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Japan’s Suzaku satellite shows how all bets are off around Cygnus X-1

by Rowan Hooper

This month, the Vermillion bird of the South — which is currently flying 550 km above Earth — meets an astronomical swan some 6,000 light-years away.

In Eastern mythology, the Vermillion bird of the South represents fire — it is a spirit creature renowned for its elegance and power. Its Japanese name, Suzaku, is also that of an X-ray-detecting satellite launched by the Japanese Aerospace Exploration Agency in 2005.

Now Suzaku’s eyes have been trained on one of the most famous objects in the galaxy — the black hole in the constellation Cygnus (meaning “Swan”), which is also sometimes known as the Northern Cross.

What Suzaku has seen there is arguably more wondrous than the most fantastical mythological story: It has observed the swan at dinner; the black hole swallowing dead stars.

Black holes are strongly embedded in our culture. We refer to them metaphorically (“that business was a black hole for money”), while the best-known of many mind-boggling facts about black holes — that nothing, not even light, can escape them — is probably familiar to most of us even if we don’t really understand what it means.

All that is to say it’s easy to take black holes for granted. But in fact, while astronomers strongly suspect they must exist, that suspicion is maddeningly hard to prove.

Strictly speaking, the things we think of as black holes are only black hole candidates. However, Suzaku is armed with telescopes that, instead of picking up visible light, detect X-rays, which are more powerful than visible light. And now, its observations have provided good evidence that these most mysterious of astronomical objects are real.

Shin’ya Yamada and colleagues at the Riken Nishina Center for Accelerator-Based Science at Wako in Saitama Prefecture directed Suzaku’s telescopes at an enigmatic object named Cygnus X-1.

Discovered in 1964, this is one of the strongest known sources of X-rays in our Milky Way galaxy, and has long been a suspected black hole. (It was the subject of one of the most famous bets in science, as we’ll see.)

Cygnus X-1 is a remarkable object. Its mass is 15 times greater than that of our sun, but it is far more compact. The diameter of our sun is 1.4 million km, while that of Cygnus X-1 is only 52 km. Earth’s diameter is 12,700 km. So in Cygnus X-1, a gigantic amount of matter has been compacted down into an incredibly small space. The more massive an object, the stronger its gravitational pull, which means that Cygnus X-1 has irresistible suction power.

Anything that gets close is sucked in. Black holes accumulate massive amounts of matter — dust, gas, rocks, even bits of planets and stars — in a huge disc that orbits the black hole, gradually getting closer. This is called the accretion disc, and it is from this that powerful X-rays are emitted as the gravity of the black hole crushes the matter in the disc. Once stuff gets too close, however, it can never break free. The point of no return is called the event horizon.

What the Riken astronomers observed is gas being sucked across the event horizon and into oblivion. Suzaku measured the temperature of the gas as it fell into the black hole, and recorded a heat spike of more than 1 billion degrees Celsius in the last few milliseconds before it vanished. This, they report in Astrophysical Journal, is strong evidence that Cygnus X-1 is indeed a black hole (DOI reference: 10.1088/2041-8205/767/2/L34).

For at least 20 years, most physicists have been happy to accept that Cygnus X-1 is a black hole. Indeed, that was what the bet was about. In 1974 the English cosmologist Stephen Hawking and his similarly renowned U.S. colleague and friend, Kip Thorne, were arguing about this faraway object.

Hawking wagered that Cygnus X-1 was not a black hole — but in 1990, he finally conceded he was wrong.

The Hawking-Thorne wager can be seen as the end point in a longer debate among scientists about black holes. Albert Einstein, whose work showed among other things that gravity could influence light (and even bend it), himself did not believe that black holes could exist.

But theoretical work supporting the idea of a super-dense object that warps space-time continued to build, and by the 1960s it was generally accepted that black holes should exist. Some physicists credit the discovery of Cygnus X-1 as the first time a black hole was found.

Incidentally, the terms of the Hawking-Thorne bet shed interesting light on the reading habits of these famous scientists. If Hawking had won, his reward would have been a four-year subscription to the fortnightly Private Eye, Britain’s foremost satirical magazine. Thorne requested, and hence received, a year’s subscription to Penthouse, a U.S. monthly that may euphemistically be termed a “men’s magazine.” (Hawking reported that Thorne’s wife was not happy with her husband’s winnings.)

Meanwhile, Suzaku continues to look deep into the galaxy. Last year, after its X-ray scopes were directed at the constellation of Orion, it observed an usually bright patch of cosmic dust known as McNeil’s Nebula.

This region turned out to be a protostar, a region of space where the magnetic fields are so intense that they are heating the surrounding area and emitting X-rays.

Kenji Hamaguchi, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said he suspected that the protostar, which is still swaddled in its birthing cloud, is a mere baby in astronomical terms at younger than a million years old — but not the sort of babe Thorne was seeing when he won his bet.

Rowan Hooper (@rowhoop on Twitter) is the News Editor of New Scientist magazine. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru (The Evolving Human).”