The Sun radiates most of its energy as light and some as X-rays. Meghnad Saha had predicted X-ray emission from the Sun. During occasional solar flare-ups, it was found that the solar X-ray output increases affecting radio reception on the Earth. The Sun has a temperature of about 5,700 K. when celestial objects are heated to millions of degrees, they emit most of their energy in the form of X-rays and only a negligible portion as visible wavelengths. However, the atmosphere of the Earth absorbs the radiation. X-rays can therefore be observed only from above the atmosphere.
X-rays reveal a dramatic, often violent, Universe. They are, in general, associated with sources and situations in the Universe, which involve large concentrations of energy or where large energy releases are taking place. In the course of its evolution, a star may radiate only a small portion of its energy but at the end of its life, it may collapse. In the last few seconds preceding such a collapse, the heat could be in the range of 10 million to 100 million degrees (Kelvin), which accounts for the X-rays.
A study of the X-ray radiation is likely to throw light on several fundamental questions. One relates to the X-ray background. Even the earliest rocket-based devices disclosed a diffuse background of X-radiation. The uniformity of the phenomenon (above 2,000 eV) suggested that most of it originates outside our galaxy. If it is proved that quasars or other discrete objects account for the integrated X-radiation, there would be no need to assume an all-pervading thin hot gas as its cause. Existence of such a gas had led many astronomers to forecast that the expansion of the Universe would be ended and presume a closed Universe.
X-rays may explain the secret of the enormous energy of quasars. The most distant quasars seem to emit only X-rays. Large changes in the X-ray intensity of quasars noted over just a few hours are taken to indicate that X-ray formations should be closer to the center of the object. There is speculation that the center of a quasar may have a black hole swallowing the stars.
The X-ray picture of the extra-galactic space is giving us a new view of the Universe. Individual X-ray sources in other galaxies have been pinpointed and studied by X-ray satellites. For example, the nearest spiral galaxy, M31, in the constellation of Andromeda (about two million light years away) compares well with our own in terms of individual bright X-ray stars. However, there seems to be a greater concentration of stars in the central region of the other galaxy than in the center of our galaxy, while relatively younger stars, gas and dust seem to mark the spiral arms of M31. Celestial X-rays also throw some light on the way galaxies form themselves into cluster each with an average of 100,000 stars. It is common to find hundreds or thousands of galaxies gravitationally associated over a relatively short distance.
With the increase in the sensitivity of the X-ray telescopes, it has been found that almost all astronomical objects, ranging from the nearby Jupiter to the most distant quasars emit X-rays. Several ideas are bound to undergo constant revision in the light of new discoveries. In particular, scientists would be looking for clues to understand the conversion of matter into energy on such an enormous scale.
X-rays were first detected in the Sun by a rocket. The US Naval Research Laboratory discovered them in 1949, using captured V-2 rockets. Following the discovery, Herbert Friedman and his colleagues scanned the X-ray output of the Sun throughout its 11-year Sunspot cycle. The Sun was found to emit only a millionth of its total energy as X-rays. The results confirmed that the solar corona (the halo seen only during a solar eclipse) has a high temperature of the order of am million degrees.
X-ray detection on photographic film was not quite satisfactory. Moreover, scientists wanted to know whether there is any X-ray source outside the solar system. In 1962 Riccardo Giacconi and his colleagues devised a special telescope and found a source outside the solar system in the constellation Scorpio, SCO X-1. The temperature of the faint blue star was 50 million degree Centigrade and its brightness fluctuated. In the succeeding years, more X-ray sources were found. In 1964 the famous Crab Nebula, was found to emit X-rays. What was more, the source was not a point but a region, as the X-ray output continued to be registered even when the Moon hid the nebula. The Crab is 3,000 light years away and has been moving at 1,600 km/s! It was soon found that its X-ray output was 10,000 times more than its radio emission.
Notable discoveries were made outside our galaxy also. They included M87, a giant elliptical galaxy and 3C 273, earlier recognized as the first quasar. Another interesting finding was the identification of a bright and flickering visible companion for SCO X-1.
The American satellite, Explorere-42, known as Uhuru, was devoted entirely to x-ray astronomy. Launched in 1970, it lasted three years and gave several interesting details. One of the most important findings of Uhuru was binary star systems in our galaxy, where matter was continuously transferred from a larger star to its smaller companion.
