was to achieve a 50% increase in the annual science budget over the following five years. Central to this plan was the concept of four ‘Cornerstones’ — large-scale missions whose scientific objectiveswould be achievable. The second Cornerstone was to be a ‘High Throughput X-ray Spectroscopy’ mission or XMM by another name.

Serious work on XMM started in 1985 with the establishment of a number of working groups. The overall configuration was developedby1987, looking very much like XMM as we know it today. Following the experience with Exosat, which demonstrated the value of a highly eccentric orbit for long uninterrupted observations of X-ray sources, XMM was to be placed in a 48-hour period orbit using the Ariane 4 launcher.

The payload now featured only four X-ray mirror systems. However a very important feature had been added — the Optical Monitor — an instrument to allow simultaneous observation of the field of view the X-ray telescopes in the UV and visible bands. This was a lesson learned from the operation and exploitation of Exosat. An important part of XMM-Newton is that all instruments work in parallel — this is an extremely important tool in making the observatory more efficient.

ESA approved the mission in this form in June 1998. One year later the selection of the instruments and the long hardware development program began. The Survey Science Centre was selected by ESA in 1995 to develop the processing of the XMM data.

Launch

The xmm-Newton probe was launched from French Guiana on 10 December 1999 and the European Space Agency's X-ray Multi-Mirror satellite became the most powerful X-ray telescope ever placed in orbit. At launch-time scientists were confident the mission would help solve many cosmic mysteries, ranging from enigmatic black holes to the formation of galaxies.

Orbit

One of the key advantages is the satellite's highly eccentric orbit, traveling out to nearly one third of the distance to the Moon, enables very long and uninterrupted observations. Peering into deep space, XMM-Newton's science payload continues to considerably increase our knowledge of very hot objects created when the Universe was very young.

Summary

XMM-Newton is ESA's second 'Cornerstone' mission. Development and construction of the spacecraft had to overcome major technologicalhurdles. Its wafer-thin X-ray mirrors are a miracle of engineering and the smoothest ever built to date. With its five X-ray imaging cameras and spectrographs, and its optical monitoring telescope, the new space observatory has been on the cutting edge of astronomy for ten years. ESA's X-ray space observatory is unique. It is the biggest scientific satellite ever built in Europe, its telescope mirrors were amongst the most powerful ever developed in the world, and with its sensitive cameras it was designed to see much more than any previous X-ray satellite.

Overview

Intro

The name of XMM-Newton stems from the design of its mirrors, a masterpiece of engineering, the highly nested X-ray Multiple Mirrors. These are enabling astronomers to discover more X-ray sources than with any of the previous space observatories. In one day, XMM-Newton sees more sources in a small region of sky than one of the earliest X-ray satellites, UHURU, found across the whole sky during its three years in operation. But the program also has a more formal name: the High-Throughput X-ray Spectroscopy Mission. Spectroscopy allows astronomers to measure a source's composition. In the same way the color of a lamp indicates what gas is used in street lighting, the instruments of XMM-Newton will reveal the chemical composition, temperature, and even the velocity of the source.

European Photon Imaging Cameras

There are three scientific instruments aboard XMM-Newton. XMM-Newton carries telescopes with CCD cameras in their focal surfaces which 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.

The mirror modules send the image beam along the telescope tube to five cameras at the extremity of the spacecraft. At the prime focus of each of the telescopes, behind six-position filters, are three European Photon Imaging Cameras (EPIC). With silicon chips that can register extremely weak X-ray radiation, these advanced Charge-Coupled Device cameras (CCD) are capable of detecting rapid variations in intensity, down to a thousandth of a second and less!

Reflection Grating Spectrometer

The same X-ray CCD cameras that are used for imaging also register the energy of incoming X-ray photons. Therefore, radiation can also be analyzed with respect to its spectral characteristics within the XMM-Newton 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-Newton 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, the so-called “Reflection Grating Spectrometers” (RGSs). The OM offers prisms for simultaneous low-resolution optical or UV spectroscopy. For a complementary analysis of the spectrum, a grating structure on two mirror modules reflects about half of the incoming rays to a secondary focus, with its own CCD camera. This Reflection Grating Spectrometer (RGS) “fans out” the various wavelengths, thus indicating, in more detail than EPIC, the exact condition of individual elements, such as oxygen and iron.

Optical/UV Monitor

The third instrument aboard XMM-Newton is a conventional but very sensitive Optical/UV Monitor (OM), which can observe simultaneously the same regions as the X-ray telescopes, but in the ultraviolet and visible wavelengths. This gives astronomers complementary data about the X-ray sources. In orbit, this 30 cm telescope is as sensitive as a four-meter instrument on the Earth's surface. 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.
Details

European Photon Imaging Cameras

The main focal plane instrument on XMM-Newton, providing imaging and spectroscopy, is the European Photon Imaging Camera (EPIC). The three EPIC cameras offer the possibility to perform extremely sensitive imaging observations over the telescope's field of view (FOV) of 30 arcmin and in the energy range from 0.15 to 15 keV with moderate spectral (E/∆E ~ 20-50) and angular resolution (PSF, 6 arcsec FWHM).

All EPIC CCDs operate in photon counting mode with a fixed, mode dependent frame read-out frequency, producing event lists, that is tables with one entry line per received event, listing (among others) attributes of the events such as the position at which they were registered, their arrival time and their energies. Another associated experiment is the EPIC Radiation Monitor (ERM). The main function of the ERM is the detection of the radiative belts and solar flares in order to supply particle environment information for the correct operation of the EPIC camera. In addition, the ERM provides detailed monitoring of the space radiative environment constituting a reference for the development of detectors to be used in futures missions. The EPIC consortium is made up of ten institutes in four nations: the United Kingdom, Italy, France and Germany.

Charge Coupled Device

EPIC consists of three cameras, at whose heart are charge-coupled devices (CCD) which register and record the energy of incoming X-ray photons and a Radiation Monitor to register the dose of radiation seen by the spacecraft as it crosses the radiation belts surrounding the Earth. Two of the cameras employ metal oxide semi-conductors (MOS) CCDs, developed jointly by Leicester University & English Electric Valve (EEV, Chelmsford UK) while the third uses a new type of CCD (PN) developed by the Max Planck Institute of Extraterrestrial Physics in Garching, Germany.

