In a close binary system, the stars are near enough to transfer matter from one to the other. Mass-flux rates of up to about a millionth of a solar mass per year have been observed by optical astronomers. A compact object in a close binary system can, therefore, receive enough matter to power it as an X -ray source,

Let us now take a closer look at how the mass transfer takes place and how the X-ray emission arises in a binary system. There are two ways in which the mass transfer can take place. In a sense these are two extremes and in practice the transfer is probably due to a combination of the two. Firstly, imagine the compact object to be in orbit around an ordinary, fluffy star. As the ordinary star burns its nuclear fuel its central regions contract, and its outer regions correspondingly expand. Eventually, as the star gets bigger, some parts of its surface layers will be attracted more to the circling compact object than itself. This surface material peels off, and falls onto the compact object and mass transfer begins. Secondly, there is mass transfer by means of a stellar wind. The bright 0 and B stars, with which some X-ray sources are known to be associated, are much more massive and much more luminous than the Sun and are believed to have dense stellar winds. These winds are so dense that a neutron star or black hole circling such a star would be able to accrete enough material from the wind to power itself as an X-ray source. In whatever way the mass transfer takes place, because the binary system is rotating, the infalling material has, in general, too much angular momentum to be able to fall straight onto the compact object, just as the Earth {luckily!) has too much angular momentum to be able to fall straight into the Sun. Instead the material forms a ‘disc’ around the compact object.

If the compact object is a white dwarf, a black hole or a neutron star with a magnetic field less than about 104 tesla, the accretion disc extends right down to the surface of the star, or, in the case of a black hole, down to the innermost stable circular orbit. The same friction that gives rise to the angular momentum transfer in the disc also produces a large amount of dissipation of kinetic energy and generation of heat. Thus, as we mentioned above, although material falling radially onto a black hole generates very little radiation, material spiralling into a black hole through an accretion disc can in fact radiate away up to about 40 per cent of its rest mass energy. This is the most efficient means known of transform¬ing matter into energy and it is not surprising that this is one possibility for the explanation of such luminous objects as X-ray sources. The material at the inner edge of a disc around a black hole just falls down the black hole without radiating further. However, if the disc has a star at the centre, about half of the radiation observed is emitted by the disc, and the other half is emitted when the accreted material at the inner edge of the disc crashes into the stellar surface, dissipating the rest of its kinetic energy as heat.

If, however, the compact object is a neutron star with a strong magnetic field the story is quite different. For the neutron stars associated with the pulsating radio sources, magnetic fields as high as I08 tesla (1012 gauss) have been suggested, and there seems no reason why such strong fields should not exist on X-ray-source neutron stars UN well. Such a strong field controls the (low of material out to a distance of several hundred times the radius of the neutron star. The accreting material is highly conducting and so can flow only a long t he lines of magnetic force and not across them. The shape of the magnetic field of a neutron star is basically similar to that of the Earth in that it is a dipole that is. there are two magnetic poles, one north and one south. As for the Earth, there is no reason to expect the magnetic poles and the axis of rotation of the star to be coincident. The accretion disc is. in this case, disrupted a large distance from the neutron star, and the infalling material is funnelled by the magnetic field onto the magnetic poles. At each pole the energy dissipated by the infalling matter produces a hot spot, about a kilometre across, which is a very luminous X-ray emitter. If these hot spots lie well away from the rotation axis, then as the star rotates we see first one spot and then the other- in other words the X-ray emission from the source is pulsed with a period between the pulses equal to, or half, that of the rotation period of the neutron star. This is indeed thought to be the case for the two pulsing X-ray sources cen X- 3 and Her X -1.

Hercules X-l pulses every 1.24 sec and undergoes eclipses lasting 0.8 hours every 1.7 days. The 1.24 sec pulse period actually varies smoothly with a 1.7-day period just as if it were orbiting an unseen companion, and this gives conclusive evidence of the association of X-ray sources with binary systems. The star HZ Herculis varies regularly in brightness by a factor 4 with a period of 1.7 days. If HZ Her were a solitary star, it would be a rather uninteresting star with a surface temperature of some 7000 K. However the close proximity of its energetic and luminous companion causes the side of it nearest the X-ray source to be heated to over 20 000 K and the rotation of the system as a whole causes the observed regular variation in brightness. This is, however, by no means the end of the story. As well as the two periodicities, Her X-l has a third 35-day period associated with it. For 10 of the 35 days the X-ray source is on, whereas in general, for the remaining 25 days, few X-rays are seen at all. Another odd thing is that whereas the X-ray source Her X-l appears to turn on and off, the heating of its companion HZ Her remains more or less constant throughout. This means that for most of the time the X-ray source manages to shine X-rays at the companion star but not at us. How it achieves this still remains something of a mystery.

The X-ray source Cygnus X-l was unidentified for some time, but in March 1971 a radio source appeared close to the expected X-ray position (a number of binary systems, e.g. Algol, are known to have weak, but variable, radio sources associated with them, though the cause of this radio emission is still unknown). The radio source, whose position could be determined much more accurately than that of the X-ray source, coincided with a very young luminous supergiant star HDE 226868. The spectral lines of the star are observed to vary sinusoidally in wavelength over a regular period, indicating that it does indeed have an unseen companion. Because the supergiant star is so massive (about 20 times more massive than the Sun), its companion must also be massive (at least six times and probably ten times more massive than the Sun), in order to be able to swing it around at such a speed. Thus the mass of the unseen companion is well above the theoretical upper limits to the masses of neutron stars and white dwarfs, and it is widely speculated that this object must be a black hole .

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