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Issue 3.17

The Astrophysics Spectator

October 4, 2006

This issues completes for the time being the discussion of compact binary stars. Two pages are added to the web site that briefly describe the broad variety of x-ray binaries and cataclysmic variables that we see.

To repeat what was said in previous weeks, we see two types of compact binary star: the cataclysmic variable and the x-ray binary. We see these systems because the compact star in the binary is pulling gas from its companion onto itself. The first type of system contains a degenerate (white) dwarf star in orbit with a low-mass fusion-powered star. These systems are very common, because stars with the mass of the Sun or less are very common, and the transition from fusion-powered star to degenerate dwarf does not disrupt the binary system. The gas that falls onto the degenerate dwarf radiates light predominately at visible, ultraviolet, and x-ray energies. As the name implies, these systems are capable of producing violent outbursts, the most brilliant of which are called classical novae.

In contrast, the compact star in the relatively rare x-ray binary is either a neutron star or a black hole. These systems are rare, because the very massive stars that creates neutron stars and black holes are very rare, and the transition of one of these stars in a binary system to a neutron star or black hole will often unbind it from its companion. But the massive amounts of energy released as x-rays and gamma-rays as gas falls onto a neutron star or a black hole make these systems very bright and easily seen across the galaxy, so we know of many such systems, despite their rarity.

While x-ray binaries emit their light at dramatically higher frequencies than do the cataclysmic variables, the x-ray binary subclasses have counterparts among cataclysmic variables. The x-ray pulsar, which is an x-ray binary containing a highly-magnetized neutron star, has the AM Herculis type binary, with its highly-magnetized degenerate dwarfs, as its counterpart. The X-ray burster, which on occasion brightens when a thermonuclear holocaust engulfs the surface of the binary's neutron star, has its counterpart in the classical nova. What makes these systems similar is the presence of the same physics in each star: interactions of gas with a magnetic field, the formation of an accretion disk around a compact object, and the nuclear fusion of hydrogen and helium. They differ through the stronger surface gravity of the neutron star relative to that of the degenerate dwarf, and in the case of the high-mass x-ray binaries, through the different mechanism of transferring mass between the stars, the capture of the companion's stellar wind. These systems are nice complements, because they allow us to study the same phenomena subject to quite different conditions.

Next Issue: The next issue of The Astrophysics Spectator is planned for release in two weeks on October 18.

Jim Brainerd


Types of Cataclysmic Variable. Cataclysmic variable binary stars are composed of a degenerate (white) dwarf star in orbit with a fusion-powered star, which is usually a main-sequence star. The binary produces energy when the fusion-powered star overflows its Roche lobe, dumping gas onto the white dwarf. How we see this energy depends on whether the magnetic field of the degenerate dwarf prevents the formation of an accretion disk. Without an accretion disk, gas fall onto the magnetic poles of the degenerate dwarf, which modulates the light from the gas striking the degenerate dwarf's atmosphere. Systems with an accretion disk can have sudden outbursts when the conditions of the accretion disk change catastrophically. Cataclysmic variables are vulnerable to thermonuclear runaway within the atmosphere of the degenerate dwarf; such events are known as classical novae. (continue)

Types of X-ray Binary. X-ray binaries are rare but bright. They are compact binaries containing either a neutron star or a black hole in orbit with a fusion-powered star. The compact object is surrounded by an accretion disk. If the compact object is a black hole, all of the light we see is from the accretion disk. If the compact object is a neutron star, roughly half of the light we see light comes from the accretion disk, with the remainder produced by gas flowing from the accretion disk into the neutron star's atmosphere. If the neutron star has a strong magnetic field, the inner edge of the accretion disk is disrupted, and the gas flows from the accretion disk onto the star's magnetic poles, creating a pulsed x-ray emission as the star rotates. These systems are called x-ray pulsars. Often the hydrogen and helium that accumulates on the surface of an accreting neutron star builds up until a thermonuclear runaway occurs. These systems are called x-ray bursters. (continue)

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