The MOS detectors register photons in the 'soft' portion of the X-ray spectrum with good energy resolution. With only a 40-micron sensitive depth of silicon, the detectors are less responsive to high energy or 'hard' X-rays. This upper part of the spectrum will be covered better by the EPIC 'PN' CCD, which has a 300-micron thickness.

The only preceding mission using X-ray CCDs (the ASCA satellite) proved its efficiency to detect hard X-rays, but also highlighted its vulnerability to radiation damage. In order to protect all the detectors, the three cameras are well shielded with a 3 cm-thick piece of aluminum. One position on each camera filter wheel can also occult the sensitive detector. All the cameras have a large radiator that cools down the CCDs to their operating temperature of -100 °C.

Each EPIC-MOS CCD consists of seven silicon chips, each made up of a matrix of 600 x 600 pixels. Each CCD reads out in a couple of seconds and the image data is processed and compressed electronically in the instrument electronic boxes, so as to be compatible with the spacecraft's telemetry, sending the data back to the Earth ground stations.

New Technology 'PN' Detector

The development of the 'PN' detector was a seven-year effort by the semi-conductor laboratory of the Max Planck Institute for Extraterrestrial Physics, followed by over two years of integration work and testing on the camera itself. The result is an EPIC 'PN' detector with a single chip, whose design represents a radical departure from the integrated circuit conception of current MOS CCDs.

The silicon wafer has been manufactured 'in-house' with extreme precision to maintain the very high resistivity of the 300 micron-thick double-sided processed silicon detectors. This fact is responsible for the efficient detection of the high energy X-rays.

As the PN-CCD is illuminated from the rear side, which does not have insensitive layers or coatings, the X-ray detection efficiency is extremely high and homogeneous from the very low to the highest XMM-Newton energies (over 90% from 0.5 to 10 keV). The parallel readout of 768 independent channels enables the camera to be operated quickly: only 80 ms are needed to acquire one picture or frame. Special readout modes allow the observation of transient objects with a time resolution of only 40 ms.

In contrast to the MOS camera, the PN-detector has a 400 x 400 pixel matrix of 6 cm x 6 cm, monolithically fabricated on a four inch high purity silicon wafer. The 36 cm2 sensitive area makes it the largest X-ray CCD detector ever built.

Reflection Grating Spectrometer

Examining the spectra of X-ray objects

Privileged visitors to the ESTEC clean rooms during the final integration of XMM-Newton noticed that after the installation of the flight-model mirror modules, there appeared to be something missing. Only two of the three mirror modules project out of the support platform. The lack of symmetry stems from the fact that only two of the mirror modules are equipped at their exit with reflection grating arrays. With the associated cameras, they are part of the Reflection Grating Spectrometer (RGS) component of the XMM-Newton mission.

Dispersive spectroscopy fans out X-ray photons much as a prism does with visible light. It is a relatively new technique and XMM-Newton is the first ever X-ray space observatory to be equipped with reflection gratings operating in the X-ray band of the electromagnetic spectrum.

Early X-ray missions carried 'Bragg-crystal' spectrometers; later missions like EXOSAT (1983) used 'transmission gratings'. XMM-Newton is the first mission to use the latest technology which makes it possible to produce large 'reflection gratings' plates, giving simultaneously a high spectral resolution and throughput.

Reflection grating

A reflection grating is a mirror with tightly controlled grooves on it, in the case of RGS about 600 grooves per mm, equivalent to 15 grooves in the width of a human hair! X-radiation reflected off the top and the valley of the grooves interfere with each other and cause a 'spectral image' whereby X-radiation of different wavelengths (or energy) are reflected under slightly different angles. The two grating arrays on XMM-Newton are each composed of 182 grating plates. Each plate consists of a silicon carbide substrate coated with a thin (2000 Ångstrom) film of gold. Measuring 10 x 20 cm, they were produced by a replication process from a mechanically ruled master. The plates, with stiffening ribs on their rear side, are integrated onto a beryllium support structure.

Once splayed out into a spectrum, the X-rays are focused on the two RGS cameras in the spectral focal plane geometrically offset with regard to the EPIC cameras. The RGS cameras are composed of a strip of nine MOS CCDs developed by the EEV Ltd. Chelmsford (UK), and under the guidance of the Sensor Technology Development group at SRON Utrecht. These back-illuminated CCDs are extremely precise as to where an X-ray photon will fall on them, and also give an indication of the energy of the incoming photon. To reduce background noise, the cameras operate at between -80 and -120 °C. This temperature is provided by the coldness of space captured by two passive radiators on the outside of the spacecraft.

X-ray Spectrography

The spectral data provided by the CCDs of the RGS cameras is typically presented as a plotted curve displaying the presence, seen as peaks or lines, of certain elements (such as iron, oxygen, silicon) in the X-ray source under observation. For astrophysical sources, the positions and sizes of the peaks in the spectrum are measures of the temperature and the relative abundance of the different elements, respectively. The data can also provide clues as to the density of the emitting gas. The main ('first spectral order') curve will be accompanied by a second or third order curve, produced by the reflection grating assembly.

A sample RGS spectrum obtained at the long-beam facility at the Panter test facility. The upper graph displays presence of elements; the lower one shows the first and higher order spectra, separated from one another by the intrinsic resolution of the CCDs.

The wavelength band chosen for RGS (5 - 35 Ångstrom) contains the K-shell transitions of oxygen, neon, magnesium, aluminum, silicon, as well as the L-shell transitions of iron. Each time one of these transitions takes place in the atom, a distinct amount of energy is released, in the form of a photon. So, different atomic transitions generate photons with different energies, which in turn show up as different spectral lines in the X-ray spectrum.

Specrtal Features

Detailed study of these spectral features allows the physical characteristics (density, temperature, ionization state, element abundances, mass motions and red shift) of the emitting region and its surrounding environment. XMM-Newton makes it possible to study these spectral features in many types of astrophysical objects like corona of stars, binary star systems, supernova remnants, clusters of galaxies and far away active galactic nuclei.

X- ray spectroscopy can also be useful to astronomers investigating gamma ray bursts, which can also be observed at X-ray wavelengths. Like the 'after-glow' observations made by the BeppoSAX satellite (Italy-Netherlands 1996), XMM-Newton will also be able to contribute to an understanding of these phenomenally powerful and mysterious bursts of gamma rays.

