X-rays are associated with high-energy phenomena. The Sun emits most of its radiation at frequencies corresponding to visible light and has an effective surface temperature of about 5800 K. X-ray photons are roughly a thousand times more energetic than the photons of ordinary light, and so to produce X-rays we need either temperatures of several million degrees, or particles with energies more than several thousand electron volts. Although ordinary stars emit most of their radiation in optical light, there are also bodies in the Universe which emit most of their radiation as X-rays. Early studies with rocket-borne detectors of these cos¬mic sources of X-rays showed a class of objects within our own Galaxy radiating up to a million times more powerfully than the Sun. With the launch of the X-ray satellite Uhuru in 1970, over a hundred such objects were rapidly discovered within the Galaxy and a few more in the Small and Large Magellanic Clouds

These X-ray stars in the Milky Way are characterized by their high luminosity and by the strength and rapidity of their variability. For example, the star Cygnus X-l (the first X-ray source to be discovered in the constellation of Cygnus) varies on a time-scale of milliseconds as well as by factors of five over a few minutes. Even more spectacular are the transient X-ray sources, sometimes called X-RAY NOVAE which appear from nowhere within a day or so and then die away over a period of months. There are also the soft gamma-ray or hard X-ray bursts; several bright ones are seen per year and although they are as luminous as the other X-ray sources they last only a matter of seconds or less. Much of the variability associated with X-ray stars is highly irregular, but some do vary in a periodic manner. The X-radiation from the sources Hercules X-l and Centaurus X-3 is regularly pulsed with periods of 1.24 and 4.84 seconds, respectively. In addition, these two sources and a number of others display regular eclipses with periods of a few days, indicating that these sources at least are members of binary star systems.

Because of the short timescales involved, the X-ray emission must originate from relatively small, compact objects. Three classes of object fit this description: white dwarfs, neutron stars and black holes. The escape velocity from a neutron star, which is the same as the velocity achieved by a particle falling radially onto its surface, is about a third of the velocity of light. A white dwarf has about the same mass as a neutron star but is about a thousand times larger, and so the escape velocity is about thirty times smaller. The velocity achieved by a particle falling onto a black hole approaches the speed of light.

What energy source powers the luminous sources of X-rays ? The pulsar in the Crab Nebula emits pulsed X-radiation with the same pulse period of 33 milliseconds as that observed at radio frequencies. Rotation is the most credible source of such regular pulses and the only known object that can rotate once every 33 milliseconds without being torn apart by centrifugal force is a neutron star. In the case of the Crab pulsar the observed slowdown of the rotation period and consequent decrease of the rotational kinetic energy of the star indicate that the source of energy is indeed the rotational kinetic energy. However the Crab pulsar is unique among X-ray stars in this respect. Centaurus X—3 has a much larger pulse period of 4.84 seconds and has therefore correspondingly less stored rotational energy. If rotation powered this X-ray source it would slow down in less than ten years. This is observed not to be happening: in fact the star is actually spinning up. Thus the bulk of the X-ray stars, unlike radio pulsars, are not powered by their stored rotational energy.

As we mentioned above, when a blob of matter falls onto a neutron star it reaches a velocity of about a third of the speed of light. This means that about a tenth of the blob’s original rest mass energy is converted into kinetic energy during infall and is dissipated as heat when the blob strikes the stellar surface. Similarly up to about 40 per cent of the rest mass energy of matter trickling into a black hole from an accretion disc is released as radiation. A hundred millionth of a solar mass falling onto a neutron star per year is sufficient to heat the star to X-ray temperatures and to provide the observed amount of radiation as well. A comparable trickle of matter through an accretion disc, heats the disc to X-ray temperatures.

A lone neutron star in interstellar space captures material from its surroundings. The density of material in space is so low that the amount of matter that can be acquired heats the star up to only about 100 000 K giving rise to a faint source of ultraviolet radiation.

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