4. BINARY X-RAY SOURCES:

The X-ray sky: With the launch of the UHURU X-ray satellite in 1970, astronomers embarked on a voyage of discovery of neutron stars and black holes that continues to this day. These objects have such great gravity that only a small amount of matter falling into them will release tremendous luminosity as it falls in. Because the radius of the object is so small and the luminosity is so great, the temperature of the photosphere must be millions of degrees or more, according to the Stefan-Boltzmann Law. At such temperatures, the objects will radiate most of their luminosity as X-rays, and that is why the X-ray sky is dominated by compact objects.

It would be great if we had a movie of the X-ray sky. If so, you would see some of these sources (neutron star binaries) flashing on and off periodically, with timescales ranging from seconds to several minutes; others (X-ray bursters) would occasionally flare up for a minute or so, becoming brighter than all the rest, then fade back, repeating again after several hours; and others (black hole binaries) would be varying in brightness wildly, on timescales ranging from milliseconds to months.

Neutron star binaries: If the more massive star in a newly formed binary system has a mass greater than about 8 solar masses, its core will probably be too massive to become a white dwarf after it evolves and passes its envelope to its lighter companion. Then, just as in a single massive star, the core will undergo advanced stages of nuclear burning to become iron, which will then collapse to form a neutron star, resulting in a supernova explosion. (Core collapse supernovae in binary systems may be the peculiar Type Ib and Type Ic supernovae, which show no hydrogen lines in their spectra.) The supernova explosion will not destroy the companion star. The explosion may, however, give the neutron star enough of a kick to eject it from the binary system. But, evidently, this doesn't happen all the time because we have observed neutron stars in binary systems with companion stars of all kinds: blue giants, red giants, white dwarfs, and even other neutron stars. These neutron star binaries give rise to a fascinating variety of phenomena.

Here are two learning centers about X-ray binaries with some nice graphics: NASA's Imagine Binaries, and Ignacio Negueruela's X-ray Binaries Page.

Here is a brief outline summary:

X-ray pulsars: Neutron stars accreting gas from a companion star. The gas flows into the neutron star in an accretion disk, but extremely powerful magnetic fields channel the flow to the north and south magnetic poles of the neutron star. The gas falls onto the neutron star with approximately 30% of the speed of light. When it strikes the surface, it converts its energy of motion into heat and radiates this heat in the form of X-rays. The energy release is tremendous: every gram of infalling gas that strikes the neutron star surface will produce the energy equivalent of an atomic bomb! The X-rays come out in beams, which rotate with the neutron star with periods ranging from less than a second to tens of minutes. We see X-ray flashes when the beams sweep past us.

A few hundred such X-ray pulsars have been found. By analyzing their orbital motions, we can infer the masses of the neutron stars. We find that all the neutron stars have masses of about 1.4 solar masses -- the Chandrasekhar limit mass of a white dwarf star. This fact confirms the notion that these objects are formed by the collapse of a massive star when the mass of its iron core reaches the Chandrasekhar limit.

Measured masses of neutron stars in binary systems. The 9 sources at the bottom are radio pulsars, while the 7 sources at the top are X-ray binaries. Within the measurement uncertainties, all masses are consistent with 1.4 solar masses. None have masses greater than 2 solar masses.

In contrast to radio pulsars, the pulse rates of these X-ray pulsars are often seen to increase. That happens because the neutron star is accreting gas from an accretion disk that is rotating faster than the neutron star, causing the star to spin up.

X-ray bursters: These sources are also neutron stars in binary systems, but most do not pulse regularly. Instead, they occasionally flare up in very bright bursts of X-rays that last from several seconds to a few minutes. See X-Ray Bursters. We think that an X-ray burst is the neutron star analogue of a classical nova: a layer of hydrogen is deposited on the surface of a neutron star by mass transfer from a companion star. When the surface layer reaches a critical temperature and density, it explodes. But the X-ray burst releases much less energy than a classical nova explosion (about 10^-5 as much). The reasons are: (1) since the gravity on the surface of a neutron star is much greater (by about 10⁵) than that on a white dwarf, a much thinner layer is required to reach the temperature and density to ignite the explosion; and (2) the surface area of the neutron star is much smaller than that of the white dwarf. Like classical novae, X-ray bursts repeat, but the interval between repetitions is much shorter -- a few hours, instead of decades.

Besides for the energy and duration of the burst, there are other important differences between X-ray bursts and classical novae. Classical novae radiate most of their energy at optical and ultraviolet wavelengths, whereas X-ray bursts radiate the energy as X-rays. In classical novae, the layer is ejected from the star by the explosion. In X-ray bursters, the gravity of the neutron star is so great that the explosion can lift the material only a few hundred meters, and then it falls down again. We think that X-ray bursters do not pulse because the magnetic fields on the neutron stars in bursters are relatively weak. Therefore, the gas that falls onto the neutron star is deposited on the entire surface, not channeled to the magnetic poles as in a pulsar.

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Last modified March 7, 2002
Copyright by Richard McCray