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       An unusual star -- suspected pulsar -- has been observed by astronomers who used two mirrored dishes, intended for solar energy research, as high-energy observatory at NASA's Jet Propulsion Laboratory.

       The star, Cygnus X-3, could have been created from supernova that may have occured within the last few centuries, but was unseen from Earth due to interstellar dust and gas obstructing the view, according to Dr. Richard C. Lamb of Iowa State University, who led the experiment. The star, peculiar X-ray source, is also the brightest high-energy gamma-ray source in the sky. It was observed to emit gamma rays in manner suggestive of pulsar or neutron star.

       The mirrored, parabola-shaped dishes, located at JPL site on Edwards Air Force Base in the California desert, were recently used to observe Cygnus X-3 at ultra-high gamma ray energies above l00 billion electron volts -- one of the highest energy astronomical observations ever made. The ll-meter (33-foot) mirrors are among the world's most sensitive detectors of highenergy gamma rays.

       The observations were conducted by Lamb and graduate student Chris Godfrey of Iowa State University, Dr. William Wheaton of JPL's gamma ray astronomy group, and Dr. Tumay Tumer of the University of California at Riverside. The results of their observations are reported in the April 8, l982 edition of the British science journal _N_a_t_u_r_e.

       Supernovae result in great outbursts of visible light. Four supernovae in our galaxy have been observed from Earth in recorded times; the last was Keppler's Supernova in l604.

       If Cygnus X-3 was supernova in recent times, the explosion of visible light could have been curtained from Earth by dust and gas. But the remnants of the supernova could be visible in gamma ray energy.

       Cygnus X-3 is an astronomical oddity among the hundreds of known X-ray sources in the Milky Way galaxy. Although it emits relatively large fraction of its X-rays in the high energy (or hard X-ray) region of the electromagnetic spectrum, it has never been observed to pulse on the few-second time scale characteristic of most of the other hard X-ray sources.

       Cygnus X-3's X-ray output peaks every 4.8 hours, which is believed to indicate that it is binary star with 4.8 hour orbital period. Theorists have speculated that it may indeed be pulsar, but one which pulses so fast, on the order of l00 pulses per second, that the pulsations have never been observed. (By comparison, the fastest known pulsar, the Crab Nebula, pulses 30 times per second.)

       Astronomers using the JPL mirrors believe that with additional observations this summer, they may be able to detect high-speed pulsation from the star.

       A pulsar is thought to be neutron star -- an extremely dense object in the last stage of stellar evolution.

       Gamma rays, from pulsar or any other source, do not penetrate Earth's atmosphere, and are not normally observable by ground-based apparatus. However, the ultra-high energy gamma rays from Cygnus X-3 were rendered visible by phenomenon known as the Cerenkov effect. faint flash of light which lasts for just few nanoseconds (billionths of second), is created when ultra-high energy gamma rays hit the atmosphere. Although the flash is too brief to be seen by the naked eye, it can be observed by fast photomultiplier tube placed at the focal point of large, upward facing parabolic mirror, like the ones at JPL's desert test site.

       The mirrors have paraboloidal form giving them focal length of about 6 meters (20 feet). They are movable in elevation and azimuth under computer control so that astronomical objects may be tracked as the Earth rotates. The very large size, good optical quality and high reflectivity of the JPL mirrors, and the availability of two, spaced about 30 meters apart, make them uniquely useful for observing Cerenkov events.

       When high-energy gamma-ray photon strikes the atmosphere, it produces shower of secondary electrons, positrons, (anti-matter electrons), and lower energy gamma-ray photons. If the photon strikes the atmosphere with sufficient energy, the secondary electrons and positrons will move at ultra-relativistic speed -- very nearly at the speed of light.

       Although Einstein's theory of relativity states that it is impossible for any material body to move at the speed of light in vacuum, the speed of light in air is very slightly less than that in vacuum. There is no prohibition, however, against travel faster than the reduced speed of light in air, so long as the ultimate speed limit is observed. So ultra-high energy particles traveling in this narrow range of speeds, (faster than the speed of light in air but slower than the speed of light in vacumm) produce "light boom" -- just as supersonic jet produces sonic boom.

       The light boom is manifest as cone of visible radiation. The cone is very flat, almost disk, and travels nearly along the path of the original photon. Long after the air shower has been absorbed by the denser air at lower altitudes, the disk of Cerenkov radiation strikes the ground and produces the flash that is observable with the JPL solar thermal concentrators.

       The observations were supported by the U.S. Department of Energy, NASA, and grant from the Caltech President's Fund.

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4/22/82 MBM
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