Latest satellite observations provide an answer to the long-standing query: What is the source of cosmic rays -- high energy radiations -- that are received on earth?
AN AMERICAN satellite has provided firm evidence that cosmic rays -- showers of energised particles from space that bombard the earth from all directions -- are produced within earth's own galaxy -- the Milky Way.
Speculation was rife on whether these radiations emanated from within the Milky Way or outside it. These alternatives form the galactic and extra-galactic hypotheses, respectively. Recent observations made by the Compton Gamma Ray Observatory satellite (Nature, Vol 361, No 6409) rules out an extra-galactic source. Launched in 1991 by the Goddard Space Flight Centre, the Compton satellite collected data on gamma rays produced when cosmic rays struck interstellar gas from the region of the Small Magellanic Cloud, a collection of stars and luminous gas located close to the outer edge of the Milky Way.
Cosmic rays fascinate physicists because the particles of which they are composed have energies that are many orders of magnitude higher than those possible in particle accelerators.
The Compton satellite contains a gamma ray telescope that provides information about both the energy of gamma rays as well as their direction. That these measurements can provide clues about the origin of cosmic rays was pointed out more than 20 years ago by Russian astrophysicist V L Ginzburg. He noted the amount of gamma rays produced when cosmic rays strike interstellar gas can be estimated if the mass of the cloud is known, and depends on whether the source of the rays was galactic or extra-galactic.
The quantities of interstellar matter in the Magellanic clouds were already known to radio-astronomers from measurements of their hydrogen gas content. Ginzburg calculated the probable amount of gamma rays caused by cosmic radiation on the Small and Large Magellanic clouds for both the galactic and extra-galactic hypotheses. But his figures could not be verified because instruments with the required sensitivity were unavailable at that time.
Now, a team of 16 scientists has applied Ginzburg's method to analyse data collected over six weeks by the gamma ray satellite when its telescope scanned the Small Magellanic Cloud. Their calculations showed the observed flux of gamma rays tallies closely with Ginzburg's findings for gamma rays of galactic origin.
This finding is expected to intensify efforts to pinpoint sources of cosmic rays received on the earth and in other galaxies. One possible source of extra-galactic cosmic rays could be galaxies with small bright centres called active galactic nuclei (AGN), which are widely believed to be powered by a rotating black hole at their centre.
However, the flux of gamma rays is barely large enough to detect because these rays constitute only one or two of every 100,000 cosmic rays. Furthermore, the flux of the gamma rays fades off sharply at those higher energies that are of greatest scientific interest because they could provide better understanding of astrophysical objects like black holes, neutron stars and AGN. Gamma rays at lower energies are already known to be emitted mainly by the sun.
Meanwhile, larger and more sensitive detection equipment is being developed to circumvent the difficulties of observing high energy cosmic rays. The largest of the new facilities is the Casa-Mia detector in the Utah desert (Science, Vol 259, No 5092). A joint project of the University of Chicago and the University of Michigan, the Casa-Mia consists of an array of 1,089 sensors spread over 0.25 square kilometres, which can detect cosmic rays in unprecedented numbers upto the highest energies.
Sensors have also been created to detect all three effects generated when cosmic rays enter the atmosphere. One of these effects is a kind of shock wave that gives rise to a cone of fluorescence called Cherenkov radiation. Another is the original particles giving rise to a shower of millions of "secondary" particles and the third is atmospheric nitrogen excited by the particles giving off a bluish glow.
New experiments like the Casa-Mia array have been probing sources in the sky such as the double stars Cygnus X-3 and Hercules X-1, which are suspected to be sources of high-energy cosmic rays. Early results indicate these sources only emit cosmic rays in brief bursts and not continuously, which means they cannot account for the bulk of the observed cosmic rays.
Scientists are now focussing their attention on confirmed sources of low-energy gamma rays like the Crab nebula. They plan to observe these sources at a higher range in the energy spectrum in the hope of discovering large amounts of gamma ray emission. As larger areas of the sky are scanned at increasingly higher energies, cosmic ray physics seems poised on the threshold of some exciting discoveries.
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