What happens when a black hole falls on hard times? An international team of astronomers have found a black hole that’s rapidly getting dimmer, and they think it’s starving.
The idea makes sense. Black holes often lie at the core of galaxies. When a black hole forms you can’t see it, but whatever falls into the black hole heats up as it falls, throwing off detectable radiation all over the EM spectrum. This leaves galactic centers aglow with spinning debris. But after a certain point, a black hole should clear its surroundings, having already accreted everything that was close enough to fall in. And after that, after the glow of gases falling into the black hole has faded, what remains would be just the black hole itself, bereft of its incandescent accretion disc.
Scientists are thrilled to have caught this fading phenomenon in action, right after it started to happen to a supermassive black hole at the core of Markarian 1018, a galaxy in the Cetus constellation. This is the first such thing we’ve ever studied. Bernd Husemann, CARS project leader and lead author of a paper discussing their observations and hypotheses, explained: “We were lucky that we detected the event just 3-4 years after the decline started, so we could begin monitoring campaigns to study details of the accretion physics of active galaxies that cannot be studied otherwise.”
Black holes don’t just hover in space glowing at a constant brightness. As they accrete material of varying densities and amounts, they flicker, with a frequency on the order of tens of thousands of years — almost like a candle guttering and flaring in a breeze. But that’s not what’s going on here. Ten years is such a short little blip on cosmic timescales, but 10 years is all it’s taken for the brightness of this supermassive black hole to dim by an order of magnitude. Scientists have actually been watching this particular black hole off and on for thirty years. Every instrument they looked with has showed a five- to ten-fold decrease in brightness over the last ten years.
Ingesting a star causes a brief, sharp flare that trails off slowly as the black hole finishes its “meal.” But the team of astronomers who have been watching this black hole have seen it do exactly none of that, either. Geometry and spectral lines have ruled out debris along the line of sight. No, the dimming the astronomers have seen suggests something else entirely.
There’s a huge ring of ingestable debris at a radius of about 24 light-days, at which distance it would take about five years to fall into the black hole. But the ring isn’t being sucked in, even though it’s within range. Why? “It’s possible that this starvation is because the inflow of fuel is being disrupted,” said Rebecca McElroy, lead author of the paper outlining the discovery. “An intriguing possibility is that this could be due to interactions with a second supermassive black hole.” Such a black hole binary system is a distinct possibility in Markarian 1018, since it’s the product of a major merger of two galaxies — each of which probably had a supermassive black hole at its core.
How far does the metaphor extend? Can a black hole starve to death? The astronomers intend on finding out. Their discovery of this phenomenon got them discretionary time on the VLA, Hubble and Chandra all, and they’ll be making continuing observations across the whole EM band to figure out just exactly what is going on at Markarian 1018.
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