We may have caught a supermassive black hole flipping its entire magnetic field

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 strong gravitational and magnetic fields.

Technically, a black hole doesn’t have a magnetic field per se, but the dense plasma surrounding the black hole like an accretion disk does. When the plasma swirls around the black hole, the charged particles it contains generate an electric current and a magnetic field.

The direction of the plasma flow does not change spontaneously, so one could imagine that the magnetic field is very stable. So imagine astronomers’ surprise when they saw evidence that a black hole’s magnetic field had undergone a magnetic reversal.

In simple terms, a magnetic field can be represented as that of a simple magnet, with a north pole and a south pole. A magnetic reversal is where the orientation of this imaginary pole flips and the orientation of the magnetic field flips. This effect is common in stars.

Our Sun reverses its magnetic field every 11 years, resulting in the 11-year sunspot cycle that astronomers have observed since the 1600s. Even Earth experiences magnetic reversals every few hundred thousand years.

But magnetic reversals were not considered likely for supermassive black holes.

In 2018, an automated survey of the sky revealed 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 glow in X-rays and ultraviolet light. A search for archival observations of the region showed that the galaxy did indeed begin to brighten towards the end of 2017.

At the time, the rapid brightening was thought to be caused by a star passing close to the galaxy’s supermassive black hole. Such a close encounter would cause a tidal disturbance event, which would tear apart the star and disrupt the flow of gas in the black hole’s accretion disk. But this new study casts a shadow over that idea.

How a black hole could undergo a magnetic reversal. (NASA Goddard/Jay Friedlander)

The team looked at observations of the galactic flare across the full spectrum of light, from radio to X-rays. One of the things they noticed was that the intensity of the X-rays was decreasing very rapidly. X-rays are often produced by charged particles spiraling into strong magnetic fields, suggesting a sudden change in the magnetic field near the black hole.

At the same time, the intensity of light in the visible and ultraviolet increased, suggesting that parts of the black hole’s accretion disk were getting hotter. None of these effects are what you would expect with a tidal disturbance event.

Instead, a magnetic reversal fits the data better. As the team showed, when a black hole accretion disk undergoes a magnetic reversal, the fields first weaken at the outer edges of the accretion disk. As a result, the disc can heat up more efficiently.

At the same time, the weaker magnetic field means fewer X-rays are produced by charged particles. After the magnetic field completes its reversal, the disc returns to its original state.

This is only the first observation of the magnetic reversal of a galactic black hole. We now know that they can happen, but we don’t know how common these reversals are. It will take more observations to determine how many times a galaxy’s black hole can become a switch.

This article was originally published by Universe Today. Read the original article.

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