The Proof Is Out There

The X-ray is her siren song, my ship cannot resist her long
Nearer to my deadly goal until the black hole gains control
Spinning, whirling, still descending, like a spiral sea unending
Sound and fury drowns my heart, every nerve is torn apart…
– from “Cygnus X-1” by Rush (1977)

Tom Bolton’s licence plate bears the designation CYG X1. On occasion, people ask if it refers to the song “Cygnus X-1” by the rock band Rush. Little do they realize that Bolton, a University of Toronto astrophysicist, is the fellow who – 30 years ago – discovered the black hole that inspired the song.

Black holes have seized people’s imaginations ever since Bolton published observational proof of their existence in 1972. They have spawned science-fiction plots about time travel and secret passageways to other dimensions, and they have fascinated anyone who ponders what the heavens might hold.

Bizarre beyond comprehension, a black hole occurs when a gigantic star implodes to become extremely small and dense, resulting in an immensely powerful gravitational pull. “Things can go in, but they can’t go out,” Bolton explains. Even light cannot escape from a black hole, making it invisible from the outside.

That was the challenge for astronomers seeking these elusive phenomena in the late 1960s and early 1970s: if black holes indeed existed, how could they be detected? Only indirect clues were available, such as strong X-rays pouring from an unidentified source or a mysterious back-and-forth motion in a star’s orbit, suggesting a gravitational tug from an unseen companion.

Bolton didn’t set out to find a black hole in the summer of 1971. Then a 28-year-old post-doctoral fellow and part-time faculty member at the University of Toronto, he simply had access to plenty of telescope time at the university’s David Dunlap Observatory in Richmond Hill, Ont., and spent several nights a week peering at the heavens through the 1.88-metre telescope inside the domed observatory. His first love was binary star systems (two stars that orbit each other, similar to the earth-moon orbit).

That summer, he grew interested in a possible binary system involving a giant blue star called HDE226868 and something else that was emitting powerful X-ray signals. He thought this invisible X-ray source, dubbed Cygnus X-1 (because it was in the constellation of Cygnus, the Swan), was likely a neutron star.

In September, he started observing HDE226868. Heading to the observatory that night, Bolton met a grad student who remarked that this binary system might contain a black hole. “No way,” Bolton told him. But the next day, looking at photographic plates of the star’s spectrum, he saw some puzzling emission lines – bright marks in unusual places. “That tweaked my interest,” he says. He decided to continue observing HDE226868.

By November, Bolton had enough data to estimate that HDE226868 was moving around its invisible partner at more than 70 kilometres per second – far, far faster than anticipated. Such tremendous speed meant that whatever body was emitting the X-rays was far too dense, and its gravitational pull was much too strong, to be a neutron star.

The only feasible option was a black hole. “At this point my excitement reached a fever pitch, and I could hardly eat or sleep,” Bolton recalls. A stream of gas seemed to be flowing from the star to Cygnus X-1, swirling around it at incredible speeds before vanishing. As a result, the gas became super-heated and produced the X-rays that had drawn astronomers’ attention.

They were heady findings, and Bolton admits he was naive not to consider that others might be racing to publish the same results. Sure enough, in December a colleague handed him a preprint of a paper by two English astronomers about Cygnus X-1 and HDE226868. But their observations were cautiously worded, and Bolton believed he had better data. He decided to submit his own results for publication. “If I was wrong, it could be my career,” he says. “On the other hand, if I was right, it could be my career!”

In December, an American colleague supplied some last-minute findings that erased any remaining doubts in Bolton’s mind. He wrote his paper; it appeared in Nature in February 1972, shortly after the English astronomers published their data. But Bolton had staked his claim: the high energies of the X-rays and the large mass of the star’s unseen companion raised “the distinct possibility that [Cygnus X-1] is a black hole.”

“Theorists have been postulating black holes, but this could be observational evidence for one,” commented University of Toronto astronomer Helen Hogg in her popular Toronto Star astronomy column.

Today, Bolton looks back on those exhilarating months as “easily the most exciting time of my career.” In December 1972, he published a detailed follow-up paper in Nature Physical Science, later included in a Harvard University Press selection of the most important astronomy papers published between 1900 and 1975. He was interviewed by The New York Times and was invited to speak at Princeton University.

Extraordinary claims require extraordinary proof, and the next few years brought arguments against Cygnus X-1. “At the time, it was tremendously exciting and tremendously controversial,” recalls John Percy, who, like Bolton, is a professor in U of T’s department of astronomy and astrophysics.

Bolton himself wondered why no other black holes were being identified. Finally, at the end of the 1970s, another candidate was proposed; today, about 10 stellar-mass black holes have been pinpointed. And with the idea of black holes firmly established, “astronomers subsequently went on and found super-massive black holes at the centre of many galaxies,” says Percy.

Bolton doesn’t claim to be an expert on black holes. “I find them, you explain them,” says the 59-year-old astrophysicist, who, according to family legend, uttered “moon” as his first word. But Bolton can rest assured – locating one of these cosmic oddities is clearly achievement enough.

Kristine Culp is a freelance writer in Toronto.