Optical Monitor

First X-ray space observatory with an optical and UV capacity

By an astronomer's yardstick, the Mullard Space Science Laboratory is but a stone's throw from the British capital. Perched on hills overlooking the undulating English countryside, this research centre (part of the Department of Space and Climate Physics of University College London) is housed in a traditional Victorian mansion. A community of scientists and engineers have participated over the years in about 40 satellite missions. From Ariel-1 in 1962 to EXOSAT in 1983 and the more recent ROSAT, Giotto, ERS Earth remote sensing craft, ISO, SOHO, Cassini and Cluster... and they are planning contributions to future missions such as ESA's far-infrared telescope, Herschel.

A 15 strong team led by principal investigator Keith Mason started working on the XMM-Newton project in 1989. As leaders of the Optical Monitor multinational consortium of research institutes, they have been responsible for providing the multi-wavelength capacity of the XMM-Newton space observatory, which is unique in its ability to view simultaneously regions of the sky in the visible, ultraviolet and X-ray ranges.

“XMM-Newton was conceived from the start with this optical telescope. Previous missions had demonstrated the need. EXOSAT, for instance, had no such instrument and attempting to get simultaneous data in the visible range using ground based observatories had a very small success rate”, explains Keith Mason.

The telescope, detectors and canister tube were largely produced in-house in the laboratory's engineering workshops, partners in Belgium and the United States providing, respectively, the power supplies and the data processing units.

How does the Optical Monitor work?

The Optical/UV Monitor Telescope (XMM-OM) is mounted on the mirror support platform of XMM-Newton alongside the X-ray mirror modules. Designed and developed at the Mullard Space Science Laboratory (MSSL), the Optical Monitor is an improved Ritchey-Chrétien telescope (a telescope design that gives a high-quality image over a relatively wide field of view) with a 30 cm aperture that has a sensitivity for imaging comparable to a 4-metre instrument on the Earth's surface. It provides coverage between 170 nm and 650 nm of the central 17 arc minute square region of the X-ray field of view, permitting routine multiwavelength observations of XMM targets simultaneously in the X-ray and ultraviolet/optical bands.

After being focused by two mirrors, the light is directed towards one of two identical filter wheels and detector chains. In each, an ultra-compact electronic image intensifier amplifies the light signal a million times before it falls on a silicon detector chip (CCD) capable of registering 100 frames per second. Detailed imaging on the central region of a view is possible using a x4 magnifier situated on the filter wheels, which also incorporate two grisms for low-resolution spectroscopy. Data is then processed and compressed before being sent back to Earth as part of the spacecraft telemetry.

OM Main Characteristics

• 2 m long telescope tube
• 30 cm Ritchey-Chretien telescope
• Focal ratio of f/12.7 and focal length of 3.8 m
• Total coverage between 170 nm and 650 nm of a 17 arc min square field of view
• A primary mirror of 0.3 m and a hyperboloid secondary mirror
• Two (redundant) filter wheels with 11 apertures: one blanked off, six broad band filters (U, B, V, UVW1, UVM2 and UVW2), one white, one magnifier and two grisms (UV and optical)
• Two (redundant) detectors: micro-channel plate intensified CCD with 384 x 288 physical pixels (Active area 256x256). Photon events centroided to 1/8 physical pixel
• Two (redundant) Digital Electronics Module

The ability to observe X-ray targets simultaneously in the visible and UV is contributing to a vast increase of our knowledge of the Universe. Keith Mason and his colleagues are particularly excited about the prospect of better understanding the physical processes in quasars. These are the brightest and most distant known objects producing radiation covering the full range of the electromagnetic spectrum.

Introduction

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, roughly from 0.1 to 10 keV. XMM-Newton works in the energy range from 0.1 to 15 keV. Apart from X-ray continuum emission, produced through processes such as, for example, thermal bremsstrahlung, a significant fraction of the total emissivity of hot thermal plasmas 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-Newton 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 hydrogen-like 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-Newton's science objectives are described here in two parts:
• General science objectives
• Prominent X-ray emitting sources

Mission Objectives

XMM-Newton 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-Newton 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-Newton offers the possibility to simultaneously study the optical/UV properties of the observed X-ray sources. The basic characteristics of XMM-Newton'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-Newton is provided in the XMM-Newton Users' Handbook.

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

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

Summary

It is a general goal of X-ray missions to detect and identify X-ray sources on the sky. XMM-Newton 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-Newton 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 ensures that the X-ray and optical images are well-aligned, making it easy to identify sources in the field of view by comparing the images from the different instruments.

XMM-Newton is detecting more X-ray sources than any previous satellite and is helping to solve many cosmic mysteries of the violent Universe, from what happens in and around black holes to the formation of galaxies in the early Universe. It is designed and built to return data for at least a decade.

XMM-Newton has been able to measure for the first time the influence of the gravitational field of a neutron star on the light it emits. This measurement provides much better insight into these objects. On launch XMM-Newton became the most sensitive X-ray observatory ever launched. Its high quality focusing mirrors and battery of instruments enable it to achieve the following:
• Investigate spectra of cosmic X-ray sources with a limiting flux of 10-15 erg cm-2 s-1
• Performing sensitive medium-resolution spectroscopy with resolving powers between 100 and 700 over the wavelength band 5 - 35Å (350 - 2500 eV)
• Broad band imaging spectroscopy from 100 eV to 15 keV (0.8 - 120Å)
• Simultaneous sensitive coverage of the wavelength band 1600 to 6000Å through a dedicated co-aligned optical monitor

Operational Orbit

After launch by Ariane-5, the XMM-Newton spacecraft was placed into a 48-hour elliptical orbit around the Earth. Inclined at 40° with a Southern apogee at 114 000 km, the perigee altitude is 7000 km. XMM-Newton's operational orbit is highly eccentric and has been chosen for two reasons. First the XMM-Newton instruments need to work outside the radiation belts surrounding the Earth. These radiation belts are filled with highly energetic particles and extend out to about 40 000 km from the Earth. The radiation of the accelerated particles can cause both damage to the science instruments and false readings. Second this type of orbit allows for the longest possible uninterupted observing time.