Neutron stars, white dwarfs and black holes are the results of catastrophic implosions when stars burn out their nuclear furnaces and can no longer have central gas pressure to sustain the weight of the mass. A light-weight star like our Sun will end its as a white dwarf, no bigger than the Earth but where a spoon of matter would weigh about a tonne! White dwarfs cannot exceed 14 solar masses. Neutron stars are middle-weight champions, not exceeding two to three solar masses, perhaps 16 km in diameter, reaching densities 100 million times that of the white dwarfs.
Next are the heavy-weight stars that collapse in a millionth of a second into a black hole from where nothing not even light, can escape. The volume of such a collapsing star becomes smaller and smaller but its density grows higher and higher. Astrophysicists say that it can be sensed only by its external gravitational pull which will be enormous as they have three to 50 solar masses in areas as small as 18-300 km. The secrets of neutron stars and black holes can be understood only through X-rays, while white dwarfs can be studied with large optical telescopes.
The first X-ray satellite, Uhuru, was useful in optically locating Hercules (Her X-1) and Centaurus (Cen X-3), binaries which showed regular X-ray pulsations.
Several findings of Uhuru were later confirmed and extended. Two Seyfert galaxies, 200 million light years away, were found to emit X-ray millions of times more powerful than Crab Nebula. In the 1970s several satellites studied X-rays from space. They included the Astronomical Netherlands Satellite (ANS), US Air Force’s Vela satellites, the British Ariel V, the Soviet Salyut-4 and the American Copernicus (1972). The last named had no conventional lenses but metal walls that focused the incoming radiation to a detector. Copernicus found, among others, evidence of time variation in an extra-galactic source. Ariel V avoided the Earth’s inner radiation belts which might distort the X-ray pictures. It investigated known sources, while another, High Energy Astronomical Observatory (HEAO-1), observed sources 10,000 times fainter than the Crab.
India’s Aryabhata (1975) had an X-ray detector as an experiment. It had two telescopes, one along the spin axis for particular sources and the other on its belly for a general scanning of the sky. Though the experiment could be given power only for five days, the results showed evidence of X-rays in the center of our galaxy. The X-ray experiments were a follow-up of the studies conducted by the Tata Institute of Fundamental Research with balloon-borne instruments aimed at catching glimpses of X-rays above the veil of the atmosphere.
As a neutron star shrinks, it rotates and in the process, electrons locked in the magnetic field are rotated at the speed of light. This results in X-radiation which can, depending on the angle of the magnetic field to the spinning star, be seen as a revolving beam of light. Such X-ray pulsars have been found in Crab and elsewhere and they vary in their emission from 70 milliseconds to 14 minutes. The ‘bursters’ and the transient sources may turn out to be only different forms of the same high-density matter.
Indian scientists from TIFR, in collaboration with Canadian investigators, made a major discovery from one of their balloon flights. They found that Scorpius X-1, the first X-ray source outside the solar system discovered in 1962, was in fact a pulsar, sending rapid pulses of X-rays at very short intervals of 2.93 milliseconds.
Though by 1977 over 400 X-ray sources had been discovered, they were among the most luminous and unusual phenomena in our galaxy and the most powerful and the nearest in other galaxies.
There is a direct proportionality between the photons (energy carried by a particle of light) emitted from a hot gas and its temperature. For example, for the Sun, the photon energy is 2.5 electron Volt (eV). If the object reaches a temperature of a million degrees (K), the photon energy would be about 500 eV or 0.5 KeV (Kilo electron Volt). Early detectors tried to register the radiation, but they lacked directionality and clearance from background noise.
Increasing the area of collection of radiation would not proportionately increase the sensitivity. As glass would absorb X-radiation, a different technique was called for. Giacconi and Rossi led a team of scientists in the U.S. to develop grazing optics. They first tried the new technique in solar X-ray photography. They succeeded in obtaining in 1963 what is regarded as the first true picture of the Sun. with the development of solid state technology, larger telescope and better X-ray TV cameras were developed in time to place them in HEAO-2, otherwise known as the Einstein Observatory, launched in 1978.