Thursday, January 16, 2014

Spanish Researchers Discover The First Black Hole Orbiting A Star

Spanish scientists have discovered the first binary system ever known to consist of a black hole and a ‘spinning’ star – or more accurately, a Be-type star. Although predicted by theory, none had previously been found. The observations that led to the discovery were performed with the Liverpool and Mercator telescopes at the Observatorio del Roque de los Muchachos (Canary Islands, Spain).

Be-type stars are quite common across the Universe. In our Galaxy alone more than 80 of them are known in binary systems together with neutron stars. ‘Their distinctive property is their strong centrifugal force: they rotate very fast, close to their break-up speed. It's like they were cosmic spinning tops,’says Jorge Casares of the Instituto de Astrofísica de Canarias (IAC) and La Laguna University (ULL). Casares is the lead author and an expert in stellar-mass black holes (he presented the first solid proof of their existence back in 1992).

The theory predicted their existence, but no one could find them so far. Using the Liverpool and Mercator telescopes of the Observatory from Roque de los Muchachos (La Palma Island, Canary Islands), a team of researchers from different Spanish centres has located the first known binary system consisting of a black hole and a spinning or Be- type star. The discovery has been published today in Nature.
 
The Be-type massive star rotates at high speed and ejects the disk of matter that is attracted by the black hole and absorbed through an accretion disk.

Be stars are quite common across the universe. In our galaxy alone, more than 80 are found making up binary systems with neutron stars. “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", says Jorge Casares, from the The Canaries’ Astrophysical Research Institute (IAC ) and from the University of Laguna (ULL), one of the discoverers and expert in stellar-mass black holes (he found out the first solid proof of their existence in 1992). This is the case of this star, known as MWC 656, located in the constellation Lacerta (the Lizard) 8,500 light years from Earth and whose surface speed exceeds 1 million kilometres per hour.

“We started studying this star back in 2010, when space telescopes detected transient gamma-ray emission coming from its direction, Marc Ribó reports, from the Institute of Cosmic Sciences at the University of Barcelona (ICC/IEEC-UB). "No more gamma-ray emission has subsequently been detected, but we found that the star was part of a binary system," he adds.

A detailed analysis of the spectrum allowed us to infer the characteristics of its companion. "It turned out to be an object with a body mass between 3.8 and 6.9 solar masses. Such an object, invisible and with such large mass, can only be a black hole, since no neutron star is stable over three solar masses”, Ignasi Ribas states, CSIC researcher at the Institute of Space Sciences (IEEC -CSIC).



The black hole orbits around the Be star and and is fed by matter ejected from the latter. "Its high speed rotation causes matter to be ejected into an equatorial disc, matter which is attracted by the black hole, forming another disc - called an “accretion disc”, as it falls. By studying the emission from the accretion disc we could analyse the motion of the black hole and measure its mass," Ignacio Negueruela states, a researcher from the University of Alicante’s Astrophysics Research Group.

Scientists believe this object to be a nearby member of a hidden population of Be stars paired with black holes. "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, such as the Gran Telescopio Canarias," Casares concludes.

The Be star spins at extremely high velocity, ejecting matter through an equatorial disc. Part of this matter falls on to the black hole forming an accretion disc. 
Credit: Gabriel Pérez - SMM (IAC)

Also participating in the study with Jorge Casares, Ignacio Negueruela, Marc Ribó and Ignasi Ribas are Josep M. Paredes, from the Cosmic Science Institute at the University of Barcelona (ICC/IECC-UB) and Artemio Herrero and Sergio Simón, both from the IAC and ULL.

Black holes, an ongoing challenge

The detection of black holes has always been a challenge from the theoretical formulation in the eighteenth century. Since they are not seen, their gravitational force prevents light from escaping -telescopes can not detect them. However, some black holes can occasionally produce high-energy radiation in the environment surrounding them and can thus be traced by X-ray satellites This is the case of active black holes, which are being "fed" by material obtained from a nearby object. If violent X-ray emission is detected from a place where nothing but a normal star is seen, a black hole might be hiding there.

Using this method, 55 potential black holes over the last 50 years have been discovered. Seventeen of them have what astronomers call a "dynamic confirmation": the feeding star has been localised, allowing for the mass of its invisible companion to be measured. If it is above three solar masses, then it is considered to be a black hole.

The biggest problem is put forth by “sleeping” black holes, such as the one found by the researchers around this B type star: "Their X-ray emission is almost absent, and so it is very unlikely that our attention would be drawn to them," Casares explains. Researchers believe there are thousands of black hole binary systems across the Milky Way, some of them also with Be-type stellar companions.


Contacts and sources:
Asociación RUVID

Citation: Casares, J.; Negueruela, I.; Ribo, M.; Ribas, I.; Paredes, J. M.; Herrero, A.; Simon-Diaz, S. A Be-type star with a black-hole companion. Nature 505, 378–381 (16 January 2014), doi:10.1038/nature12916
http://www.nature.com/nature/journal/v505/n7483/full/nature12916.html









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