Stellar black hole binary

Artist's view of a black hole and a star in a binary system. Scientists think a small black hole called M 101 ULX-1 may be shining brightly because of its companion star's stellar wind. (Jingchuan Yu / November 29, 2013)

A bright black hole in the Pinwheel galaxy has been shining us on, astronomers say – this intergalactic trickster puts out light like a big black hole but it’s really quite tiny. M 101 ULX-1, described in the journal Nature, may force scientists to keep hunting for more "intermediate" black holes – and rethink their understanding of them.

Black holes are thought to be remains of dead stars whose entire mass has collapsed to a tiny point. They warp space-time so badly that not even light can escape. The small ones created by single stars can be up to roughly 30 times the mass of our sun. The supermassive ones at the centers of galaxies can be billions of solar masses.

But astronomers also think there’s a class of black hole that falls in between those mass-sucking monsters that are too big to be made from a single star, and too small to be the enormous eye of a spiral galaxy.

Such intermediate black holes range in mass from roughly 100 to 1,000 suns, and could be what formed the seeds of those supermassive galactic nuclei. This makes them a fascinating target for astronomers, who could witness the growth and evolution of the kind of black hole that powers a spiral galaxy’s heart.

Since black holes don’t let light escape, it makes them very difficult to study. But many of them, such as the supermassive black holes, may also give off massive bursts of X-ray light from their accretion discs as all the matter falling in rubs together and generates friction.

But researchers have also identified another X-ray source – these so-called ultraluminous X-ray sources, or ULXs – that aren’t associated with these supermassive black holes. These ULXs, powerful enough to shine out from whole other galaxies, are thought to be beacons from intermediate black holes. Based on its high-octane light show, M 101 ULX-1 was thought to be one of them.

But after studying it with the Hubble Space Telescope, the Chandra X-ray Observatory and the Gemini telescope, an international team of astronomers concluded that this strange, bright source was probably only 20 to 30 solar masses – which would probably make it more like a single-star black hole.

It’s unclear why such a small black hole is so bright. The astronomers think that the black hole has a companion star whose stellar wind – gusts of charged particles blowing out from the star’s body and onto the black hole’s accretion disk – could be powering the bright beacon. Scientists previously thought the stellar wind would be too weak to have such an effect. 

So is it back to the drawing board on intermediate black holes?

“Several aspects of our understanding of ULXs and, indeed, of black-hole formation, may need to be revised,” Kip Kuntz, an astronomer at Johns Hopkins University who was not involved in the study, wrote in a commentary on the paper.

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