X-ray Transients

This is an X-ray image of a ROSAT observation of a portion of the Andromeda Galaxy. The source shown in the circle is RX J0045.4+4154, a recently discovered recurrent X-ray transient.

A rocket-borne experiment in April 1967 found an intense X-ray source in Centaurus. This source had not been detected a year or so before, and observations after April showed a steady decline in the source luminosity until it completely disappeared in late September. The source was named Centaurus X-2, and the word "transient" was associated with its behavior. Two years later, in 1969, another source was seen in the Vela 5B satellite data that exhibited similar "not there, then there, then not there" behavior. By mid-1973, enough sources had been discovered with similar characteristics, that a new class of sources, the X-ray transients, was firmly entrenched in X-ray astronomy.

The precise definition of an X-ray transient has undergone significant evolution since the early 1970s, primarily because of ever-increasing sensitivity of X-ray telescopes and detectors. Originally, the definition was highly based on obserations, and included the following criteria:

• A transient had to have a rapid rise time (the time it takes to reach its peak luminosity) of less than a week, and gradual decline of approximately 1 to 8 weeks
• The ratio of maximum X-ray flux to minimum flux had to be 1,000:1 or more
• The source could not reappear on time scales less than two years

The ten-year light curve of the transient source V0332+53 as seen by the Vela 5B all-sky monitor. The source became very bright in 1973, and was not seen again until 1983.

X-ray transients seem to divide themselves naturally into two classes: those associated with high-mass X-ray binaries and those associated with low-mass X-ray binaries. The high-mass X-ray binaries contain a neutron star or black hole paired with a massive star (usually more than five times the mass of our sun). Often, the stellar companion is a Be star, which sometimes sheds material from its equatorial region. In these systems, the transient event is characterized by having more higher energy X-rays in the spectrum. The low-mass X-ray binaries contain a neutron star or black hole orbiting around a cooler, low-mass star. These transient events often generate more lower-energy X-rays.

How do they do it?

For a binary to be a bright X-ray emitter, there must be some source of material that the neutron star can accrete. In high-mass X-ray binaries, a companion Be star provides a well-known potential source of the material for the neutron star. Be stars are known to possess a circumstellar envelope that is strongly concentrated in the equatorial plane. Furthermore, these stars are known to eject large blobs of material from their equatorial region suddenly, probably because these stars rotate so quickly that they are near their break-up point.

Winds from the normal star can also provide material, especially for the more massive binaries. Changes in the wind density or velocity can then lead to a neutron star being able (or unable) to accrete the material due to the existence of a centrifugal barrier around a rapidly rotating neutron star. This barrier arises from the interaction of the neutron star's magnetic field, which directs material to the neutron star, and the material in the wind. The magnetic field rotates at the same rate as the spinning neutron star. If the accretion rate is small enough, the size of the magnetic field grows. If the magnetic field is large enough, then the velocity at the edge of the field is larger than the orbital velocity of material in the wind, and the material cannot enter the magnetic field. Hence, accretion onto the neutron star is cut off and the X-ray production is turned off. X-ray production can be turned back on if the wind velocity or density increases, shrinking the size of the magnetic field. Thus, while the wind is persistent, the X-ray production can turn on and off, creating a large change in source luminosity.

What is happening to change the accretion rate in a low-mass system is not nearly so clear. But we know that something must be changing the accretion rate. It could be that accretion occurs only during a small part of the binary orbit, when one star enters inside the Roche lobe of the other star. If these systems have extremely long orbital periods so that this does not happen very often, it could make the source look like a transient. Or perhaps the binary orbit is precessing, and only during a certain phase of the long-period precession do all the right conditions occur that would lead to significant accretion. The most popular model for low-mass binary transients is the creation of an instability in the accretion disk. In essence, it is thought that material can accumulate "quietly" in the disk for some time until there is enough material for a thermal instability to take over and enhance the rate of accretion onto the compact object.

Last Modified: October 2010