Orbital Insertion

XMM-Newton reached its final operational orbit about eight days after being launched by Ariane-5. The satellite was initially injected into a temporary orbit, with a perigee of 850 km and an apogee of 114 000 km, and then utilized its own propulsion system to raise the perigee. Forty minutes after the satellite was released from the launcher upper stage, telemetry from XMM-Newton confirmed that the solar arrays had deployed. After checking the satellite's health and its correct orientation, engineers at the Mission Control Centre waited almost one day (22 hours) until XMM-Newton reached its first apogee. At that precise moment they ordered the first of four (eventually five) firings of XMM-Newton's thrusters, eight small jets using hydrazine propellant. Each boost occurred at apogee, progressively raising the perigee to 7000 km. Meanwhile the telescope tube was emptied of any residual gases (outgassing), the sunshield deployed, and finally the doors of the mirror modules opened.

Second, a highly eccentric orbit offers the longest possible observation periods - less interrupted by the frequent passages in the Earth's shadow that occur in a low orbit. In addition, the orbital period of XMM-Newton is exactly two times the Earth rotation period to maintain optimal contact between XMM-Newton and the ground stations tracking the satellite. This allows XMM-Newton data to be received in real-time and for it to be fed to the Mission Control Centers.

XMM-Newton's elliptical orbit is tilted at 40° to the Earth's equator, with its apogee in the Southern Hemisphere. During its orbit the spacecraft rises to nearly one third of the distance to the Moon. At apogee XMM-Newton is 114 000 km away from Earth and moving at its slowest. At perigee the velocity is nine times faster as it passes the Earth at an altitude of 7000 km.

The orbital parameters evolve as the mission progresses. As an example, the perigee altitude will vary between 7000 km and 22 000 km, while the apogee altitude will vary between 114 000 km and 100 000 km.

Orbital parameters at start of operational orbit
Period48 h
Perigee Altitude7000 km
Apogee Altitude114,000 km
Eccentricity0.79
Inclination40°
Science

Comming Soon

Introduction:

The most prominent feature of the WMAP spacecraft is a pair of back-to-back telescopes that focus the microwave radiation from two spots on the sky roughly 140° apart and feed it to 10 separate differential receivers that sit in an assembly directly underneath the optics. Large ”elephant ear“ radiators provide cooling for the sensitive amplifiers in the receiver assembly. The bottom half of the spacecraft provides the services necessary to carry out the mission including command and data collection electronics, attitude (pointing) control and determination, power services and a hydrazine propulsion system. The entire observatory is kept in continuous shade by a large deployable sun shield that also supports the solar panels.

The WMAP instrument consists of a set of passively cooled microwave radiometers (connected to radiator panels with metal straps) with 1.4 x 1.6 meter diameter primary reflectors to provide the desired angular resolution. Measuring the temperature of the microwave sky to an accuracy of one millionth of a degree requires careful attention to possible sources of systematic errors. The avoidance of systematic measurement errors drove the design of WMAP:

• The instrument has five frequency bands from 22 to 90 GHz to facilitate separation of galactic foreground signals from the cosmic background radiation.
• WMAP is a differential experiment: if you would like to know whether one piece of wood is longer than another, it is better to put the pieces directly next to each other than to measure them separately with a ruler. WMAP measures the temperature difference between two points in the sky rather than measuring absolute temperatures.
• An orbit about the Sun-Earth L2 libration point that provides for a very stable thermal environment and near 100% observing efficiency since the Sun, Earth, and Moon are always behind the instrument's field of view.
• A scan strategy that rapidly covers the sky and allows for a comparison of many sky pixels on many time scales.

The WMAP spacecraft
Other News

Soft Gamma-ray Repeaters (SGRs)

XMM-Newton has caught the fading glow of a tiny celestial object, revealing its rotation rate for the first time. The new information confirms this particular object as one of an extremely rare class of stellar zombie - the dead heart of a star that refuses to die.

There are just five so-called Soft Gamma-ray Repeaters (SGRs) known, four in the Milky Way and one in our satellite galaxy, the Large Magellanic Cloud. Each is between 6 to 19 miles (10 to 30 km) across, yet contains about twice the mass of the Sun. Each one is the collapsed core of a large star that has exploded, collectively called neutron stars.

What sets the SGRs apart from other neutron stars is that they possess magnetic fields that are up to 1,000 times stronger. This has led astronomers to call them magnetars.

NASA's Compton Gamma Ray Observatory discovered SGR 1627-41 in 1998 when it burst into life emitting around a hundred short flares during a six-week period. It then faded before X-ray telescopes could measure its rotation rate. Thus, SGR 1627-41 was the only magnetar with an unknown period.

Last summer, SGR 1627-41 flared back to life. But it was located in a region of sky that the European Space Agency's (ESA) XMM-Newton was unable to point at for another 4 months. This was because XMM-Newton has to keep its solar panels turned toward the Sun for power. So astronomers waited until Earth moved along its orbit, carrying XMM-Newton with it and bringing the object into view. During that time, SGR 1627-41 began fading fast. When it came into view in September 2008, thanks to the sensitivity of the Energetic Particles and Ion Composition (EPIC) instrument on XMM-Newton, it was still detectable.

A team of astronomers made the observations and revealed that the object rotates once every 2.6 seconds. "This makes it the second fastest rotating magnetar known," said Sandro Mereghetti, INAF/Istituto di Astrofisica Spaziale e Fisica Cosmica, Milan, one of the team members.

Theorists are still puzzling over how these objects can have such strong magnetic fields. One idea is that they are born spinning very quickly, completing one rotation every 2-3 milliseconds. Ordinary neutron stars are born spinning at least ten times more slowly. The rapid rotation of a newborn magnetar, combined with convection patterns in its interior, gives it a highly efficient dynamo, which builds up an enormous field.

With a rotation rate of 2.6 seconds, this magnetar must be old enough to have slowed down. Another clue to the magnetar's age is that it is still surrounded by a supernova remnant. During the measurement of its rotation rate, XMM-Newton also detected X-rays coming from the debris of an exploded star, possibly the same one that created the magnetar. "These usually fade to invisibility after a few tens of thousand years," said Mereghetti. "The fact that we still see this one means it is probably only a few thousand years old."

If it flares again, the team plans to re-measure its rotation rate. Any difference will tell them how quickly the object is decelerating. There is also the chance that SGR 1627-41 will release a giant flare. Only three such events have been seen in the last 30 years, each from a different SGR, but not from SGR 1627-41.

