A Supermassive Black Hole Just Flipped its Entire Magnetic Field
Black holes are incredibly powerful cosmic engines. They are the source of energy for quasars and other active galactic nuclei (AGNs). This is because matter interacts with its enormous gravitational and magnetic fields.
Technically, a black hole does not have its own magnetic field, but the dense plasma surrounding it as an accretion disk does. The charged particles in the plasma generate an electrical current and a magnetic field as it swirls around the black hole. The direction of the plasma flow does not vary spontaneously, so the magnetic field must be highly constant. Imagine astronomers’ surprise when they discovered evidence of a magnetic reversal in a black hole’s magnetic field.
In basic terms, a magnetic field can be imagined as that of a simple magnet, with a north and south pole. A magnetic reversal occurs when the orientation of that imaginary pole and the magnetic field flip. This is a regular phenomenon among stars. Our Sun’s magnetic field flips every 11 years, causing the 11-year cycle of sunspots that astronomers have observed since the 1600s. Even the Earth undergoes magnetic reversals every few hundred thousand years. Magnetic reversals, on the other hand, were thought to be unlikely for supermassive black holes.
In 2018, an automated sky survey found a sudden change in a galaxy 239 million light-years away. The galaxy, known as 1ES 1927+654, had brightened by a factor of 100 in visible light. Swift Observatory photographed its brightness in x-rays and ultraviolet shortly after its discovery. A search of archival observations of the region revealed that the galaxy began to brighten toward the end of 2017.
This rapid brightening was assumed to be caused by a star passing close to the galaxy’s supermassive black hole at the time. A near encounter like this would create a tidal disruption event, ripping the star apart and disrupting 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 could undergo magnetic reversal. Jay Friedlander/Goddard NASA’s Space Flight Center. Credit: NASA’s Goddard Space Flight Center/Jay Friedlander
The researchers examined data from the galactic flare across the entire light spectrum, from radio to x-ray. One thing they noticed was that the strength of the x-rays quickly dropped. X-rays are frequently created by charged particles spiraling within powerful magnetic fields, hence a sudden change in the magnetic field surrounding the black hole was suggested.
At the same time, the intensity of visible and ultraviolet light rose, implying that areas of the black hole’s accretion disk were becoming hotter. Neither of these effects is what you’d expect from a tidal disruption event.
Instead, a magnetic reversal fits the data better. As the researchers demonstrated, when a black hole accretion disk undergoes a magnetic reversal, the fields weaken first towards the accretion disk’s outer edges. As a result, the disk can heat up more efficiently. At the same time, the smaller magnetic field means that charged particles create fewer x-rays. The disk returns to its original state once the magnetic field has finished its reversal.
This is the first observation of a galactic black hole’s magnetic reversal. We now know they can occur, but we don’t know how common they are. More data will be required to determine how many times a galaxy’s black hole may serve as a switch hitter.
Reference: Laha, Sibasish, et al. “A radio, optical, UV and X-ray view of the enigmatic changing look Active Galactic Nucleus 1ES~ 1927+ 654 from its pre-to post-flare states.” arXiv preprint arXiv:2203.07446 (2022).