(ORDO NEWS) — Black holes are powerful cosmic engines. They provide energy for quasars and other active galactic nuclei (AGNs). This is due to the interaction of matter with their powerful gravitational and magnetic fields.
Technically, the black hole itself does not have a magnetic field, but the dense plasma that surrounds the black hole in the form of an accretion disk does. As the plasma swirls around the black hole, the charged particles in it create an electric current and a magnetic field.
The direction of the plasma flow does not change spontaneously, so it can be assumed that the magnetic field is very stable. So imagine astronomers’ surprise when they saw evidence that the black hole’s magnetic field had reversed.
In general terms, the magnetic field can be thought of as an ordinary magnet with a north and south pole. A magnetic reversal is when the orientation of this imaginary pole changes and the orientation of the magnetic field changes. This effect is often found in stars.
Our Sun reverses its magnetic field every 11 years, resulting in an 11-year sunspot cycle that astronomers have been observing since the 1600s. Even the Earth undergoes a change in the magnetic field every few hundred thousand years.
But magnetic reversals were not considered likely for supermassive black holes.
In 2018, an automated survey of the sky detected a sudden change in a galaxy 239 million light-years away. Known as 1ES 1927+654, the galaxy became 100 times brighter in visible light.
Shortly after its discovery, the Swift Observatory recorded its glow in X-rays and ultraviolet rays. A search for archival observations in this region showed that the galaxy did indeed start to brighten in late 2017.
At the time, it was thought that this rapid increase in brightness was caused by the passage of a star near the galaxy’s supermassive black hole.
Such a close encounter would cause tidal disruption that would tear the star apart and also disrupt the flow of gas in the black hole’s accretion disk. However, a new study casts a shadow on this idea.
The team studied the results of observations of the galactic flare in the entire spectrum of light – from radio to X-ray. In particular, they noticed that the intensity of the X-ray emission decreased very quickly. X-rays are often generated by charged particles spiraling in intense magnetic fields, so this indicates a sudden change in the magnetic field near a black hole.
At the same time, the intensity of light in the visible and ultraviolet ranges has increased, suggesting that part of the black hole’s accretion disk has become hotter. None of these effects are what one would expect from a tidal disruption event.
Instead, the 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 disc can be heated more efficiently.
At the same time, the weakening of the magnetic field means that the charged particles produce less X-rays. As soon as the magnetic field completes its reverse movement, 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 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 black hole in a galaxy can become a switch.
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