These super flares can supply as much energy to Earth as solar flares, even though they are halfway across the galaxy, whereas the Sun is at our celestial doorstep. "These are intriguing objects; we have much still to learn about them," said Mereghetti

Black hole boasts heavyweight jets

Astronomers studying a black hole in our Galaxy with ESA's XMM-Newton observatory have made a surprising discovery about the cocktail of particles that are ejected from its surroundings.

Stellar-mass black holes are often found feasting on material from a companion star. Matter flows from the star towards the black hole, circling in a disc around it with a temperature so high that it emits X-rays.

The black hole can be a fussy eater: instead of swallowing all of the material, it sometimes pushes a fraction of it away in the form of two powerful jets of particles. Because these jets release mass and energy into the surroundings, the black hole has less material to feed on.

By studying the composition of the jets, we can learn more about the feeding habits of black holes.

Observations at radio wavelengths have already found that black hole jets contain electrons moving at close to the speed of light. But, until now, it was not clear whether the negative charge of the electrons is complemented by their anti-particles, positrons, or rather by heavier positively-charged particles in the jets, like protons or atomic nuclei.

In a new study, astronomers have used XMM-Newton to study a black hole binary system called 4U1630-47, well known to show outbursts of X-rays over periods of months and years.

"In our observations, we found signs of highly ionised nuclei of two heavy elements, iron and nickel," says María Díaz Trigo of the European Southern Observatory in Munich, Germany, lead author of the paper published in the journal Nature.

"The discovery came as a surprise – and a good one, since it shows beyond doubt that the composition of black hole jets is much richer than just electrons."

The team of astronomers observed 4U1630-47 with XMM-Newton on two occasions in September 2012, and compared the results with near-simultaneous radio observations from the Australia Telescope Compact Array.

Although the two sets of observations described by Dr Díaz Trigo and collaborators were separated by only a couple of weeks, the results were surprisingly different.

In the first set of observations, the astronomers detected X-rays from the accretion disc, but did not see anything in radio waves – a sign that the jets were not active.

But in the second set, they detected the source both in X-rays and radio waves, so they knew the jets had been reactivated in the meantime.

When scrutinizing the X-ray data from the second batch of observations, the astronomers also found tell-tale signs of iron nuclei moving both towards and away from XMM-Newton, providing confirmation that the ions belong to the two jets, pointing in opposite directions.

The astronomers also found evidence of nickel nuclei in the jet pointing towards XMM-Newton.

"From these 'fingerprints' of iron and nickel, we could show that the speed of the jet is very high, about two-thirds of the speed of light," says co-author James Miller-Jones from the Curtin University node of the International Centre for Radio Astronomy Research in Perth, Australia.

"Moreover, the presence of heavy atomic nuclei in black hole jets means that mass and energy are being carried away from the black hole in much larger amounts than we previously thought, which may have an impact on the mechanism and rate by which the black hole accretes matter," adds co-author Simone Migliari from the University of Barcelona, Spain.

This is the first time that heavy nuclei have been detected in the jets of a relatively typical stellar-mass black hole.

There is only one other X-ray binary that shows similar signatures from atomic nuclei in its jets – a source known as SS 433. This black hole system, however, is characterized by an unusually high accretion rate, which makes it difficult to compare its properties to those of more ordinary black holes.

The new observations of 4U1630-47 will help astronomers learn more about the physical mechanism that launches jets from a black hole's accretion disc.

"While we now know a great deal about black holes and what happens around them, the formation of jets is still a big puzzle, so this observation is a major step forward in understanding this fascinating phenomenon," says Norbert Schartel, ESA's XMM-Newton Project Scientist.

Unique pair of hidden black holes discovered by XMM-Newton

A pair of supermassive black holes in orbit around one another have been spotted by XMM-Newton. This is the first time such a pair have been seen in an ordinary galaxy. They were discovered because they ripped apart a star when the space observatory happened to be looking in their direction.

Most massive galaxies in the Universe are thought to harbor at least one supermassive black hole at their centre. Two supermassive black holes are the smoking gun that the galaxy has merged with another. Thus, finding binary supermassive black holes can tell astronomers about how galaxies evolved into their present-day shapes and sizes.

To date, only a few candidates for close binary supermassive black holes have been found. All are in active galaxies where they are constantly ripping gas clouds apart, in the prelude to crushing them out of existence.

In the process of destruction, the gas is heated so much that it shines at many wavelengths, including X-rays. This gives the galaxy an unusually bright center, and leads to it being called active. The new discovery, reported by Fukun Liu, Peking University, Beijing, China, and colleagues, is important because it is the first to be found in a galaxy that is not active.

"There might be a whole population of quiescent galaxies that host binary black holes in their centers," says co-author Stefanie Komossa, Max-Planck-Institut für Radioastronomie, Bonn, Germany. But finding them is a difficult task because in quiescent galaxies, there are no gas clouds feeding the black holes, and so the cores of these galaxies are truly dark.

The only hope that the astronomers have is to be looking in the right direction at the moment one of the black holes goes to work, and rips a star to pieces. Such an occurrence is called a 'tidal disruption event'. As the star is pulled apart by the gravity of the black hole, it gives out a flare of X-rays.

In an active galaxy, the black hole is continuously fed by gas clouds. In a quiescent galaxy, the black hole is fed by tidal disruption events that occur sporadically and are impossible to predict. So, to increase the chances of catching such an event, researchers use ESA's X-ray observatory, XMM-Newton, in a novel way.

Usually, the observatory collects data from designated targets, one at a time. Once it completes an observation, it slews to the next. The trick is that during this movement, XMM-Newton keeps the instruments turned on and recording. Effectively this surveys the sky in a random pattern, producing data that can be analyzed for unknown or unexpected sources of X-rays.

On 10 June 2010, a tidal disruption event was spotted by XMM-Newton in galaxy SDSS J120136.02+300305.5. Komossa and colleagues were scanning the data for such events and scheduled follow-up observations just days later with XMM-Newton and NASA's Swift satellite.

The galaxy was still spilling X-rays into space. It looked exactly like a tidal disruption event caused by a supermassive black hole but as they tracked the slowly fading emission day after day something strange happened.

The X-rays fell below detectable levels between days 27 and 48 after the discovery. Then they re-appeared and continued to follow a more expected fading rate, as if nothing had happened.

Now, thanks to Fukun Liu, the behavior can be explained. "This is exactly what you would expect from a pair of supermassive black holes orbiting one another," says Liu.

