Discovery of Astronomical X-Rays

Cygnus X-1, the First Star Identified as a Likely Black Hole

Aug 26, 2007

The discovery, in the early 1960s, of the first celestial X-ray sources led to the first star identified as a likely black hole candidate, Cygnus X-1.

Cygnus X-1

Cygnus contains the classic example of a likely black hole, the evolutionary endpoint for the most massive stars. This classic black hole candidate goes by the name Cygnus X-1 because it was the first X-ray source discovered in the constellation Cygnus and among the first X-ray sources discovered.

Discovery of Celestial X-Rays

On June 18, 1962 one minute before midnight an Aerobee rocket was launched from White Sands Missile Range in New Mexico. During its brief 120 km flight, the rocket reached an altitude of 225 km. The portion of the flight higher than 80 km, which was above enough of Earth's atmosphere for the intended measurements, lasted a scant 350 seconds, just shy of 6 minutes.

The rocket carried three Geiger counters that were designed to detect celestial X-rays from the Moon and other possible celestial sources. Two worked. With this pioneering flight the team consisting of Riccardo Giacconi, Herbert Gursky, Frank Paolini, and Bruno Rossi launched the field of X-ray astronomy.

They discovered an X-ray source in Scorpius that is now known as Scorpio X-1. The X-ray detectors also found a possible second X-ray source in the general direction of Cygnus and Cassiopeia. They probably observed Cygnus X-1, but the positional information at this stage was not very accurate. Subsequent rocket and satellite flights that continue to this day, have more thoroughly studied these X-ray sources and the rest of the X-ray sky.

Confirmation of Cygnus X-1

Two years later, in 1964, a team consisting of Herbert Friedman, Stuart Bowyer, Edward Byram, and Talbot Chubb confirmed that the possible second source in the 1962 flight was in Cygnus. It then officially became Cygnus X-1, but the position was still uncertain.

It is simply not as easy to accurately measure the direction from which X-rays originate as it is for optical light or radio waves. The constraints of a rocket flight lasting only a few minutes exacerbate this difficulty. X-ray satellites with longer duration flights allow more accurate position determinations than brief rocket flights but are still not as accurate as optical or radio observations.

UHURU - First X-Ray Satellite

In December 1970 NASA launched the first satellite specifically for X-ray astronomy, UHURU. Cygnus X-1 was a prime UHURU target. In 1971 the UHURU X-ray satellite measured a reasonably accurate position for Cygnus X-1. However the error range for this position was still large enough that the range of possible positions was a rectangular section of sky containing between 200 and 300 stars visible with the Mt Palomar 200" telescope. The position needed to be determined more accurately. How?