One of the most intense X-ray sources in the sky went dark, and so far scientists cannot explain why.
The black hole responsible for creating the lights-out mystery is found in GRS 1915 + 105, a star system 36,000 light-years from Earth that contains both a normal star and the second heaviest known black hole in the Milky Way.
That heavyweight is 10 to 18 times the mass of the Sun and is only second in mass to Sagittarius A* (or SgrA*), the supermassive black hole at the galactic center. The region around the black hole GRS 1915 + 105 generally glows with intense X-ray light as it feeds on its companion star. As the material surrounds the cosmic drain, the particles within rub against each other, generating energy before falling into darkness at the center of the black hole. That swirling material is the black hole's accretion disk, which is illuminated by X-rays as the black hole devours more and more sustenance.
But the researchers saw something surprising starting in July 2018: the light from the GRS 1915 + 105 system began to dim. Then in early 2019 the light dimmed even more and no one had seen anything like it before.
So what is going on?
“We suggest that this state should be identified as the 'hidden state,'” the researchers wrote in a new paper published Jan. 1 in the arXiv database , which has not yet been peer-reviewed.
In other words, something got between the light source and the Swift X-ray telescope that has been monitoring the object, obscuring the telescope's view.
There is still a lot of light coming from the bright region near the black hole's event horizon, which astronomers sometimes call the "engine," as well as the larger "accretion disk" of falling matter. But that light doesn't reach Earth like it used to.
Mayura Balakrishnan, an astronomy graduate student at the University of Michigan , said in a statement:
"The geometry of obscuration (the precise nature of the structure that blocks the light) is difficult to discern."
No existing telescope can work out the details of the distant system, so Balakrishnan and his co-authors had to make inferences about how the light from GRS 1915 + 105 changed from day to day between 2018 and 2019.
Black holes with large companion stars are sometimes dimmed because their companion stellar winds can push clouds of gas in front of their lights.
Balakrishnan said:
"In the case of GRS 1915 + 105, the companion star is low mass and does not have massive stellar winds that would create the obscuring gas observed."
The researchers concluded that "there is a lot of gas in some structure that scatters and blocks the light coming from the central engine and the accretion disk."
In other words, whatever is blocking the light is likely coming from the accretion disk itself.
However, the nature of that structure remains a mystery. GRS 1915 + 105 is interesting to astronomers because its X-ray engine resembles a scale model of the engines that power many supermassive black holes at the centers of distant galaxies. The difference is that the fuel for supermassive black holes comes from matter clouds in their galactic nuclei, while GRS 1915 + 105 draws its fuel from a neighboring star. So understanding what is happening to this star eater could shed light on what is happening to the heaviest objects in the universe.
The research findings have been published on arXiv.org.