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Runaway Star Might Explain Black Hole's Disappearing Act
At the center of a far-off galaxy, a black hole is slowly consuming a disk of gas that swirls around it like water circling a drain. As a steady trickle of gas is pulled into the gaping maw, ultrahot particles gather close to the black hole, above and below the disk, generating a brilliant X-ray glow that can be seen 300 million light-years away on Earth.
This illustration shows a black hole surrounded by a disk of gas. In the left panel, a streak of debris falls toward the disk. In the right panel, the debris has dispersed some of the gas, causing the corona (the ball of white light above the black hole) to disappear. Photo: NASA/JPL Caltech
These collections of ultrahot gas, called black hole coronas, have been known to exhibit noticeable changes in their luminosity, brightening or dimming by up to 100 times as a black hole feeds.
But two years ago, astronomers watched in awe as X-rays from the black hole corona in a galaxy known as 1ES 1927+654 disappeared completely, fading by a factor of 10,000 in about 40 days. Almost immediately it began to rebound, and about 100 days later had become almost 20 times brighter than before the event.
The X-ray light from a black hole corona is a direct byproduct of the black hole's feeding, so the disappearance of that light from 1ES 1927+654 likely means that its food supply had been cut off.
In a new study in the Astrophysical Journal Letters, scientists hypothesize that a runaway star might have come too close to the black hole and been torn apart. If this was the case, fast-moving debris from the star could have crashed through part of the disk, briefly dispersing the gas.
Nearly every galaxy in the universe may host a supermassive black hole at its center, like the one in 1ES 1927+654, with masses millions or billions of times greater than our Sun.
They grow by consuming the gas encircling them, otherwise known as an accretion disk. Because black holes don't emit or reflect light, they can't be seen directly, but the light from their coronas and accretion disks offers a way to learn about these dark objects.
The authors' star hypothesis is also supported by the fact that a few months before the X-ray signal disappeared, observatories on Earth saw the disk brighten considerably in visible-light wavelengths (those that can be seen by the human eye). This might have resulted from the initial collision of the stellar debris with the disk.
Although a wayward star seems the most likely culprit, the authors note that there could be other explanations for the unprecedented event. One remarkable feature of the observations is that the overall drop in brightness wasn't a smooth transition: Day to day, the low-energy X-rays NICER detected showed dramatic variation, sometimes changing in brightness by a factor of 100 in as little as eight hours.
In extreme cases, black hole coronas have been known to become 100 times brighter or dimmer, but on much longer timescales. Such rapid changes occurring continuously for months was extraordinary. (NASA)
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