X-ray Transients

This is an X-ray image of a ROSAT PSPC observation of a portion of M31. 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 which 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 detection sensitivity. Originally, the definition was highly observation-biased, e.g., a transient had to have a rapid rise time (< 1 week) and gradual decline (~ 1 - 8 weeks), the maximum to minimum X-ray flux ratio had to be of order 1000, the source could not reappear on time scales less than 2 years, and so on. Today, the definition is much less restrictive. Today, a source is defined as a transient if it has periods of enhanced X-ray emission which typically last longer than a week, but which are not representative of the usual observed emission from the source. Recurrences can be periodic or aperiodic, but there is no obvious correlation between recurrence time and the outburst luminosity amplitude.

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 2 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 5 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 has to be some source of material which the neutron star can accrete. The identification of the optical companion of many of the massive transients as Be stars provides a well-known potential source of the material for the neutron star. In particular, Be stars are known to possess a circumstellar envelope which is strongly concentrated in the equatorial plane. Furthermore, these stars are known to suddenly eject large blobs of material from their equatorial region... probably due to the fact that these stars rotate so fast that they are near their break up point.

Winds from the normal star can 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 the neutron star. 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 might 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 which would lead to significant accretion. A more popular model for low-mass binary transients is the creation of an accretion disk instability. 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 accretion rate onto the compact object.