Just when you thought that we knew everything there was to know about black holes, astronomers have discovered an anomaly.

The history of black holes in the modern sense goes back to 1916, just after Einstein published his general theory of relativity, when fellow physicist Karl Schwarzschild published a solution of the Einstein equations giving the gravitational field of a spherical object. His solution predicted that if mass exceeded a certain density, then not even light could escape from the surface. And since nothing can accelerate beyond the speed of light, this meant that such objects would effectively trap everything that falls into them.

In the decades that followed, the theory of black holes was expanded with generalizations to black holes that rotate and that carry electromagnetic fields, and with the discovery that quantum mechanics allows black holes to evaporate, as well as detailed models of have giant stars can eventually collapse into these mysterious objects. On the observational side, it has been found that supermassive black holes form the core of most galaxies, and three years ago a team of telescopes working in unison were even able to create a low resolution photograph of a distant black hole.

And now something bizarre has happened.

In the galaxy known as 1ES 1927+654, there is a supermassive black hole at the center. And towards the end of 2017 astronomers started noticing that it was actually getting brighter!

( As an aside here, black holes themselves do not emit enough light or radiation to be detected directly. However their extreme gravitational fields pull in matter from the surrounding galaxy, and as it gets torn apart by the gravitational field it emits an intense jet of radiation, and sometimes even accelerated jets of matter that just escaped falling in the black hole. It is this radiation that we refer to when talking about the brightness of a black hole)

In this case it took only a matter of months for the intensity of the radiation to grow by two orders of magnitude, and astronomers and astrophysicists are at a loss to explain why that should happen. No known theories of black holes have predicted such a phenomenon.

However there is a new theory for why this might have happened, and it is just as intriguing. An international team of astrophysicists have suggested that the magnetic field of this particular black hole may have flipped over, and during that change it would have been emitting much more radiation than usual.

The latest study seems to support the hypothesis. The group collected observations from seven arrays of telescopes, both ground based and in orbit, tracking the radiation being emitted from 1ES 1927+654 as it grew brighter and then eventually dimmed again. And it matched the predictions of the model.

When the black hole pulls in matter from the surrounding galaxy, it also pulls in the magnetic fields produced by that matter. As a result, the accretion disc around the black hole will have a definite north and south pole, just like the Earth does. In this case the incoming matter may have had enough of a magnetic field that was opposite in orientation that it effectively cancelled out the existing field, and then caused it to reverse polarity.

One important prediction of this process is the type of radiation being produced. In most astrophysical processes, the intensity of ultraviolet light and x-rays tend to rise and fall together. However the reversing magnetic field would suppress x-ray production while increasing production of UV radiation. And that was the key observation from the seven telescopes that effectively proved this hypothesis.

Before this, it was assumed that such supermassive black holes were relatively static. The magnetic field that formed when the black hole first came into existence would remain basically unchanged throughout its lifetime. This new discovery seems to suggest that these active galactic nuclei (as they are known in the academic literature) are actually much more dynamic than we thought.

By last summer, the intensity coming from the galactic nuclei had returned to its normal levels, leaving no evidence that anything had happened. But the implications for our understanding of both galactic nuclei and black holes in general will be felt for decades yet to come.