Spanish Research: Black hole that doesn't emit x-rays discovered near massive star

Trailed intensity image of the two lines constructed from the phase binned spectra. 

Two orbital cycles are displayed for clarity. 

The colour scale indicates counts normalised to the continuum, with the black colour corresponding to 0.98 and the white colour to 1.08 in Fe II and 1.16 in HeII. 

Credit: Nature 

Researchers in Instituto de Astrofísica de Canarias, Universidad de Alicante, Universitat de Barcelona, and Institut de Ciències de l’Espai (IEEC-CSIC), Spain have discovered a black hole that doesn't reveal itself through x-ray radiation thrown off by material that is being sucked into it.

In their paper published in the journal Nature, team members from several research institutions throughout Spain, report that the black hole appears to exist as a companion (binary) to a massive Be star that spins so fast it's surrounded by a gas disk.

I. Negueruela

Up until now, virtually all black holes have been discovered via x-ray radiation signals—as material is pulled in past the point of no return, radiation is flung out into space where it is noted by space scientists here on Earth.

In this new effort, the research team was able to identify the black hole because of its behaviour, rather than its signature.

Many Be stars have been found to have companions—most of the time they are supernova remnants (neutron stars) but never before has a Be star been found to have a black hole as a companion.

The star, named MWC 656 is really big—approximately 10 to 16 times as massive as our sun. It spins really fast too (approximately 671,000 mph) which the researchers say, explains why the black hole next to it doesn't emit any radiation.

J. Casares

They suggest that because the star is spinning so fast, it casts gas into a disk surrounding its equator which in turn is cast off towards the black hole, but rather than being pulled in, the gas joins an accretion disk that surrounds the "mouth" of the black hole, moving so fast (due to the angular momentum of the gas cast off from the star) that it can't be pulled in. Thus the disk simply continues to grow larger.

The black hole is pretty big too (approximately 3.8 to 6.9 more massive than our sun) which likely puts it in the category of stellar mass black holes—those that come into existence when a star runs out of fuel.

The discovery of the "silent" black hole suggests that many more like it might exist, which will undoubtedly lead researchers to look for more, now that they know what to look for.

More information: A Be-type star with a black-hole companion, Nature 505, 378–381 (16 January 2014) J. Casares, I. Negueruela, M. Ribó, I. Ribas, J. M. Paredes, A. Herrero & S. Simón-Díaz DOI: 10.1038/nature12916