Liu had been working on models of black hole binary systems that predicted a sudden plunge to darkness and then the recovery because the gravity of one of the black holes disrupted the flow of gas onto the other, temporarily depriving it of fuel to fire the X-ray flare. He found that two possible configurations were possible to reproduce the observations of J120136.

In the first, the primary black hole contained 10 million solar masses and was orbited by a black hole of about a million solar masses in an elliptical orbit. In the second solution, the primary black hole was about a million solar masses and in a circular orbit.

In both cases, the separation between the black holes was relatively small: 0.6 milliparsecs, or about 2 thousandths of a light year. This is about the width of our Solar System.

Being this close, the fate of this newly discovered black hole pair is sealed. They will radiate their orbital energy away, gradually spiraling together, until in about two million years time they will merge into a single black hole.

Now that astronomers have found this first candidate for a binary black hole in a quiescent galaxy, the search is inevitably on for more. XMM-Newton will continue its slew survey. This detection will also spur interest in a network of telescopes that search the whole sky for tidal disruption events.

"Once we have detected thousands of tidal disruption events, we can begin to extract reliable statistics about the rate at which galaxies merge," says Komossa.

There is another hope for the future as well. When binary black holes merge, they are predicted to release a massive burst of energy into the Universe but not mostly in X-rays. "The final merger is expected to be the strongest source of gravitational waves in the Universe," says Liu.

Gravitational waves are ripples in the space-time continuum. Astronomers around the world are currently building a new type of observatory to detect these ripples. ESA are also involved in opening this new window on the Universe. In 2015, ESA will launch LISA Pathfinder, which will test the necessary technology for building a space-based gravitational wave detector that must be placed in space. The search for elusive gravitational waves is also the theme for one of ESA's next large science missions, the L3 mission in the Cosmic Vision programme.

In the meantime, XMM-Newton will continue to look out for the tidal disruption events that betray the presence of binary supermassive black holes candidates.

XMM-Newton line detection provides new tool to probe extreme gravity

21 June 2010

A long-sought-after emission line of oxygen, carrying the imprint of strong gravitational fields, has been discovered in the XMM-Newton spectrum of an exotic binary system composed of two stellar remnants, a neutron star and a white dwarf. Astronomers can use this line to probe extreme gravity effects in the region close to the surface of a neutron star.

Stellar remnants are the last evolutionary step of the life of stars which, after having burned their nuclear fuel, collapse into very compact and exotic objects - white dwarfs, neutron stars and black holes, depending on the mass of the stars. With an enormous mass contained in a very restricted space, these objects are extremely dense; in particular, neutron stars and black holes give rise to very strong gravitational fields and thus prove to be excellent testbeds for Einstein's theory of general relativity.

Stars often come in pairs, and neutron stars and black holes are no exception, often being found as one component of a binary system. Due to the strong gravitational attraction that the compact remnant exerts on its companion, material from the latter flows onto the remnant forming an accretion disc. As the material in the disc spirals around the remnant, it is heated up to millions of degrees - because of internal friction - and produces copious amounts of X-rays. These systems are thus referred to as X-ray binaries.

The object of this study, 4U 0614+091, is a very special X-ray binary, consisting of two remnants, namely a neutron star accreting mass from a white dwarf. The fact that the companion star is also a compact object is evident from the exceptionally short orbital period of the system: in fact, the two objects orbit around each other in about 50 minutes, which identifies the source as an Ultra-Compact X-ray Binary (UCXB).

Due to their compact nature, it is virtually impossible to directly image the immediate vicinity of a neutron star and its accretion disc. Fortunately, spectroscopy of these systems yields plenty of information to fill the gap and represents a unique tool to investigate the dynamics of the accretion process in X-ray binaries. The material surrounding the neutron star, irradiated by X-rays, reflects this radiation and, during the process, ions of heavy elements, such as oxygen and iron, that are present in the disc leave their imprint on the spectrum of the reflected light as characteristic emission lines. The profile of these so-called 'fluorescent' lines is deeply influenced by the strong gravitational field of the compact remnant, hence their detection is extremely important for testing the strong regime of general relativity.

"The only line so far observed in X-ray binaries was the iron line, which corresponds to an energy of about 6.4 keV," explains Oliwia Madej, a PhD student at the Netherlands Institute for Space Research (SRON) and Utrecht University who led the study that detected, for the first time, a broad line of oxygen in the spectra of 4U 0614+091. This line is at a lower energy than the iron one - about 0.7 keV - and represents not only an additional diagnostic of the inner parts of the system, but actually a more powerful one. "The advantage is that instruments are able to collect more photons at the energy of the oxygen line than at the energy of the iron line, resulting in a better quality spectrum," she adds.

The outstanding result relies on both low- and high-resolution spectra of 4U 0614+091 collected by XMM-Newton. "The high-resolution of the spectra delivered by the Reflection Grating Spectrometers (RGS) was crucial for isolating the long-sought-after oxygen signature amongst the plethora of spectral features," comments Norbert Schartel, XMM-Newton Project Scientist.

The line, which is intrinsically narrow, appears broadened towards lower as well as higher energies. Relativistic effects are responsible for the broadening towards lower energies through a combination of gravitational redshift - as photons lose energy as they escape the strong gravitational field of the neutron star- and relativistic Doppler effect. "The broadening towards higher energies is interpreted, instead, in terms of photons scattering off the very hot electrons present in the disc and gaining energy through this process," explains Peter Jonker from SRON, one of Madej's PhD supervisors.

By studying the profile of the oxygen line in very great detail, it is possible to infer a wealth of information about the accretion disc within a few to a few tens of neutron star radii, corresponding to a distance of only a few kilometers to several tens of kilometers from the neutron star's surface. Probing these regions allows us to test Einstein's general relativity in an extreme environment, where the gravity is immensely stronger than in our Solar System.

"It is amazing how Nature provides us with astronomical sources that are exceptional laboratories to study how matter behaves in such a strong gravitational field, so dense that one teaspoonful would weigh a billion tons on Earth," comments Schartel. "Our role is to figure out better and better tools to observe these sources and uncover all the information they conceal."

X-ray discovery points to location of missing matter

11 May 2010

Using observations with ESA's XMM-Newton and NASA's Chandra X-ray Observatory, astronomers have announced a robust detection of a vast reservoir of intergalactic gas about 400 million light years from Earth. This discovery is the strongest evidence yet that the 'missing matter' in the nearby Universe is located in an enormous web of hot, diffuse gas.

