Artist illustration of the supermassive
black hole in 1ES 1927+654 before the flare. Credit: NASA/Sonoma State
University, Aurore Simonnet
Black holes are powerful cosmic engines. They provide the energy
behind quasars and other active galactic nuclei (AGNs). This is due to the
interaction of matter with its powerful gravitational and magnetic fields.6
Technically, a black hole doesn’t have a magnetic field on its
own, but the dense plasma surrounding the black hole as an accretion disk does.
As the plasma swirls around the black hole, the charged particles within it
generate an electrical current and magnetic field. The direction of the plasma
flow doesn’t change spontaneously, so one would imagine the magnetic field is
very stable. So, imagine the surprise of astronomers when they saw evidence
that a black hole’s magnetic field had undergone a magnetic reversal.
In basic terms, a magnetic field can be imaged as that of a simple
magnet, with a north and south pole. A magnetic reversal is where the
orientation of that imaginary pole flips, and the orientation of the magnetic
field flips. This effect is common among stars.
In 2018, an automated sky survey found a sudden change in a galaxy
239 million light-years away. Known as 1ES 1927+654, the galaxy had brightened
by a factor of 100 in visible light. Soon after its discovery, the Swift
Observatory captured its glow in x-rays and ultraviolet. A search of archival observations of the
region showed the galaxy actually started to brighten toward the end of 2017.
At the time it was thought this rapid brightening was caused by a star passing close to the galaxy’s supermassive black hole. Such a close encounter would cause a tidal disruption event, which would rip the star apart as well as disrupt the flow of gas in the black hole’s accretion disk. But this new study casts a shadow on that idea.
How a black hole might undergo
magnetic reversal. Credit: NASA’s Goddard Space Flight Center/Jay Friedlander
The team looked at observations of the galactic flare across the
full spectrum of light from radio to x-ray. One of the things they noticed was
that the intensity of x-rays dropped off very quickly. X-rays are often
produced by charged particles spiraling within intense magnetic fields, so this
suggested a sudden change in the magnetic field near the black hole. At the
same time, the intensity of light in visible and ultraviolet increased which
suggested that parts of the black hole’s accretion disk were getting hotter.
Neither of these effects is what you’d expect with a tidal disruption event.
Instead, a magnetic reversal better fits the data. As the team
showed, as a black hole accretion disk undergoes a magnetic reversal, the
fields weaken at the outer edges of the accretion disk first. As a result, the
disk can heat up more efficiently. At the same time, the weaker magnetic field
means that fewer x-rays are produced by charged particles. Once the magnetic
field completes its reversal, the disk returns to its original state.
This is only the first observation of the magnetic reversal of a galactic black hole. We now know they can occur, but we don’t know how common these reversals are. It will take more observations to determine just how many times a galaxy’s black hole can become a switch hitter.
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