[Music throughout] At the end of 2017,
a galaxy 236 million light-years away began
a rare and dramatic transformation. It’s an event
astronomers are still puzzling over, one set off by changes near
its central black hole. They first explained this
as a tidal disruption event. That’s when a star wanders so close
to a supermassive black hole that it’s torn apart.
A new study of observations spanning the entire event
suggests a different cause. The trigger may have been
a flip in the magnetic field in the disk of material around the
black hole. The Sun’s magnetic field reverses polarity
about every 11 years. On longer timescales, even Earth’s magnetic
field flips. Starting in December 2017,
the galaxy began to brighten in visible and ultraviolet light.
The source of this brightening appeared to be the disk of material around
the supermassive black hole at the galaxy’s center.
It peaked three months later at nearly 100 times
its previous brightness.
That’s when regular monitoring with NASA’s Swift satellite began.
As the visible and UV brightened, X-rays from the galaxy dimmed.
By August 2018, the higher-energy X-rays had
disappeared completely. A few months later,
the high-energy X-rays came back, even brighter than before.
They returned to normal within a year.
These X-rays come from a cloud of superhot particles near the black hole.
It’s a feature called the corona, which is formed by the strong magnetic field.
The lack of higher-energy X-rays means that this structure
was essentially gone. Based on observations from Swift,
Europe’s XMM satellite, and ground-based optical and radio telescopes,
here’s what may have happened.
The visible and UV flare results when the flow of matter into the black
hole increases. This may have started when the magnetic field in
the disk’s outer regions began to flip.
The weakened magnetic field can no longer support the corona, which vanishes.
The flipped magnetic field gains strength, restoring
the X-ray corona, but the inward flow of matter is still
high, so this emission is stronger than it was before the flare.
Finally, the corona and disk return to their states before
the flare, now with a flipped magnetic field.
Rapid changes in UV and visible light have only
been observed in a few dozen active galaxies like this one.
But this is the first time X-rays have been seen to drop out
as other wavelengths brighten. These surprising events
offer a tantalizing glimpse at the extreme forces at work
near an actively feeding supermassive black hole.
[Music]
Telescope images courtesy of: Associated Universities Inc., ASTRON, Gran Telescopio Canarias, Italian Institute of Astrophysics
NASA