Black holes are powerful cosmic reactors. They supply the energy for quasars and other active galactic nuclei (AGNs). This is due to the interplay between matter and its enormous gravitational and magnetic forces.
A black hole technically lacks a magnetic field, but the dense plasma surrounding it as an accretion disc does possess a magnetic field. As plasma spirals around a black hole, the charged particles inside it create an electrical current and magnetic field.
The direction of plasma flow does not spontaneously vary, hence the magnetic field is likely rather stable. Imagine the researchers' amazement when they discovered evidence that the magnetic field of a black hole had undergone a magnetic reversal.
A magnetic field may be conceptualized as a magnet with a north and south pole. A magnetic reversal occurs when the direction of the imaginary pole and the magnetic field both reverse. This occurrence is prevalent among stars.
The Sun reverses its magnetic field every 11 years, which produces the 11-year cycle of sunspots recorded by scientists since the 1600s. Even the Earth experiences magnetic reversals every few hundred thousand years. However, magnetic reversals were not considered to be probable for supermassive black holes.
In 2018, an automated sky scan detected an abrupt alteration in a galaxy 239 million light-years distant. The galaxy known as 1ES 1927+654 has become 100 times brighter in visible light.
Swift Observatory caught its x-ray and ultraviolet light emission shortly after its detection. An examination of the region's archival data revealed that the galaxy began to brighten near the end of 2017.
|How a black hole might undergo magnetic reversal. Credit: NASA’s Goddard Space Flight Center/Jay Friedlander
At the time, it was believed that this sudden brightening was produced by a star passing near the galaxy's supermassive black hole. Such a near encounter would result in a tidal disruption event, which would shatter the star and block the flow of gas in the black hole's accretion disc. However, this recent research puts doubt on this theory.
The researchers examined data of the cosmic flare over the whole spectrum of light, from radio to x-ray. One of the things they saw was that the strength of x-rays decreased rapidly. X-rays are typically created by charged particles swirling under powerful magnetic fields, therefore this indicated an abrupt shift in the magnetic field surrounding the black hole.
Simultaneously, the visible and ultraviolet light intensities rose, indicating that portions of the black hole's accretion disc were heating up. Neither of these outcomes is consistent with a tidal disruption event.
Instead, the findings are best explained by a magnetic reversal. As the researchers demonstrated, when a black hole accretion disc experiences a magnetic reversal, the fields diminish first near the accretion disk's outer edges.
Consequently, the disc may heat up more effectively. Charged particles create fewer x-rays as a result of the reduced magnetic field. Once the magnetic field's reversal is complete, the disc returns to its initial condition.
This is the very first detection of the magnetic reversal of a galactic black hole. Now that we know they are possible, we do not know how often these reversals are. It will need further studies to calculate the number of times a galaxy's black hole may flip positions.
Reference(s): Peer-Reviewed Research Paper