With permission from the one at the center of the galaxy Messier 87, the first to be photographed in 2019, Cygnus X-1, in the constellation Swan, is probably the most famous black hole in the history of astrophysics. To begin with, because it was the first discovered and one of the closest to Earth, but above all because it starred in a famous bet between the British physicist Stephen Hawking and the American and then future Nobel laureate Kip Thorne. Against his own work, Hawking in 1974 gambled a four-year subscription to the erotic magazine ‘Penthouse’ that it was not one of these exotic objects. It was not until 1990, when more observations of Cygnus X-1 had already been made, that it acknowledged its defeat.
Probably Hawking, who died in 2018, would have been interested to know the results of the new study published this Thursday in the journal ‘Science’, although they add more salt to the wound of its failure. Because the research, led by an international team of astronomers, suggests that Cygnus X-1 is more massive than previously believed. It has around 21 solar masses, which is a 50% increase over previous estimates. This makes it the most massive stellar-mass black hole ever detected without the use of gravitational waves. In addition, it is further away: it is located 7,200 light years from our planet and not 6,100 as previously believed.
The object was discovered in 1964 when a pair of Geiger counters, an instrument for measuring radioactivity, were transported aboard a suborbital rocket launched from New Mexico. Now, it has been studied with the help of the Very Long Baseline Array, a radio telescope the size of a continent made up of ten plates scattered across the United States, along with a clever technique for measuring distances in space.
“If we can see the same object from different locations, we can calculate its distance from us by measuring how far the object appears to move relative to the background,” explains James Miller-Jones, a professor at Curtin University and the International Center for Research at Radio astronomy (ICRAR). If you place your finger in front of your eyes and see it with one eye at a time, you will notice that your finger seems to jump from one place to another. It is exactly the same principle , he clarifies.
The researchers observed a full orbit of the black hole for six days and used observations taken from the same system with the same set of telescopes in 2011. ‘This method and our new measurements show that the system is further away than previously thought, with a black hole that is significantly more massive, “concludes the study’s lead author.
That this black hole is so massive challenges what astronomers believed about how these cosmic wells form. Stars lose mass in their surroundings through stellar winds that blow from their surface. But to make such a heavy black hole, we need to reduce the amount of mass that bright stars lose during their lifetime, ”explains Ilya Mandel of Monash University and the ARC Center of Excellence for the Discovery of Gravitational Waves (OzGrav).
Close to the speed of light
The black hole in the Cygnus X-1 system began its life as a star approximately 60 times the mass of the Sun and collapsed tens of thousands of years ago, says the scientist. “Incredibly, it is orbiting its companion star, a supergiant, every five and a half days at only one-fifth the distance between Earth and the Sun. Furthermore, Cygnus X-1 spins incredibly fast, very close to the speed of light and faster than any other black hole found to date.
Next year, the world’s largest radio telescope, the Square Kilometer Array (SKA), will begin construction in Australia and South Africa. Scientists hope that this and other next-generation facilities will help understand these extreme regions of space. This is a great time to be an astronomer. Studying black holes is like shedding light on the best kept secret in the Universe, it is a challenging but exciting area of research, “says Professor Miller-Jones