Golden age of X-ray astronomy
23 July 1999
The Shuttle launch of NASA's Chandra X-Ray Observatory on 23 July heralds a golden age of space astronomy. Because next December the European Space Agency's XMM satellite will follow the US telescope into space. Many of the world's astronomers are directly involved and have observing time on both missions. In six months' time, the scientific community will have not one but two amazing discovery machines to probe the hot spots of the sky producing X-rays, to which one must add the forthcoming Japanese spacecraft, Astro-E.After 30 years of exploration by X-ray satellites of moderate power, it was time to look harder with very powerful telescopes. NASA's Chandra and ESA's XMM have been conceived to clear up many mysteries that remain, and move on from vague theories to hard facts.
Although the size, appearance and operational orbits of both spacecraft are similar, the roles of the two new X-ray telescopes are different. Chandra is the "gourmet" of the two, examining X-ray sources in sharp detail. For its part, XMM is a "Gargantua", with a prodigious appetite for cosmic X-rays. With its 4500 cm2 of X-ray collecting area, it will simultaneously image and analyse bright sources very quickly and detect many extended faint sources for the first time.
Chandra carries a high-resolution mirror assembly, composed of two pairs of four nested grazing incidence mirrors. The largest of the 8 mirrors is 1.2 m in diameter and 0.8 m long. NASA's mirrors are made of 'Zerodur' glass, 15 mm thick, provided by Schott Glaswerke, Mainz, Germany. The Chandra mirrors are coated with iridium.
To make its own X-ray "high throughput" telescope as greedy as possible, the European Space Agency and European industry have pushed telescope-making beyond the limits of known technology. XMM carries three mirror assemblies, or 'modules', each with 58 mirrors, nested like Russian dolls. The mirror shells are made of wafer-thin nickel, with a reflective surface of gold. Altogether XMM's high-precision optics will catch more X-rays than all previous satellites put together.
Chandra carries two science instruments, a wide field High Resolution Camera (HRC) composed of 69 million minute lead-oxide glass tube detectors, and an Imaging Spectrometer (ACIS) using charge-coupled device (CCD) arrays to detect X-rays. Each instrument can serve as an imager to "take pictures" or a spectrometer to measure X-ray energy levels.
At the prime focus of each of the XMM telescopes are three European Photon Imaging Cameras (EPIC). With advanced CCDs, these cameras will be capable of detecting rapid variations in intensity, down to a thousandth of a second and less! One the cameras (pn) will be able to take "colour" images of the sky at energy levels beyond 15 keV.
Chandra is equipped with diffraction gratings. These can be inserted into the path of the incoming rays, fanning them out to create a spectrum. One grating has been provided by the Massachusetts Institute of Technology (MIT).
The Netherlands Space Research Organisation (SRON) and the Max Planck Institute, Germany have also developed transmission gratings for Chandra. Both European Institutes are also deeply involved in XMM, with the spectrometer Principal Investigator coming from SRON.
Conversely, Columbia University, NY is a co-investigator for XMM's Reflection Grating Spectrometre (RGS) sponsored by NASA and an American firm, Perkins Elmer, contributed to the manufacture of the reflection grating arrays for the European mission.
To address the growing need for a multi-wavelength understanding of our universe, XMM is equipped with a conventional but very sensitive Optical/UV Monitor (OM) which will observe simultaneously the same regions as the X-ray telescopes but in the ultraviolet and visible wavelengths. In orbit, this 30-cm telescope will be as sensitive as a 4-m instrument on the Earth's surface.
After spacecraft and science instrument checkout, Chandra is scheduled in three weeks' time to target its first X-ray source, supernova remnant Cas A. The two new X-ray telescopes, with their complementary qualities, will certainly compete to make discoveries. But together XMM, Chandra and Japan's Astro-E will lift the veil on the exotic and violent X-ray hot spots, each participating in the global effort to make sense of the Universe we inhabit.
