A MEETING WITH THE UNIVERSE

Chapter 4-8

Quasars and the X-Ray Background

As NASA launched successively more powerful X-ray observatories, more and more cosmic X-ray sources were found. In addition we investigated a background of cosmic X-rays that seem to come from all directions in space. As it became quite clear that these background X-rays were very uniformly distributed, astronomers wondered whether they were produced by a vast collection of point sources at great distances, so numerous that individual ones could not be seen, or whether the emission source is genuinely spread out over all of intergalactic space. To answer this question, the X-ray astronomers took very long exposures of portions of the sky. With one type of instrument they confirmed that the background radiation is remarkably uniform; with another they found a surprisingly large number of quasars that emit X-rays.

Quasars, first discovered in the early 1960s, appear almost like stars, although vastly more distant than any individual star that we can see. They were called "quasi-stellar radio sources" (shortened to "quasars") when they were detected with radio telescopes. They emit tremendous amounts of energy from very small volumes. The most distant quasars seen are so distant that they are receding at more than 90 percent of the speed of light. As bright, concentrated radiation sources, they resemble the nuclei of Seyfert galaxies, but are far more luminous.

Remarkably, the visual brightness of some quasars can change by a factor of two in about a week, and noticeable changes can occur even in one day. Since a source of light cannot change brightness significantly in a time shorter than it takes light to cross it, these quasars cannot be much bigger than one light day across, only twice the size of our solar system. The problem, then, is to explain how a quasar can produce vastly more energy than a galaxy in such a small region.

The brightest quasar as seen from Earth, 1.5 to 3 billion light years away, is called 3C273, from its number in a catalogue of radio sources. It is speeding away from us at nearly 16 percent of the speed of light. It has been detected as a source of radio, infrared, visible light, ultraviolet, X-ray, and even gamma ray emission. Some of the new quasars found by the HEAO-2 satellite are much further away, with recession velocities of two-thirds the speed of light.

Beacons of distant space.
One of the most powerful known energy sources, a quasar and a Seyfert galaxy (at right) are shown here as photographed in visible light. 3C273, the first quasar found, has a noticeable jet, extending toward the lower left. Most of the radiation from a Seyfert galaxy comes from the central nucleus. Thus, at a sufficiently great distance, a Seyfert galaxy might resemble a point like quasar. Both 3C273 and NGC 4151 (the Seyfert galaxy) are strong sources of infrared waves, X-rays, and gamma rays. Although the two objects are about equally bright in visible light as seen from Earth, the quasar is perhaps 80 times as far away, and hence about 6400 times more luminous.

Another distinctive type of X-ray source is closely related to the quasars and may even be an unusual type of quasar. They are called "BL Lac" objects after the first known member of the class, BL Lacertae, which for years was listed as an ordinary variable star of the Milky Way in astronomers' catalogues. More recently, we learned that it is in fact a distant object in extragalactic space, which resembles a star until studied in detail. The visible light of BL Lac objects is dominated by radiation from high-velocity electrons spiraling in magnetic fields. By contrast, in ordinary quasars there are prominent visual emissions from hot gas. However, in their compact dimensions, rapid intensity variations, and emissions of X-rays, gamma rays, infrared and radio waves, the BL Lac objects resemble quasars and constitute yet another mystery of distant space that we are only beginning to explore. Quasars, galaxies, and the X-ray background seem to be linked to gether. By making some reasonable guesses about how quasars and galaxies evolve-that is, how they change their X-ray brightness as they grow older - it is possible to account for all of the observed background radiation. But this idea is not easy to confirm.

The imaging X-ray telescopes launched thus far are sensitive only to relatively "soft" X-rays, with energies of about one kilovolt, in physicists' terms. At higher energies there is a "bump" of enhanced intensity in the X-ray spectrum of the X-ray background near 40 kilovolts, but none of the quasars observed so far have such bumps in their spectra. If this bump is not from quasars, what is it from? One suggestion is that very young galaxies formed shortly after the creation of the universe would have a great many pulsars and neutron stars in them to supply the necessary X-rays. This is one of several possible descriptions of the very early universe to be tested by the Cosmic Background Explorer mission planned for mid-1980s. There is also a diffuse background of gamma rays, with a bump at the energy of 1000 kilovolts. Is it due to the quasars once again or to something even more exotic?