X-ray Binaries

If you have X-ray eyes and look up at the sky, it would be a very different and unusual sight. You will be blinded by a few hundred very bright X-ray stars, concentrated mostly towards the center of our galaxy. To an optical astronomer an X-ray binary is at first sight nothing special. But in X-rays they appear as blindingly bright stars, that we now know are black holes and neutron stars devouring material from a companion star. The brightest X-ray source in the sky, called Sco X-1, is an X-ray binary although probably the most famous X-ray binary system is Cygnus X-1, which is thought to be a black hole.

This shows the sounding rocket that was launched around midnight on 1962, June 18/19 and discovered the first X-ray sources outside our solar system. This discovery by R. Giaconni and his collaborators began the field now known as X-ray astronomy. The first point X-ray source discovered was an X-ray binary, known as Sco X-1.

Because X-ray binaries are so bright they were the first X-ray stars to be discovered by X-ray astronomers using very simple X-ray detectors launched into space in the 1960s. The original observations of X-ray binaries were made using brief 5 min sounding rocket flights. In such a brief interval it was hard to get sufficient detail to prove their nature. It was not until the launch of the Uhuru satellite by NASA in late 1969 that their binary nature was confirmed. The longer observations possible with an orbiting observatory revealed eclipses by the companion star and pulsations from the rotating neutron star. These X-ray observations, coupled with optical identifications established that the bright X-ray sources are binary systems containing a neutron star or black hole orbiting around a normal star, like our own sun. The neutron star or black hole captures material from the normal star and in doing so releases large amounts of energy as the material falls into the intense gravitational field (similar to the bang that results if a book is droped onto the floor). The study of these X-ray binary systems is fascinating because it allows us to probe extreme conditions not found on earth. The Rossi X-ray Timing Explorer (RXTE) was recently launched by NASA, with a prime goal the study of these systems in fine detail.

This artists impression of an X-ray binary shows the intense gravitational field of a neutron star drawing matter from a companion star. Because the two stars orbit each other, the material has angular momentum and this causes the material to spiral into the neutron star in an accretion disk.

The neutron star or black hole contains the mass of our sun, but compressed into a sphere 20 km (32 miles) across (similar to the size of a large city like Washington DC). When anything falls into such an intense gravitational field, it will release energy equivalent to 10% of its mass. This is the most efficient way to produce energy known and is ten times more than that released in a nuclear explosion. Any material captured by the compact object, from its companion star, will be heated up to many millions of degrees and will shine as X-rays. In optical light all that is seen is the light of the normal star, and the accretion disk. Until X-ray observations from space were possible, these systems did not seem particularly unusual, and went unnoticed.

Many X-ray binaries are seen all the time, and are now well know friends to X-ray astronomers. But others are transient X-ray stars that come and go over a few months. These transients sometimes are so bright they outshine the brightest permanent source, Sco X-1. Because of this when a new transient system appears, a flurry of activity begins with astronomers eager to capture every aspect of the outburst before it fades.

Even though there are only a few hundred X-ray binaries in our galaxy, they show an amazing variety of different behaviour. During the 1970s it became clear that a defining characteristic is the nature of the companion mass donor star. If this star is old, then the system is called a Low Mass X-ray Binary, where low mass means the mass of the companion star is similar to or less than that of our own sun. In this case X-rays are seen only if the two stars are very close to each other so the normal star fills what is called its Roche Lobe. When this happens material is squeezed off the normal star by the intense gravity of the compact object, like squeezing a tube of toothpaste.

The Low Mass X-ray Binary systems can have very short binary periods indeed and in fact the shortest binary period known is the X-ray binary 4U1820-30 where the two stars orbit each other every 11 minutes! This system is so small it will easily fit between the earth and the moon. To give some estimation of the scale the picture is shown in scale against our Sun.

The orbital lightcurves of Low Mass X-ray Binary systems. When these are viewed from the side, the companion star will get in the way, and cause an eclipse. This results in the X-ray source turning off for a few minutes. The eclipses tell us the orbital period. Also seen is lots of dipping activity which usually preceeds the eclipse. These dips are caused by the splash of material from the companion star, where it hits the accretion disk.

