X-RAY ASTRONOMY: Newton unveils high-energy universe

May 1, 2000
A new x-ray space observatory, nicknamed Newton, took its first pictures in February 2000, giving new views on the universe

A new x-ray space observatory, nicknamed Newton, took its first pictures in February 2000, giving new views on the universe. The images confirm that the X-ray Multi-Mirror Mission (XMM) spacecraft, its x-ray telescopes, and other instruments are functioning. Initial inspection of the images showed some unique x-ray views of several celestial objects.

The XMM satellite was launched by the European Space Agency (ESA) on December 10, 1999, aboard Ariane 504 from Kourou, French Guiana, and brought to its operational orbit the following week. The project involves groups from universities and industry across Europe. The XMM is the largest scientific satellite ever built in Europe, at 10 m long and just less than 4 tons in weight, carrying the largest x-ray telescope ever built.

The satellite is the second cornerstone mission of the ESA Horizon 2000 Project and the second of three recently planned international x-ray observatories. The first, NASA's Chandra X-ray Observatory, went into space last summer and is operating well. The third, Japan's ASTRO-E, was lost at liftoff in February when a rocket failed to lift it into sustainable orbit. XMM will detect much fainter emissions than Chandra, although with less detail.

The XMM mission's scientific data are being received, processed, and dispatched to astronomers by the XMM Science Operations Centre in Madrid, Spain. First analyses of the initial series of short and long exposures confirm that the spacecraft is extremely stable and working as expected.

The XMM satellite carries a range of instruments, including three parallel-mounted x-ray telescopes, the European photon imaging camera (EPIC), an optical monitor (OM), and reflection grating spectrometers (RGSs). The telescopes use Wolter mirrors to focus the x-rays onto the detector. Each of the telescopes has its own photon imaging camera to record the acquired data. Two of the cameras also are equipped with RGSs to measure the wavelength and, hence, energy, of the x-ray radiation. The instruments were developed by a consortium of university and industrial groups.

Far-viewing

EPIC is the charge-coupled-device (CCD) camera system that will collect images for the XMM spacecraft. The CCD system includes two separate designs (one PIN and two MOS devices), which will be positioned in the focal plane of each of the three x-ray telescopes. The principal investigator for this instrument is Martin Turner of the X-ray Astronomy Group at Leicester University (Leicester, England); the EPIC consortium comprises 10 institutes in four nations: the UK, Italy, France, and Germany.

The RGS is the first mission-based spectrometer to use large reflection gratings, simultaneously giving a high spectral resolution and high throughput. These gratings contain 600 grooves/mm, equivalent to 15 grooves in the width of a human hair. Measuring 10 x 20 cm, the grooves were produced by a replication process from a mechanically ruled master and were designed and procured through Columbia University (New York, NY).

The principal investigator for the RGS is Bert Brinkman, based at the Space Research Organization Netherlands (Utrecht, The Netherlands). His co-investigator is Steven Kahn from Columbia University. Other institutes in the consortium include the Mullard Space Science Laboratory (Dorking, England), part of University College London, and the Paul Scherrer Institute (Villigen, Switzerland).

The OM is a telescope that can study the sky in visible and ultraviolet light. It was built by a consortium of institutes from the UK, the USA, and Belgium, led by K.O. Mason from the Mullard Space Science Laboratory. Although its 30-cm-diameter main mirror is modest in comparison with ground-based telescopes, when the OM is lifted above the Earth's atmosphere its performance will rank alongside the biggest and best. Optical Surfaces Ltd. (Kenley, England) has been responsible for producing both of the OM mirrors.

These mirrors have been coated with aluminum to an extremely high precision; on average the largest 'bump' on their surfaces is no more than 1 nm, or less than half the width of a single atom of gold. (If these mirrors were expanded to cover the Atlantic, the largest ripple would be less than 1 mm high.) The mirrors, which took four months to polish, were made in an underground workshop to minimize vibration and provide a stable temperature environment. Such precision is necessary if the telescope is to achieve its goal of finding the optical counterparts to the hundreds of thousands of new x-ray sources that will be discovered by XMM.

Until now, it has been extremely difficult to follow up discoveries of many x-ray stars and galaxies because they are often very faint in visible light. The OM will overcome this problem by studying the same area of sky as the observatory's x-ray instrument but in the visible and near-UV. The OM also includes a spectrometer, enabling astronomers to learn more about the temperature and composition of the objects in view.

If all goes well, XMM will revolutionize x-ray astronomy. Perhaps 1 million new x-ray sources could be discovered, ranging from exploding stars to hot interstellar gas and huge disks of material being swept into the jaws of a massive black hole.

About the Author

Bridget Marx | Contributing Editor, UK

Bridget Marx was Contributing Editor, UK for Laser Focus World.

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