Number 51, October 10, 1991 by Phillip F. Schewe and Ben Stein
THE COSMIC X-RAY BACKGROUND , first detected in 1971, consists perhaps largely of radiation from discrete quasars. British scientists analyzing data from the German x-ray satellite Rosat looked at a very select far-distant field of the sky and found 39 x-ray sources; follow-up studies with an optical telescope showed that 24 of these were quasars. The scientists conclude that at least 30% (but perhaps almost all) of the x-ray background in the 1-keV energy range arise from quasars. (Nature, 26 Sept. 1991.)
LEVELS OF STRATOSPHERIC OZONE OVER ANTARCTICA are at a record low. NASA scientists reported that the total ozone mapping spectrometer (TOMS) on board the Nimbus-7 satellite measured an ozone level of only 110 Dobson units (units indicative of the atmosphere's ability to absorb light) on October 6, 1991, as compared to a level of 500 units which is typical at other times. Scientists fear that decreased ozone levels, and a consequent increase in solar ultraviolet radiation reaching the earth's surface, may lead to increased skin cancers. (The New York Times, 10 October 1991.) Indeed, levels of solar ultraviolet radiation measured at Palmer Station on the Antarctic Peninsula in December 1990 were twice as large as in previous years. (Geophysical Research Letters, October 1991.)
THE EXTREME ULTRAVIOLET (EUV) is a wavelength range (5-25 nm) impossible to see through the atmosphere with telescopes on the ground. The EUV detector on the Rosat satellite has therefore opened up a whole window on astronomy. British scientists in charge of Rosat's EUV survey have reported more than 700 sources, whereas only a dozen were previously known. One consequence is a downward revision of the estimated density of hydrogen in the region of the solar system; it was thought that such hydrogen would limit EUV radiation, much of it coming from hot stars, from reaching the earth. (New Scientist, 28 Sept. 1991.)
GREENLANDISH MUONS suggest the feasibility of using the Antarctic ice mass as a neutrino telescope. A Berkeley-Irvine-Wisconsin collaboration has observed Cerenkov light from secondary muons---presumably formed when high-energy (1 TeV or more) neutrinos interact with atoms inside the earth---in phototubes placed 217 m below the Greenland ice sheet. They regard the observed muon detection rate as evidence that ice is sufficiently transparent to Cerenkov light and that therefore an even more ambitious arrangement located at the South Pole, and referred to as the Antarctic Muon and Neutrino Detector Array, would succeed in locating astrophysical neutrino sources with fairly good angular resolution (one degree). AMANDA, not to be confused with the DUMAND neutrino detector being built on the ocean floor near Hawaii, would employ strings of phototubes lowered 1 km down holes drilled into the Antarctic ice. (Nature, 26 Sept. 1991.)
THE JAPAN NATIONAL LARGE TELESCOPE , an 8-meter, single-mirror telescope, will be built on Mauna Kea in Hawaii. The $100-million facility should be operational by 1999. (50th anniversary issue, Sky and Telescope, Nov. 1991.)
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