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Observational Astronomy

What We Learn from X-rays

X-ray astronomy is the branch of astronomy that studies the hot regions of our universe. At first it may seem implausible that any region visible to us would be this hot; the core of the Sun, at a temperature of 1.6×107°, produces a black-body radiation field characterized by photons of several keV, the lower end of the x-ray band. Could nature produce such conditions in the open? The answer is yes. In fact, the coronas of the Sun and other stars, with their temperatures of several million degrees, produce x-rays, as do the shock waves associated with supernovae, the surfaces of young neutron stars, the accretion disks and accretion shocks associated with degenerate dwarfs, neutron stars, and black hole candidates in binary systems, the hot gas in clusters of galaxies, and the massive black hole candidates at the cores of galaxies. Our sky is bright with these various x-ray sources. In fact, many sources of x-rays are only visible in the x-ray band: dust in the galaxy absorbs the light produced by these sources at other frequencies. Above several hundred eV, the universe becomes transparent to high-energy radiation, which gives us a clear view of the hot regions of our universe.

As with optical observations, x-ray observations provide us with the images, spectra, and time-dependent luminosities of astronomical objects. In principle we could also measure the polarization of the x-rays from a source, just as we can measure the polarization of optical light, but no current x-ray observatory measures polarization.

Of the pieces of information we can obtain from an x-ray observatory, the spectrum is the most illuminating of the physical conditions at the source. In most astronomical x-ray sources, the primary spectrum is a very broad continuum that is normally not a black body spectrum. In some cases, the spectrum is created by a thermal distribution of electrons that are insufficiently-dense to create the high flux characteristic of a black body with the temperature of the electrons; the shape of the spectrum in this case is dependent on the precise mechanism that produces x-rays, such as bremsstrahlung, which is radiation created when an electron collides with an ion, cyclotron radiation, which is electromagnetic radiation emitted from an electron spiraling in a strong magnetic field, or Compton scattering, in which an x-ray is created when a lower-energy photon is scattered by a fast-moving electron. In other cases, the electrons themselves are out of thermal equilibrium, so that the energy is distributed among the electrons in a way that cannot be described by a thermal distribution; the continuum spectrum in this case is set by both the mechanism creating the x-rays and the precise distribution of energy among the electrons. A continuum that deviates from a black body spectrum therefore contains a wealth of information about conditions at the source.

On top of a continuum spectrum can be seen emission and absorption lines. In the observations of neutrons stars accreting material from a companion star, these lines are often cyclotron resonance lines—lines at a frequency equal to the orbital frequency of an electron in a magnetic field. By measuring the frequency of a cyclotron resonance line, we have a measure of the magnetic field strength at the source. For a teragauss magnetic field (1012 Gauss; Earth's magnetic field at its surface is less than 1 Gauss), the cyclotron resonance line appears at about 12 keV.

The most useful lines are atomic transition lines. We are all familiar with the atomic lines produces at optical frequencies by the outermost electrons in an atom. For transitions involving the inner electrons of many atoms, the lines appear at x-ray energies. The elements involved in producing these lines range from carbon to iron. Carbon, oxygen, and nitrogen are often stripped of all but one of their electrons, a state that is as “hydrogen-like.” An atom in a hydrogen-like state produces a pattern of lines that resemble the lines of hydrogen in their relative spacing, but shifted to higher energies. More common and important for their effect are the atoms left with two electrons, a state termed as “helium-like.” As with hydrogen-like atoms, helium-like atoms produce atomic transition lines that resemble helium lines, but again shifted to higher energies. Finally, iron plays a large role in producing lines because it can keep hold of more of its electrons than other elements, which allows iron to produce a more complex pattern of transition lines; in particular, a set of lines known as the “iron K complex,” because they involve electrons in the so-called K energy shell, often appear in x-ray spectra at 6 to 7 keV. The accurate observation of these various x-ray lines in a source gives the observer information on the composition, level of ionization, density, temperature, and velocity of the region producing the x-rays. For instance, by measuring the redshift and width of x-ray lines in an accretion disk, we have a measure of the orbital velocity and temperature of the inner regions of the accretion disk.

Measurement of the variation of the x-ray luminosity of an object provides a wealth of information about the dynamics and physical processes in a system. The spin of an object will often appear as a periodic fluctuation in the object's x-ray luminosity. The eclipse of an object by an orbiting companion will appear as a periodic dip in the x-ray luminosity. Instabilities or fluctuations in the flow of material through an accretion disk can appear as random or quasi-periodic fluctuations in the x-ray luminosity, which provides information on the physics of fluid flown onto an accreting source.

Images play less of a role in understanding astronomical objects, because many x-ray sources appear as point sources. But if a source is extended, as is the case with supernovae shock waves or x-ray emitting regions of galactic clusters, an image of where x-rays arise can help clarify the complex structure that often arises in these systems, and the ability to locate precisely an x-ray source can help avoid confusion if two x-ray sources are in the field of view. Finally, the ability to locate the precise position of a point source enables observers to find optical and radio counterparts. The optical radiation from an x-ray source arises from a different set of processes, and often from different parts of a system than x-rays, so the ability to make these observations provides much more information about the physical conditions at the x-ray source. For instance, the x-rays from an x-ray binary star are produced from the inner edge of an accretion disk and the surface of a neutron star, but the optical emission is produced by the outer regions of the accretion disk and the surface of the companion star.

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