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"Ordinary" black hole discovered 12 million light-years away

The team used the Chandra X-ray Observatory to make six 100,000-second-long exposures of Centaurus A, detecting an object with 50,000 times the X-ray brightness of our Sun.
Centaurus A black hole
The yellow arrow in the picture identifies the position of the black hole transient inside Centaurus A. The location of the object is coincident with gigantic dust lanes that obscure visible and X-ray light from large regions of Centaurus A. Other interesting X-ray features include the central active nucleus, a powerful jet, and a large lobe that covers most of the lower-right of the image. There is also a lot of hot gas. In the image, red indicates low energy, green represents medium energy, and blue represents high energy light. Credit: NASA/Chandra
An international team of scientists has discovered an “ordinary” black hole in the galaxy Centaurus A, which is 12 million light-years distant. This is the first time that a normal-sized black hole has been detected away from the immediate vicinity of our galaxy.

Although exotic by everyday standards, black holes are everywhere. The lowest-mass black holes are formed when massive stars reach the end of their lives, ejecting most of their material into space in a supernova explosion and leaving behind a compact core that collapses into a black hole. There are thought to be millions of these low-mass black holes distributed throughout every galaxy. Despite their ubiquity, they can be hard to detect as they do not emit light, so they are normally seen through their action on the objects around them — for example by dragging in material that then heats up in the process and emits X-rays. But despite this, the overwhelming majority of black holes have remained undetected.

In recent years, researchers have made some progress in finding ordinary black holes in binary systems by looking for the X-ray emission produced when they suck in material from their companion stars. So far, these objects have been relatively close, either in our Milky Way Galaxy or in nearby galaxies in the Local Group — a cluster of galaxies relatively near the Milky Way that includes Andromeda.

The team used the orbiting Chandra X-ray Observatory to make six 100,000-second-long exposures of Centaurus A, detecting an object with 50,000 times the X-ray brightness of our Sun. A month later, it had dimmed by more than a factor of 10 and then later by a factor of more than 100, thus becoming undetectable.

This behavior is characteristic of a low-mass black hole in a binary system during the final stages of an outburst and is typical of similar black holes in the Milky Way. It implies that the team made the first detection of a normal black hole so far away, and for the first time opens up the opportunity to characterize the black hole population of other galaxies.

“So far we’ve struggled to find many ordinary black holes in other galaxies, even though we know they are there,” said Mark Burke from the University of Birmingham, United Kingdom. “To confirm (or refute) our understanding of the evolution of stars, we need to search for these objects, despite the difficulty of detecting them at large distances. If it turns out that black holes are either much rarer or much more common in other galaxies than in our own, it would be a big challenge to some of the basic ideas that underpin astronomy.”

The group now plans to look at the more than 50 other bright X-ray sources that reside within Centaurus A, identifying them as black holes or other exotic objects, and gain at least an inkling of the nature of a further 50 less-luminous sources.

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