Astronomers crack a black hole’s ID

Details providing the first ever complete description of a black hole have been wrestled from the gravitational clutches of the X-ray binary system Cygnus X-1. The breakthrough identifing the cosmic cannibal was made by a team of astronomers at the Harvard Smithsonian Center of Astrophysics (CfA) and San Diego State University.

Artist's impression of Cygnus X-1
Artist's impression of Cygnus X-1 with the black hole siphoning material from its stellar companion. Image: NASA/ESA/Martin Kornmesser.

Cygnus X-1, consisting of one of these exotic objects paired up with a massive blue O-star from which it greedily feasts, shot to fame when it was the centre of a bet between Stephen Hawking and fellow astrophysicist Kip Thorne, a professor of theoretical physics at the California Institute of Technology.

Treating the bet as an insurance policy, Hawking fluttered his theory that Cygnus X-1 did not contain a black hole and, with the scientific work on the system proving otherwise by 1990, inevitably lost the bet. “For forty years, Cygnus X-1 has been the iconic example of a black hole,” said Thorne. “However, despite Hawking’s concession, I have never been completely convinced that it really does contain a black hole – until now.”

The research team that has well and truly locked down the binary system’s identity was led by Harvard’s Mark Reid, who put an uncertainty about the mysterious X-ray belching object to bed once and for all. Enlisting the help of the National Science Foundation’s Very Long Baseline Array (VLBA), Reid, along with his Harvard colleague Lijun Gou and San Diego State University’s Jerome Orosz, used the continent wide radio telescope system to make the first direct trigonometric parallax measurement of the distance to Cygnus X-1, marking the beginning of slotting the pieces of the black hole puzzle into place.

“Because no other information can escape from a black hole, knowing its mass, spin, and electrical charge gives a complete description of it,” said Reid who is the lead author of one of the three papers about Cygnus X-1 published in The Astrophysical Journal. “The charge of this black hole is zero, so measuring its mass and spin make our description complete.”

And with their new distance measurement of 6,070 light years from Earth, parked between previous estimates of 5,800 to 7,800 light years, determining measurements for its spin and mass is exactly what the astronomers did, in the process completing the so-called ‘No hair’ theorem that says a black hole can be completely characterised by these three external properties (the ‘hair’ is a metaphor for all the other information about the matter being guzzled on by the black hole).

Rounding off the description with X-ray data from Chandra, the Rossi X-ray Timing Explorer and the Advanced Satellite for Cosmology and Astrophysics along with visible-light observations, the team measured a hefty mass of 14.8 times that of the Sun spinning at a rapid speed of 800 times per second.

“To measure the mass of the black hole and O-star [the companion star], we measure the true orbital velocities and the size of the orbit,” Orosz, who is lead author of two of the remaining papers, told Skymania News. “We can measure the radial velocities of the O-star from the optical spectra. Since the binary orbit is inclined to our line of sight, the radial velocities will be less than the actual orbital velocities. If we know the angle of inclination, then we can make a simple correction to find the actual orbital velocity.”

The extent of the Very Long Baseline Array
The extent of the National Science Foundation's Very Long Baseline Array (VLBA) that helped identify Cygnus X-1. (Credit: NRAO/AUI).

Due to the O-star becoming distorted by the gravity of its companion it is pulled into a teardrop shape, moving in its orbit in such a way that the apparent brightness flickers from one magnitude to the next. “These brightness changes are called ‘ellipsoidal variations’ and we have a pretty good model to explain them,” added Orosz. “What is normally done in binaries like Cygnus X-1 is the brightness changes over the orbit are used to measure the inclination of the binary. However, we don’t know in this case how close the O-star is to its ‘critical surface’ [basically its maximum distortion] and a fairly wide range of inclination angles can lead to the same observed ellipsoidal variations. Each inclination angle leads to a different correction factor for the radial velocities, which result in different radii for the O-star and different masses for the star and black hole.”

So how did the team get around this problem? “This is where parallax distance comes in,” Oroz told us. “Using the inverse square law and the Stefan-Boltzman law [the luminosity of a star is proportional to the square of its radius and the 4th power of its temperature], we can figure out the radius of the O-star. Once we know what the size of the O-star should be, we can figure out which range of inclination angles to choose from the light curve models. The correct range of inclinations then gives the correct mass measurement.”

Despite completing the characterisation of an object that has eluded astronomers for centuries, the team did not stop there; they now think they might know how Cygnus X-1 formed. Tracing the movement of the system through the Milky Way Galaxy with the addition of VLBA observations made during 2009 and 2010, the astronomers believe that the explosion of a supernova is not responsible for the black hole’s birth due to its sluggish speed through the cosmos.

“If there is an asymmetric ejection of a significant amount of mass at high speed during a supernova explosion, then the stellar remnant will recoil,” said Reid. “Alternatively if a star in a tight binary system explodes and loses enough mass to unbind the system, both stars can move apart at roughly the same escape speeds but since the binary is still intact, this didn’t happen for Cygnus X-1.” Instead, their results support the theory of a 100 solar mass star undergoing a ‘dark collapse’ into a black hole without an explosion.

While Cygnus X-1 has let some of its mystery slip out of its grasp, it is still refusing to give too much away as it clutches tightly to the secrets that make black holes enchanting. “The way black holes are formed in close binary systems is not well understood,” said Orosz. “Obviously having examples of close binary systems with well measured measured masses helps constrain various theories. Also the understanding of how stars explode and what types of stars explode in supernova has gaps. Having a list of well-measured black holes [and neutron stars] helps constrain things.”

Earlier this year, astronomers reported they had “weighed” a black hole in a nearby galaxy. And the youngest known black hole was identified a year ago.

Paul Sutherland

Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.
Paul Sutherland

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Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.

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