Black Hole Discovered Orbiting Spinning Star
Scientists have discovered the first ever binary star system comprised of a black hole and spinning, or Be-type, star.
Be-type stars are relatively common, with more than 80 found in binary systems with neutron stars in the Milky Way alone.
"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," said Jorge Casares of the Instituto de Astrofísica de Canarias (IAC) and La Laguna University.
Known as MWC 656, the Be star is located in the Lizard constellation some 8,500 light years from Earth and spins so fast its surface speed exceeds 1 million kilometers per hour.
"We started studying this star back in 2010, when space telescopes detected transient gamma-ray emission coming from its direction," explained Marc Ribó, of the Institut de Ciències del Cosmos of Barcelona University. "No more gamma-ray emission has subsequently been detected, but we found that the star was part of a binary system."
By analyzing its spectrum, the scientists were able to infer traits regarding its companion.
"It turned out to be an object with a mass between 3.8 and 6.9 solar masses. An object like that, invisible to telescopes and with such large mass, can only be a black hole, because no neutron star with more than three solar masses can exist," said Ignasi Ribas, of CSIC in the Instituto de Ciencias del Espacio.
The black hole orbits the Be star, feeding off matter the matter the star sloughs off.
"The high rotation speed of the Be star causes matter to be ejected into an equatorial disc. This matter is attracted by the black hole and falls on to it, forming another disc -- called an 'accretion disc.' By studying the emission from the accretion disc we could analyse the motion of the black hole and measure its mass,' Ignacio Negueruela, a lecturer at the University of Alicante, said.
The researchers hypothesize that the object is a member of a hidden group of Be stars paired up with black holes.
"We think these systems are much more common than previously thought, but they're difficult to detect because their black holes are fed from gas ejected by the Be stars without producing much radiation, in a 'silent' way, so to speak," Casares said.
To get around this, the researcher said he and his team plan on using bigger telescopes, such as the 10.4-meter Gran Telescopio Canarias, to detect similar binary systems in the Milky Way and neighboring galaxies.