This missing matter - which is different from dark matter - is composed of baryons, the particles, such as protons and electrons, that are found on the Earth, in stars, gas, galaxies, and so on. A variety of measurements of distant gas clouds and galaxies have provided a good estimate of the amount of this 'normal matter' present when the Universe was only a few thousand million years old. However, an inventory of the much older, nearby Universe has turned up only about half as much normal matter, an embarrassingly large shortfall.

The mystery then is where does this missing matter reside in the nearby Universe? This latest work supports predictions that it is mostly found in a web of hot, diffuse gas known as the Warm-Hot Intergalactic Medium (WHIM). Scientists think the WHIM is material left over after the formation of galaxies, which was later enriched by elements blown out of galaxies.

"Evidence for the WHIM is really difficult to find because this stuff is so diffuse and easy to see right through," said Taotao Fang of the University of California at Irvine and lead author of the latest study. "This differs from many areas of astronomy where we struggle to see through obscuring material."

To look for the WHIM, the researchers examined X-ray observations of a rapidly growing supermassive black hole known as an active galactic nucleus, or AGN. This AGN, which is about two thousand million light years away, generates immense amounts of X-ray light as it pulls matter inwards.

Lying along the line of sight to this AGN, at a distance of about 400 million light years, is the so-called Sculptor Wall. This 'wall', which is a large diffuse structure stretching across tens of millions of light years, contains thousands of galaxies and potentially a significant reservoir of the WHIM if the theoretical simulations are correct. The WHIM in the wall should absorb some of the X-rays from the AGN as they make their journey across intergalactic space to Earth.

Using new data from Chandra and previous observations with both Chandra and XMM-Newton, absorption of X-rays by oxygen atoms in the WHIM has clearly been detected by Fang and his colleagues. The characteristics of the absorption are consistent with the distance of the Sculptor Wall as well as the predicted temperature and density of the WHIM. This result gives scientists confidence that the WHIM will also be found in other large-scale structures.

Several previous claimed detections of the hot component of the WHIM have been controversial because the detections had been made with only one X-ray telescope and the statistical significance of many of the results had been questioned.

"Having good detections of the WHIM with two different telescopes is really a big deal," said co-author David Buote, also from the University of California at Irvine. "This gives us a lot of confidence that we have truly found this missing matter."

In addition to having corroborating data from both Chandra and XMM-Newton, the new study also removes another uncertainty from previous claims. Because the distance of the Sculptor Wall is already known, the statistical significance of the absorption detection is greatly enhanced over previous 'blind' searches. These earlier searches attempted to find the WHIM by observing bright AGN at random directions on the sky, in the hope that their line of sight intersects a previously undiscovered large-scale structure.

Confirmed detections of the WHIM have been made difficult because of its extremely low density. Using observations and simulations, scientists calculate the WHIM has a density equivalent to only 6 protons per cubic meter. For comparison, the interstellar medium - the very diffuse gas in between stars in our Galaxy - typically has about a million hydrogen atoms per cubic meter.

"Evidence for the WHIM has even been much harder to find than evidence for dark matter, which is invisible and can only be detected indirectly," said Fang.

There have been important detections of possible WHIM in the nearby Universe with relatively low temperatures of about 100 000 degrees using ultraviolet observations and relatively high temperature WHIM of about 10 million degrees using observations of X-ray emission in galaxy clusters. However, these are expected to account for only a relatively small fraction of the WHIM. The X-ray absorption studies reported here probe temperatures of about a million degrees where most of the WHIM is predicted to be found.

XMM-Newton weighs up a rare white dwarf and finds it to be a heavyweight

04 September 2009

XMM-Newton observations of the X-ray pulsator RX J0648.0-4418, timed to also cover the phase when the source was expected to be eclipsed by its companion, have resulted in a solid, model-independent mass estimation of this object. It appears to be a rare, ultra-massive white dwarf, whose continued study promises to provide sensitive tests for stellar evolution theories. Sandro Mereghetti and colleagues present these results in the 4 September issue of Science.

The X-ray pulsator RX J0648.0-4418 and the subdwarf star HD49798 form a binary system with unique properties. The subdwarf star is a bright object in the optical and UV bands, and is well characterized. The orbital period of the system is accurately known, and the discovery in 1996 of a 13.2 s periodicity in X-rays made it clear that the companion must be either a neutron star or a white dwarf.

Binary systems are key test cases for stellar evolution models

Binary systems can be considered as a kind of astrophysical laboratory. Measuring the orbital motion of the two objects in the system allows one to derive information about the mass of the objects themselves in a model-independent manner which is based solely on the application of Kepler's laws, and which does not rely on any assumptions about the state, dimensions, or evolution of the system. This sort of measurement provides astrophysicists with an independent piece of data that can be used to constrain stellar evolution models.

Often though, only information about one of the objects in the system (usually the brighter one) is known. This can be used to derive the mass function of the system, a combination of the two masses and the inclination of the orbital plane with respect to the line of sight.

When similar information about the other object can be obtained, either spectroscopic ally or by measuring the time delays in the pulsations induced by the orbital motion, then the two masses can be calculated - if a good estimation of the inclination can be obtained. The latter can be constrained quite accurately if one of the objects is periodically eclipsed by the other.

A model-independent mass estimate points to a rare, massive white dwarf

This is exactly what Mereghetti and colleagues have managed to do. By making use of recent XMM-Newton observations timed to coincide with the expected eclipse of the X-ray source, the authors have been able to accurately determine the mass function of the X-ray source, and obtain a well constrained estimate of the inclination of the orbital plane.

Armed with these data, and applying them to the equation of orbital motion, they were able to conclude that the X-ray source is a rare, ultra-massive (at least 1.2 solar masses) white dwarf. This makes RX J0648.0-4418 one of the most massive white dwarfs known to date.

Important implications for studies of stellar evolution

It is not clear how such a system has formed, but it seems clear that RX J0648.0-4418 is accreting matter from its companion, and that this may eventually push the mass of RX J0648.0-4418 to exceed the Chandrasekhar limit and possibly explode as a supernova of Type Ia. This hypothesis has wide-ranging repercussions in astrophysics, because it suggests that this might be another evolutionary path for Type Ia supernovae.

