Researchers Find First Black Hole Orbiting A “Be” Star
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John P. Millis, PhD for redOrbit.com – Your Universe Online
Black holes, by their very definition, are difficult to find. Their density is so great, that light cannot be given off directly, and photons that stray too close are absorbed into the gravitational well. So researchers have to rely on indirect evidence that a black hole is present by searching for their effects on other nearby objects.
Astronomers have long believed that black holes should exist in binary around stars. Systems known as microquasars – consisting of a black hole around a large main sequence or giant star – have been discovered. But in these systems, the binary orbit is so tight that the black hole is siphoning off material from its companion onto an accretion disk at a very high rate, lighting up the system in X-rays.
However, in systems where the accretion rate is low – such as if the accretion were not gravitationally driven, or the orbital spacing is relatively large – the radiation signals would be much more subtle.
But now astronomers believe they have found just such a system. The black hole in question is orbiting an object known as a Be-type star, which is unusual because of its incredibly high rate of rotation. As Jorge Casares, astronomer from the The Canaries’ Astrophysical Research Institute (IAC ) and the University of Laguna (ULL), explains, “Their distinctive property is their strong centrifugal force: they rotate very fast, close to their break-up speed, as if they were cosmic spinning tops.”
These Be-type stars are well known, and more than 80 such systems have been found with binary objects such as neutron star. Yet none had ever been found with a black hole as part of the system. The object in question, known as MWC 656, is found in the constellation Lacerta about 8,500 light-years from Earth.
Back in 2010, the object drew interest from astrophysicists because transient gamma-ray signals were found to emanate from the system. However, the gamma-ray flux subsequently ceased, which signaled that perhaps the Be-star at the core was in binary orbit with another object.
After doing a detailed analysis of the system’s spectrum, using the Mercator telescopes of the Observatory from Roque de los Muchachos (La Palma Island, Canary Islands), the team was able to ascertain the mass of the companion: some 3.8 to 6.9 times the mass of our Sun. Coupled with the knowledge that the object did not radiate on its own – otherwise it would have been visible in the previous optical studies – astronomers concluded that it must be a black hole.
The exciting aspect of this discovery is that it confirms that such systems exist, and that this may only be the tip of the iceberg. “We think these systems are much more common than previously thought, but they are difficult to detect because their black holes are fed from gas ejected by the Be stars without producing much radiation, that is, in a “silent” way. However, we hope to detect other similar binary systems in the Milky Way and other nearby galaxies by using bigger telescopes,” concludes, Casares.