| | | | | | |
| | |||||
|
CXC in Context: A Revolutionby Martin Elvis, Harvard-Smithsonian Center for Astrophysics, Cambridge, MACXC has been a long time, 20 years, in coming. This has made it hard to see how revolutionary CXC is. Some even describe CXC as the end of the line in X-ray technology; a never to be repeated venture to high resolution. This view is wrong. To see why, we need to see where X-ray astrophysics is going. It should not be controversial to say that X-ray Astrophysics has barely begun. Today's X-ray satellites have scored many successes; yet astronomers still extract very little of the information carried to us by X-ray photons. The excellent imaging and detailed spectral capabilities routinely available in optical astronomy, have not even begun to be used in X-ray astronomy. CXC is our first step into that world. Eventually X-ray telescopes will be built that have all three qualities needed to let us use the full power of physics on celestial sources of X-rays: many square meters of mirror area; imaging like Hubble Space Telescope; and the ability to discriminate features in spectra separated by only 1 part on 10,000 of wavelength. Then the riches of atomic physics in the X-ray band known from the Sun, laboratories, and theory, can be exploited for all classes of X-ray sources, so we can truly understand how they work. Getting there is the problem. CXC takes the first step toward this goal by combining two of the needed qualities: excellent imaging and detailed spectral ability. CXC will show us where X-ray astrophysics can go. In this context we can recognize CXC for what it is - a revolution. I will try to give you an idea just how revolutionary. The focusing ability of the CXC optics puts CXC in a separate league from all other X-ray missions - past, present or planned. This better than planned performance is thanks to Hughes-Danbury, Kodak, and the CXC Mirror Scientist, Leon van Speybroeck. The spread of a beam of X-rays due to the CXC mirrors is one hundredth that of the previous X-ray telescope. Add to this the spectral resolution of CXC and you have an X-ray observatory of unprecedented sensitivity and resolution, angular and spectral. The deepest CXC surveys will reach twenty times fainter than any previous X-ray telescope. This is uncharted territory. We can confidently expect a thousand X-ray sources in every patch of sky the size of the Moon. And each CXC exposure takes an image of just about that size, though usually less deep. Whenever one can image 100 times the detail of any previous telescope extraordinary things will be found. The improvement brought by CXC is equal to that of going from telescopes on Earth to the Hubble Space Telescope. Hubble images often give me the feeling of looking up the answer in the back of the book. These images tell us that there is structure on every scale in astrophysics. Many of the same Hubble objects are also bright X-ray sources. Surely they won't lose all their structure when we look with X-rays? CXC is the first X-ray telescope that has good imaging and good spectra at the same time. This makes it possible to isolate structures, even the sinuous shock fronts in supernova remnants and clusters of galaxies, and extract their distinctive spectra, free of confusing glare from their surroundings. With CXC's orders of magnitude advances in imaging power, in fine spectral discrimination, and in both simultaneously, we can expect surprises: complex spectra will show up in unusual places; many spectra will show features that are simply unknown, since laboratory work on Earth has covered only a little of the atomic structure which we will encounter with CXC; and many sources now thought of as simple will show complex images, even whole new types of source. For example, Hubble has shown that in Orion, the disks around newly formed stars - which will eventually form planets - are being evaporated by nearby bright stars. Since the bright stars are also bright X-ray sources, it is a simple prediction that these evaporating proto-planetary disks will also be shining in fluorescent X-rays. This will give us a whole new way to study what these disks are made of and, some day, how they are rotating. Both technologically and scientifically CXC is like Hubble: both achieved 10 times improved resolution by using heavy, rigid mirrors that were limited in size by their weight. But both demonstrate that high resolution is possible, not end of the line. Scientifically, both let us see how complex, yet comprehensible, the universe is. There will be no going back to less resolution, once we have seen Hubble and CXC images. How do we get to more area with high resolution? Where should we push on first? A first step is NASA's proposed Constellation-X mission. With approximately 3 square meters of mirror area it will will be thirty times bigger than CXC, while maintaining good spectral resolution. However angular resolution will be limited. What is next? The stumbling block are X-ray mirrors: we need 10 square meters of effective area, yet must maintain arcsecond resolution and be light in weight. Work on this challenging goal is beginning in Europe, under the "XEUS" banner. Discussions in the NASA community are just beginning. Certainly if we do not begin to develop the technology for such a mission X-ray astronomy will wait another 20 years before fulfilling the promise of the CXC revolution. After launch CXC will quickly become a user driven observatory. Calibration observations will be public at once. From the 5th month onward 70% of the time will be for Guest investigators, increasing later to 85%. This time is competed for eagerly: for every observation accepted, 6 must be rejected. But don't despair, CXC is designed to last a long time, so there will be other chances. CXC is here at last. Enjoy it. [ Press Index ] |
||||||||
Revised: February 20, 2008
|
||||||||