The demonstration that this unique binary system consists of a fast-spinning, ultra-massive white dwarf in a nearby binary system (it is at a distance of 650 parsecs) which therefore can be studied in detail, provides astrophysicists with another important test bench to use in developing models of stellar evolution.

2000 publications: another milestone for XMM-Newton

17 February 2009

The XMM-Newton mission passes another milestone this month with the publication of its 2000th scientific paper in peer-reviewed scientific journals. This is another indication that the mission continues to be one of the world's foremost astronomical observatories.

XMM-Newton - a pre-eminent astronomical observatory

Since it was launched, in December 1999, XMM-Newton has consistently demonstrated its role as one of the most important astronomical observatories of our times. This is reflected in the consistently high oversubscription to each call for observing time (typically by a factor of seven in time), publication rate (currently running at 300 a year), and the frequent citation of refereed papers (XMM-Newton scientific results are typically cited four times more often than the average refereed paper in the astronomical literature). Estimates of the number of scientists who use XMM-Newton vary between 1500 and 2000. And these scientists have been using XMM-Newton to tackle a wide variety of astrophysical questions as evidenced by the large number of scientific papers.

Addressing a diverse array of objects and topics

Among the astronomical objects covered in the 2000-plus papers have been comets and planets, quasars and clusters of galaxies, neutron stars and black holes, as well as diffuse X-ray emission from the cosmic background and interstellar regions.

Among the recent scientific highlights are:

The unambiguous detection of quasi-periodic oscillation in a supermassive black hole, RE J1034+396, which has important implications for the theoretical understanding of Active Galactic Nuclei

Discovery of 2XMM J083026+524133, the most massive cluster of galaxies known in the Universe – the high effective area of XMM-Newton makes it particularly suited to detecting extended objects such as these galaxy clusters

Detection of a 2.6 s oscillation from the magnetar SGR 1627-41, providing another key insight into this rare and unusual class of stellar object

X-ray observations of NGC 346 which, as part of a multi-wavelength analysis, provided an important key in unraveling the star-formation mechanism at play within this region of the Small Magellanic Cloud

The unraveling of complex behavior in the double pulsar PSR J0737-3039 – pulsars are a natural laboratory for the study of high-energy interaction processes and yield information not only on astrophysical process such as the physics of the interacting pulsars winds or magnetospheres of neutron stars, but also on more general physical questions relating to the equation of state of super-dense matter and to magneto-hydrodynamics

Discovery of the most massive cluster of galaxies known in the distant Universe

25 August 2008

XMM-Newton has discovered a rare, very massive cluster of galaxies at a distance of about 7700 million light years (or z~1). The object, designated 2XMM J083026+524133, was discovered during a systematic analysis of the 2XMM X-ray source catalogue. In a paper, to appear in Astronomy & Astrophysics, Georg Lamer and colleagues present the discovery and analysis of this exceptional cluster of galaxies.

The 2XMM catalogue is based on ~3500 observations performed with XMM-Newton's EPIC cameras between February 2000 and March 2007. The catalogue covers ~360 square degrees (about 1% of the entire sky) and contains ~192,000 individual X-ray sources.

A team of astronomers at the Astrophysikalisches Institut Potsdam, led by Georg Lamer, searched the catalogue for distant and bright clusters of galaxies. These large collections of gravitationally bound galaxies have a tenuous but very hot intra-cluster gas component with a temperature of up to 100 million Kelvin, causing thermal emission at X-ray wavelengths.

Lamer et al. selected all extended sources in the 2XMM catalogue and correlated these with data from the Sloan Digital Sky Survey (SDSS) to search for optical counterparts. About a third of the sky covered in the 2XMM catalogue is also covered by the SDSS. Within this region the majority of the selected X-ray sources were found to have an optical counterpart in the SDSS data. However, for a small number of X-ray sources no visible optical counterpart was detected. These sources were considered candidates for distant clusters of galaxies at redshifts above ~0.8. The large distance would render the optical light too faint to have been detected by the SDSS.

The brightest of the selected X-ray sources without an SDSS counterpart was 2XMM J083026+524133. The source was observed for an accumulated total of nearly 24 hours by XMM-Newton, gathering sufficient X-ray photon counts for a good X-ray spectrum to be obtained. Lamer et al. determined the best fit spectrum using a plasma model for a cluster of galaxies. The fitted spectrum implies a redshift of 0.99 ± 0.03 and a temperature of 8.2 ± 0.9 keV or ~95 million K for the intra-cluster gas.

This makes 2XMM J083026+524133 the hottest, most X-ray bright cluster of galaxies at redshifts z ≥ 1, with a bolometric luminosity Lbol of 1.8 × 1045 erg s-1. Using a mass model and the measured temperature, Lamer et al. also derive a mass of about 5.6 × 10exp14 Msun for this cluster of galaxies, equal to ~1000 times the mass of our Milky Way galaxy.

Ground-based follow-up

Follow-up observations were performed in May 2008 with the world's largest telescope, the Large Binocular Telescope (LBT) on Mount Graham in southeast Arizona, USA. Two broad filters were used to make deep observations of the field around 2XMM J083026+524133 in the R band (Sloan r' ~550-690 nm) and Z band (Sloan z ~850-1000 nm (upper cut-off defined by the detector and not by the filter)).

These optical and near-infrared images provided the sought after counterpart and support the identification of 2XMM J083026.2+524133 as a cluster of galaxies because:

there is an increased density of red galaxies within the X-ray contours of the extended source from the 2XMM catalogue

the magnitudes and colors of these galaxies are consistent with a redshift of ~1

Cosmological models

Clusters of galaxies at a redshift of ~1 or higher are routinely identified with XMM-Newton data and in dedicated surveys with other observatories. However, 2XMM J083026.2+524133 is a factor of ~100 brighter in X-rays than the majority of the other known clusters of galaxies at these redshifts, making it an important find.

The evolution of the number of high-mass, high-luminosity clusters of galaxies, similar to 2XMM J083026.2+524133, over the age of the Universe, strongly depends on cosmological parameters. Large X-ray surveys such as the 2XMM catalogue can provide the necessary observational constraints on these parameters through the observed number of these massive clusters at large distances. The counts can be compared with the number of massive clusters of galaxies in the local Universe to characterize their number evolution with time. This in turn has implications for the validity of cosmological models.