High energy astronomy

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X-ray astronomy is one of the youngest fields in observational astronomy. Since X-rays do not penetrate the Earth's atmosphere, the history of X-ray astronomy is the history of high altitude (balloon) and space astronomy. Early X-ray observations were done with sounding rockets (which provided very brief flights with only a few minutes of data taking) and high altitude balloons. Of course, the balloons still do not rise above all the atmosphere, and, in the X-ray part of the spectrum, even the little bit that is left matters.

One problem in observing X-rays is that it is very hard to make a mirror that works for these short wavelengths, less than 1 nm. That is because the typical spacing between atoms in a solid is about 0.1 nm. So the incoming radiation sees a rough surface, with reflections off each atom producing a scattering in some essentially random direction. There is one possibility. If we arrange for the X-rays to come in at a very shallow angle, only a degree or two, with the surface, the atoms appear to be closer together, and we achieve normal reflection. This is called grazing incidence. Of course, being constrained to only grazing angles makes it difficult to design a telescope that will collect and focus a reasonable amount of radiation. A diagram of the imaging system in one X-ray satellite is shown in Fig. 4.33(b).

X-ray satellites are able to provide both continuous and spectral line observations. Originally, the spectral information came from detectors similar to those used by high energy physicists, called proportional counters, which register the energy of the photons as they hit. Better spectral resolution was obtained by using a type of grating called a Bragg crystal, in which the 'slits' are the individual atoms in a solid. More recently, X-ray astronomers have been able to use solid state detectors that give good spectral resolution.

More recently a number of satellites have opened our eyes to the X-ray sky. One of the satellites is shown in Fig. 4.33(a). A number of the

(b)

The Atacama Large Millimeter Array (ALMA). (a) Views of the site. (b) Artist's conception of the arrangement of telescopes. (c) Artist's conception of the antenna appearance. [NRAO/AUI/NSF and ESO]

The Atacama Large Millimeter Array (ALMA). (a) Views of the site. (b) Artist's conception of the arrangement of telescopes. (c) Artist's conception of the antenna appearance. [NRAO/AUI/NSF and ESO]

earliest satellites incorporated X-ray detectors, which shared space with other experiments. The first satellite devoted entirely to X-ray observa tions was Uhuru (launched in 1970). It was also the first to survey the whole sky, and, compared to previous missions, had very sensitive detectors. It found 339 objects, showing astronomers that many different types of objects give off strong X-ray emission. Following Uhuru, which was a relatively small satellite, NASA launched a number of larger satellites in the High Energy Astronomy Observatory (HEAO) program. The second in the HEAO series was the Einstein Observatory, which was the first to utilize the grazing incidence imaging, and so produced the first real X-ray images. The Einstein images had a profound impact on our thinking about many types of astronomical objects. We went from being able to probe small sections of objects to forming whole images. In many ways it was like having a blindfold removed. The next major jump in sensitivity and angular resolution was the Roentgen Satellite (ROSAT), launched in 1990 (Fig. 4.33a). Chandra was launched in 1999, and provides sub-arcsecond imaging (Fig. 4.33c) and grating spectrometry, so it does high quality imaging spectroscopy (Fig. 4.33d).

All of the telescopes that we have discussed so far have been for electromagnetic radiation. High energy phenomena also make their presence known in other ways. One way is by the emitting beams of cosmic rays, charged particles, often with very high energies. They also give off neutrinos, subatomic particles that are very difficult to detect. They also give off gravitational radiation, distortions in the fabric of spacetime, which again are very difficult to detect. We will talk about detecting each of these in the chapters that discuss their astrophysical origin.

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X-ray satellites. (a) ROSAT. (b) Imaging system in the Chandra X-ray satellite, utilizing grazing incidence. (c) Chandra image of the Crab Nebula showing the great detail achievable in current X-ray systems. (d) Chandra test spectrum, showing the good spectral resolution. [NASA]

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