In this artists impression of the most famous X-ray binary, the black hole system Cygnus X-1. In this case a black hole with a mass of 10 solar masses orbits an O star called HD226868. Because the companion star to the black hole is so much more massive than our sun, this system is classified as a high mass X-ray binary. The wind of the star HD226868 is captured by the black hole, and it appears as a bright X-ray source.

It is often asked how a black hole can be observed, if nothing (including X-rays) can escape its gravity. The answer is that we indeed cannot directly observe the black hole. We can only observe the effects of the gravity of the black hole on material nearby. In X-ray binaries the material from the companion spirals in towards the black hole and is heated by the friction of the material rubbing against itself. The material orbits faster and faster as it falls in, and close to the black hole it approaches close to the speed of light. In these inner regions, the temperature of the matter is extremely high, approaching several million degrees. It is this hot material that X-ray astronomers observe. Without X-ray astronomy these systems would not be recognised as unusual. However, once X-ray astronomers find suitable candidates, optical astronomers then go to their telescopes and measure the motion of the optical star, and weigh the X-ray source. If the weight is more than 3 times the mass of our sun, then it is too heavy to be a neutron star and it must then be a black hole.

Cygnus X-1 was the first black hole to be found in the early 1970s, and so is the most famous black hole in our galaxy, X-ray astronomers have over the past discovered many more examples and there are now about 20 such systems. It appears that most of the black holes are transient systems, where the mass flow from the companion star only happens once every few years. The black hole seems to be gradually sucking the life out of its companion, but only a little bit at a time. Fortunately there only seem to be a few hundred of these systems in our galaxy, and none have (so far) been found close enough to the Earth to cause us any cause for concern.

If the X-ray binary contains a neutron star, it is may be seen to pulse in X-rays or give off bursts of X-rays. In this artists impression the material is funnelled by the strong magnetic field of the neutron star onto its magnetic poles. The material falling in from the companion will hit a hard surface and this will shine in X-rays as an intense hot spot. When X-ray observatories, like NASA'ss RXTE observe these systems then the hot spot from the magnetic pole will flash as the neutron star rotates. The magnetic field on neutron stars can be as high as 1,000 billion times that on the earth. Such strong magnetic fields, seem to cause the X-rays to be beamed, so that the X-ray pulsar appears as a flashing lighthouse. These are the strongest magnetic fields known anywhere in the universe, and X-ray astronomy is the only way to study them. A basic quest of science is to test the laws of physics under all conditions. Unexpected discoveries can lead to break throughs in our understanding of the laws of nature.

An artists impression of the flow of material onto a neutron star. The material is funneled by the intense magnetic field onto the netron star pole.

Not all neutron star systems have such intense magnetic fields, and it appears that in many of the low mass X-ray binary systems the field is much weaker. In these cases the flow of material onto the neutron star is more erratic, with the pulsations unstable. Many of these low magnetic field systems show what are called quasi-periodic oscillations (QPO). These oscillations seem to originate from the interaction of the neutron star with the surrounding accretion disk, or from instabilities in the captured material caused by the overwhelming intensity of the X-ray emission.

Quasi-Periodic Oscillations (QPO) seen from the low mass X-ray binary system called GX5-1. The solid line shows the intensity lightcurve in X-rays. The blobs below it are the QPO. These QPO are most intense when the source intensity is faint.

The new material (mostly hydrogen and helium) builds up on the neutron star surface and eventually reaches critical mass. There is then a brief flash, or burst of X-rays as a thermonuclear explosion takes place. Up until recently it was thought that in X-ray pulsar systems the nuclear burning is continuous and that a pulsar would never show a thermonuclear explosion. But RXTE recently discovered a bursting pulsar called GRO J1744-28 that seems to be a half way house between the two types of system. This system is currently fascinating X-ray astronomers and is giving important new information on how the neutron star interacts with its surroundings.

This animation and lightcurve illustrates the thermonuclear flashes from a neutron star in an X-ray binary. Material accumulates on the neutron star until it reaches a critical mass, and then it explodes giving a brief flash of X-rays. Typically the time between explosions is a few hours.