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Degenerate Objects

Types of X-ray Binary

X-ray binaries are much more complex systems than cataclysmic variables, because a binary follows one of two evolutionary paths to survive the birth of a neutron star or a black hole. One path creates the low-mass x-ray binary, in which the compact object is fed by its smaller companion through Roche lobe overflow, as happens in cataclysmic variables. The other path creates the high-mass x-ray binary, in which the compact object feeds itself by capturing the companion's wind. The principal difference between the two types of binary is that the low-mass systems have very steady mass flow from the donor star to the compact object, while the high-mass systems have episodic mass flow. Because the orbit of a high-mass system is very eccentric, the system can flare up when the stars come close together. These binaries are called x-ray transients.

Much of the behavior that we see among x-ray binaries is set by the physics at the compact object and within the accretion disk rather than by the precise mechanism causing gas to flow between the stars in the system. The compactness of the neutron star and the black hole dictates that compact binaries containing these objects will emit light predominately at x-ray and gamma-ray energies. The gravitational potential energy released when an object falls onto a neutron star is around 15% of the object's rest mass. Even more energy can be released from an object falling onto a black hole. For hydrogen, this implies a characteristic temperature of over 100 MeV (1012° Kelvin), which is well in the gamma-ray range. The actual temperature one sees is much lower than this, generally in the of hundreds of keV, because gravitational potential energy is converted into radiation slowly in the accretion disk, and the energy released when matter falls onto the neutron star is absorbed by the star's atmosphere. Compared to the 0.7% of the rest mass released in the fusion of hydrogen into helium, one sees that free-fall onto a neutron stars or a black holes is a very effective means of converting rest-mass energy into light. A consequence is that the power released by an x-ray binary can be as much as 10 million times that generated by the Sun. Because this massive energy is released in the x-ray and gamma-ray bands, we can see x-ray binaries from across the Galaxy.

We know of many pairs of systems that look similar despite one containing a neutron star and the other containing a black hole candidate. The behavior of these systems is set by their accretion disks. If the central object is a black hole, all of the radiation we see is from the accretion disk, but if the central object is a neutron star, roughly half of the radiation comes from the accretion disk and the remainder comes from the neutrons star.

Some types of systems are defined by the characteristics of their neutron star, characteristics that are not shared with black holes. In particular, neutron stars can possess a strong magnetic field, and they provide a platform for a thermonuclear runaway.

The x-ray pulsar, which can be either a high-mass or a low-mass binary, is a system containing a strongly magnetized neutron star; it is the x-ray binary counterpart to the AM Herculis type binaries of the cataclysmic variables. But while the white dwarf's magnetic field in an AM Herculis binary has a surface strength of over 107 Gauss, the surface field of the neutron star in an x-ray pulsar is over 1012 Gauss; by way of comparison, Earth's surface magnetic field is around 0.3 Gauss. A x-ray pulsar's magnetic field is so strong that an electron completes one turn of its spiral in the magnetic field in only 10-20 seconds. The light emitted by these electrons is characterized by this time scale, which can appear as an emission line in the x-ray band. It is the observation of such lines that gives us a direct measure of the magnetic field.

The x-ray pulsar is entirely different from the generic pulsar, which is also known as the radio pulsar. The latter systems are isolated neutron stars that convert the energy from their spin into light. The x-ray pulsar creates energy through accretion, pulling gas onto its magnetic poles. Because all the energy is released at the poles, we see the x-rays pulsate as the neutron star rotates and the orientation of the magnetic pole changes.

X-ray bursters, which have only been seen as low-mass binaries, are systems containing a neutron star that occasionally experiences a thermonuclear detonation of its atmosphere. As with the classical novae among the cataclysmic variables, hydrogen and helium build up in the atmosphere of the neutron star in an x-ray burster until the whole atmosphere explodes. But while classical nova outbursts are separated by hundreds of years, x-ray burster outbursts are typically separated by hours or days. The outbursts last for tens of minutes.

In many x-ray binaries, the temperature of the atmosphere is high enough to promote the continuous thermonuclear burning of this gas. This is an issue of the rate gas flows to the neutron star. If the gas flow is high, the temperature in the atmosphere is high enough to promote the steady fusion of hydrogen and helium, but if the gas flow is low, the atmospheric temperature is low, and no fusion occurs. In an x-ray burster, with its low accretion rate, hydrogen and helium build up in the atmosphere of the neutron star. Eventually the pressure at the base of this light-element layer reaches a point that thermonuclear fusion occurs despite the low temperature. This initial burning raises the temperature, which causes increased nuclear burning, until the whole atmosphere of the star is engulfed in flame. We see this event as an outburst of x-rays.

The interesting point about x-ray bursters is that the total energy emitted between bursts is related to the amount of energy released in a burst. The energy released between bursts is gravitational potential energy, while thermonuclear energy is released during the burst. Between bursts we see the liberation of 15% of the rest mass energy of gas falling to the neutron star, but during the bursts we see the 0.7% of the rest mass energy liberated in the thermonuclear fusion of hydrogen into helium. This means that the energy released during a burst should be about 5% of the energy released between bursts.

You may be asking yourself whether a counterpart to the dwarf novae exists. It does, in a system called the Rapid Burster. This is an x-ray burster that experiences both the thermonuclear outbursts common to other burster and outbursts attributed to an accretion disk instability. These accretion-driven outbursts are separated by times ranging from several seconds to about an hour. The bursts themselves can last from a couple of seconds to about ten minutes. Because freefall generates so much more energy than thermonuclear fusion at the surface of a neutron star, the accretion-driven outbursts can contain substantial amounts of energy.

The x-ray binary universe parallels the cataclysmic variable universe, exhibit the same phenomena, but at higher light frequencies, higher luminosities, and shorter time scales.

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