Black holes are powerful cosmic engines. They provide the energy behind quasars and other active galactic nuclei (AGN). This is due to the interaction of matter with its powerful gravitational and magnetic fields.
Technically, a black hole does not have a magnetic field on its own, but rather the dense plasma that surrounds the black hole as an accretion disk. As the plasma spins around the black hole, the charged particles within it generate an electric current and magnetic field.
The direction of the plasma flow does not spontaneously change, so one could imagine that 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 represented as that of a simple magnet, with north and south poles. A magnetic reversal is where the orientation of that imaginary pole changes and the orientation of the magnetic field changes. This effect is common among stars.
Our Sun reverses its magnetic field every 11 years, driving the 11-year sunspot cycle that astronomers have observed since the 17th century. Even the Earth undergoes magnetic reversals every few hundred thousand years.
But magnetic reversals were not thought to be likely for supermassive black holes.
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 brightened by a factor of 100 in visible light. Shortly after its discovery, the Swift Observatory captured its X-ray and ultraviolet glow. A search of archival observations of the region showed that the galaxy actually began to shine towards the end of 2017.
At the time it was thought that this rapid increase in brightness 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 and also disrupt the flow of gas in the black hole’s accretion disk. But this new study casts a shadow on that idea.
The team analyzed observations of the galactic flare across the spectrum of light, from radio to X-rays. One of the things they noticed was that the intensity of the X-rays decreased very quickly. X-rays are often produced by charged particles spiraling within strong magnetic fields, so this suggests a sudden change in the magnetic field near the black hole.
At the same time, the intensity of visible and ultraviolet light increased, suggesting that parts of the black hole’s accretion disk were heating up. Neither of these effects is what you would expect with a tidal disruption event.
Instead, a magnetic reversal fits the data better. As the team showed, when a black hole’s accretion disk undergoes a magnetic reversal, the fields weaken first at the outer edges of the accretion disk. As a result, the disk can be heated more efficiently.
At the same time, the weaker magnetic field means that charged particles produce fewer X-rays. Once the magnetic field completes its reversal, the disk returns to its original state.
This is only the first observation of a galactic black hole’s magnetic reversal. We now know that they can occur, but we don’t know how common these reversals are. More observations will be needed to determine how many times a galaxy’s black hole can become ambidextrous.
This article was originally published by Universe Today. Read the original article.