Exceptionally bright black hole with manners questions theory
It eats amazingly tidily and it’s way too luminous for its mass. That’s two things about the Pinwheel Galaxy’s black hole ULX-1 that could change our understanding of these high gravity objects.
Astronomers have, for the first time, directly measured the mass of a mysterious source of X-ray light in the Pinwheel Galaxy, one of our nearest spiral neighbours.
An underweight black hole’s eating habits – which sees it chomping down on gas and dust in a rapid, yet calm and orderly fashion – could change our understanding of how these high gravity objects consume matter.
ULX-1, which rests some 22 million light-years away in the Pinwheel Galaxy M101, is the driving force behind an energetic X-ray source. Yet the type of X-rays this black hole gives off – which can either be of high or low energy type rays – has left the team of astronomers who made observations at Gemini Observatory and led by Jifeng Liu, of the National Astronomical Observatories of China, baffled.
Since ULX-1 is on the small side, Liu and his team suspected that it would give off high energy rays – or hard X-rays – since its bigger cousin’s produce more of the softer, lower energy rays. But they were wrong and they found quite the opposite.
“It has elegant manners,” says research team member Stephen Justham of ULX-1. “Such lightweights must devour matter at close to their theoretical limits of consumption to sustain the kind of energy output observed. We thought that when black holes were pushed to these limits, they would not be able to maintain such refined ways of consuming matter.”
Justham adds: “We expected them to display more complicated behaviour when eating so quickly. Apparently we were wrong.”
Our current models suggest that when a black hole dines on the matter that falls into it and radiates energy, the X-rays mainly come from the accretion disk that surrounds it. Meanwhile the punchier, high energy X-rays stream from a type of “corona” around the disk. Futhermore, as the rate of accretion gets closer to some theoretical limit of consumption, the strength of emission from the corona is supposed to get stronger and become more complex.
An artist’s visualisation of the environment around M101 ULX-1, showing a stellar-mass black hole (foreground) with accretion disk. Gas from the Wolf-Rayet star (background) feeds the black hole’s voracious appetite.
“Theories have been suggested which allow such low-mass black holes to eat this quickly and shine this brightly in X-rays but those mechanisms leave signatures in the emitted X-ray spectrum, which this system does not display,” says Liu. “Somehow this black hole, with a mass only 20 to 30 times the mass of our Sun, is able to eat at a rate near to its theoretical maximum while remaining relatively placid. It’s amazing. Theory now needs to somehow explain what’s going on.”
To find out the mass of the black hole, the researchers employed Gemini North’s Multi-Object Spectrograph at Hawaii’s Mauna Kea to track the motion of its companion star; a variety of Wolf-Rayet that’s been given the task of feeding ULX-1 by emitting exceptionally strong stellar winds that its hungry partner has been found to be munching on a greater proportion of than originally realised.
“Although this isn’t the first Wolf-Rayet black hole binary ever discovered, at some 22 million light-years away, it does set a new distance record for such a system,” says co-author of the paper published in the science journal Nature, Paul Crowther of the University of Sheffield. “The Wolf-Rayet star will have died in a small fraction of the time it has taken for light to reach us, so this system is now likely a double black hole binary.”
With the new observation presents a new frustration for astronomers hoping to find the elusive “intermediate-mass” black hole – which have masses roughly between 100 and 1000 times the mass of the Sun – in M101 ULX-1; the full name of the source which shines a million times brighter than the Sun in X-rays due to the black hole and its accretion disk, with the companion star contributing ultraviolet light. Despite only potential candidates and no definite detections, experts believe that they are likely to have gone into hiding, spitting out ultra-luminous X-ray sources (ULXs). Originally thinking that M101 ULX-1 was one of these potential candidates, astronomers thought that it would have only been a matter of time before they were clutching evidence for an intermediate black hole. However black hole experts are understandably frustrated.
“Astronomers hoping to study these objects will now have to focus on other locations for which indirect evidence of this class of black holes has been suggested, either in the even brighter ‘hyper-luminous’ X-ray sources or inside some dense clusters of stars,” advises research team member Joel Bregman of the University of Michigan.
Images courtesy of Jifeng Liu (top) and Lynette Cook (bottom)