Astronomy: X Rays from Scorpio

Ever since 1949, when X rays from the sun were first detected, astronomers have been probing the skies for other X-ray sources outside the solar system. Their search was not rewarded until 1962, when more sensitive instruments picked up the first X-ray emissions from outside the solar system. But until this year, only one additional visible object had been definitely identified as an X-ray producer: the familiar Crab Nebula.* Though their relatively crude instruments sensed X rays from about two dozen other vaguely defined areas of the sky, astronomers have been un able to tell which, if any, of the known celestial bodies were producing them. Now X-ray astronomy seems to be coming of age. The strongest X-ray source has been located and identified as a visible object, a previously photographed but seemingly insignificant blue star in the constellation Scorpio.

In order to pinpoint the source of the mysterious Scorpio X rays, a group of scientists led by Physicist Riccardo Giacconi, of Cambridge’s American Science & Engineering, Inc., lofted a NASA Aerobee rocket 150 miles above the earth—well above the atmospheric blanket that X rays cannot penetrate.

In the rocket’s nose was a complex instrument package designed by Physicist Herbert Gursky and containing a sensitive X-ray scanner and a small camera pointed at Scorpio for 55 sec. of the brief ballistic flight. By measuring the changing intensity of X rays detected by the scanner and coordinating the scanner with the camera, Giacconi’s group was able to locate Scorpio’s X-ray source about 1,000 times as accurately as any previous studies. They also determined the angular size of the radiating object itself, and concluded that the X-ray source would probably appear as a bluish, starlike object of the 13th magnitude (visible with a 6-in. telescope).

Infant Star. Armed with this information, observers at the Tokyo Astronomical Observatory and California’s Mount Palomar Observatory focused their large telescopes on the proper position in the sky. Immediately they spotted their quarry: a blue, starlike object with a magnitude of 12.6. “It was really Giacconi’s show all the way,” says Mount Wilson and Palomar Astronomer Allan Sandage. “Identification was terribly easy after he provided the precise location.”

The newly identified object, designated ScoX-1 by astronomers, is in the Milky Way Galaxy; it is between 300 and 3,000 light-years from the earth, and could be about 100 million miles in diameter. Though it has some of the spectral characteristics of a nova, a star that suddenly flares up, the X-ray-emitting envelope of gas surrounding it is apparently not expanding. This leads Giacconi to speculate that ScoX-1 may be a cloud of gas condensing into an infant star, or an existing stellar system surrounded by a gas cloud. “Or,” he says, “we may be looking at an entirely new type of celestial object.”

* The glowing remnants of a supernova, or exploding star, observed by the Chinese in 1054 A.D.