We are used to thinking of black holes as cosmic plugholes, sucking in all the material that surrounds them, but their appetite actually sometimes leads them to spit out what they were trying to eat. Rarely have we seen it as fast as this, though.
In a discovery published in Nature, two stellar mass (star-sized) black holes have been observed devouring their companion stars so quickly that the process throws some of the material away from the black hole at almost a quarter of the speed of light. While strong outflows have been observed in supermassive black holes, this has never been observed in stellar mass objects like these.
The discovery was possible thanks to the European Space Agency’s (ESA) XMM-Newton space observatory, which studies the sky in X-rays. The two black holes are significantly brighter than other X-ray-emitting objects and, for this reason, they earn the nickname ultra-luminous X-ray sources.
The X-rays are produced by the material falling onto the black holes. Due to friction, this material is heated up and it starts emitting a lot of light, including a lot of X-rays. For the black holes NGC 1313 X-1 and NGC 5408 X-1, which are respectively 12 and 15 million light-years from us, the radiation pressure from the hot gas generates the high-speed winds observed.
“This is the first time we’ve seen winds streaming away from ultra-luminous X-ray sources,” said Dr. Ciro Pinto from Cambridge’s Institute of Astronomy, the paper’s lead author, in a statement. “And the very high speed of these outflows is telling us something about the nature of the compact objects in these sources, which are frantically devouring matter.”
But the appetite of these black holes is mysterious, as it appears to be a violation of classical physics. The radiation pressure the gas experiences from being so hot should overcome the gravity that pulls it towards the black holes, meaning it is not eaten, but that is not the case, indicating that these black holes might be bigger than we thought.
Heavier and rarer black holes, like the ones that produced the LIGO gravitational wave detection, might be responsible, and even highly magnetized neutron stars might produce a similar X-ray signature in the sky.
The team is still going through the XMM-Newton archive in the hope of finding more of these types of objects, and they are planning future follow-up observation of sources with radio and optical telescopes.
“With a broader sample of sources and multi-wavelength observations, we hope to finally uncover the physical nature of these powerful, peculiar objects,” said Pinto.