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Astronomers determine full description of a black hole
GEMMA LAVENDER
ASTRONOMY NOW
Posted: 22 November 2011


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They say that nothing can escape a black hole, but a complete description of the basic structure of these dense masses has escaped a black hole’s clutches, enabling a team of astronomers to characterise one of them, Cygnus X-1, for the first time. The finding has allowed them to reconstruct the object’s history since its birth around six million years ago.

Ever since the binary system, which contains a black hole in a tight tango with a massive blue O-type star companion from which it feasts, was discovered to be spewing X-rays nearly half a century ago, astronomers have sat up and taken notice of Cygnus X-1 but have only been able to make estimates about the object. That was until a team of astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) and San Diego State University yielded the most accurate results to date shedding light onto the system, unravelling its longstanding mysteries that has kept them in the dark for so long. “Because no other information can escape from a black hole, knowing its mass, spin and electrical charge gives a complete description of it,” says Mark Reid of the CfA.


An artist’s conception of Cygnus X-1. The Black hole draws material from its companion star [right] into a hot, swirling disk. Image: Chandra X-ray Observatory/NASA.

The charge of the black hole is zero, so the only outstanding characteristics to be measured were its mass and spin. With the assistance of the National Science Foundation’s Very Long Baseline Array – a continent-wide radio telescope system – Reid and his team set to work on making a direct trigonometric measurement of the distance to Cygnus X-1, which served as a basis for uncovering the long-awaited details of the black hole that nestles in the far away system. “One needs to know the distance to a source to convert the measured flux at the Earth into the star’s true luminosity [inverse square law],” says Reid, who is the lead author of one of the three papers about Cygnus X-1 published in The Astrophysical Journal. “This information is crucial to model the binary system to get stellar masses, sizes and temperatures.”

With their VLBA observations, the team were left with a distance of 6,070 light years; a measurement that has landed between previous estimates of 5,800 to 7,800 light years. Using this new, precise distance to Cygnus X-1 along with the help of scientists at the helm of the Chandra X-ray Observatory, the Rossi X-ray Timing Explorer, the Advanced Satellite for Cosmology and Astrophysics and visible-light observations made over more than two decades, the team were within sight of completing the puzzle of the system’s black hole. “To measure the mass of the black hole and O-star [the companion star], we need to measure the true orbital velocities and the size of the orbit,” says Jerry Orosz of San Diego State University, who is lead author of the remaining two papers alongside Lijun Gou also from CfA. “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 orbit velocity.”


Scientists think that it is unlikely that the black hole in Cygnus X-1 was created by a supernova as illustrated in this artist’s impression. Image: ESA/Hubble.

However, since the gravity of the black hole distorts the companion star, tugging it into a teardrop shape, the binary components’ dance leads to changes that the astronomers find useful. “As it [the O-star] moves in its orbit we see its cross section [or projection] on the sky change, which leads to a change in apparent brightness,” says Orosz. The brightness changes that the scientist speaks of are called ellipsoidal variations and in binary systems like Cygnus X-1, it is the alteration in luminosity over the orbit that allows scientists to measure the inclination of the binary. “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 inclination can lead to the same observed ellipsoidal variations. Each inclination angle [40 degrees versus 30 degrees say] 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,” he says.

In order to obtain the correct range of inclinations that lead to the correct mass measurements the team were then able to put their accurate distance to good use. “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 fourth power of its temperature], we can figure out the radius of the O-star,” says Orosz. “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 measurements and their uncertainties.” With their extensive research, the team found that the black hole weighed a hefty 15 solar masses with a turbo spin of more than 800 revolutions per second.

That was not all: in addition to measuring the distance to the binary system, the VLBA observations made during 2009 and 2010 also analysed Cygnus X-1’s movement through the Milky Way galaxy. Since the slow movement of the binary system implies that the black hole was not a product of a supernova, astronomers believe that its creation could have possibly been the result of the dark collapse of a giant progenitor star that initially had a mass of more than 100 times that of the Sun before losing it in a vigorous stellar wind. “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,” says 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. Our results support these suggestions.”

“It is amazing to me that we have a complete description of this asteroid-sized object that is thousands of light years away,” says Gou. “This means that astronomers have a more complete understanding of this black hole than any other in our Galaxy.”

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