Pole shift of superlatives: astronomers may have observed an active black hole during a pole reversal for the first time – the entire magnetic field of a supermassive black hole changed its orientation by 180 degrees within a few months. Evidence of the flip was a drastic 100-fold increase in the brightness of this active galactic nucleus and a mysterious pause in energetic X-rays.
While the supermassive black hole at the center of our Milky Way is currently rather inconspicuous and inactive, this is at active galaxy cores different: Because these black holes absorb large amounts of material, they are surrounded by a rapidly rotating accretion disk of hot, highly radiating plasma. observations of the Event Horizon Telescope also prove that the rapid movement of charged particles acts like a dynamo and generates strong magnetic fields – so far, so known.
Mysterious increase in brightness
“We normally expect such black holes to evolve over millions of years,” explains co-author Nicolas Scepi from the Joint Institute for Laboratory Astrophysics (JILA) in Boulder. Although there are always shorter bursts of radiation when more material is being sucked in, the radiation emission of such a galaxy core changes only slowly on a larger scale.
The behavior of the supermassive black hole in the center of the galaxy 1ES 1927+65, which is around 236 million light years away, is all the more surprising. In December 2017, its already active center began to emit more and more visible and ultraviolet light. By March 2018, its brightness had increased by around 100 times. At that time, astronomers initially suspected that it was a so-called Tidal Disruption Event might act – the rupturing of a star by the black hole.
X-ray radiation does not fit into the picture
Strangely enough, while UV and visible radiation increased, the high-energy X-rays from the black hole continued to weaken. “This is the first time that X-rays have disappeared completely while the other wavelengths have increased,” says first author Sibasish Laha of NASA’s Goddard Space Flight Center. It was not until autumn 2018 that the active galaxy core gradually began to normalize once more: its brightness decreased once more and X-ray radiation increased in return.
But what was the cause? To find out, Laha and his team once once more evaluated all available observation data for 1ES 1927+65 before and during the event and also made new observations in the X-ray and radio range. In total, the astronomers were able to reconstruct the changes in the black hole with seven telescopes and in all wavelengths. This revealed, among other things, that in addition to the X-ray emission, the radio emission of the active galaxy core also decreased during the event.
Pole reversal at a black hole.© NASA/Goddard
Magnetic field reversal instead of star death
Both together, Laha and a colleague provided a strong indication of what may have happened in the galaxy 1ES 1927+65: The supermassive black hole underwent a magnetic field reversal. “A magnetic polarity reversal, where the north pole becomes the south pole, appears to us to be the best explanation for the observations,” says co-author Mitchell Begelman of the University of Colorado Boulder.
The astronomers explain how this actually happens: “The magnetic field first weakens on the outside of the accretion disk,” says Begelmann. “This leads to more heating and the increased emission of visible and ultraviolet light.” Over the course of regarding three months, the magnetic intensity also decreases further inwards and finally drops so far that the corona of the black hole collapses – the cloud of hot, high-energy particles that are responsible for X-ray radiation.
At the peak of the polarity reversal, the black hole’s magnetic field collapses completely, stopping the “engine” for radio emission as well. A short time later, the magnetic field begins to regenerate in reverse polarity and following some time the intensity is high enough once more for X-ray and radio radiation to increase once more.
Not an isolated case
According to the astronomers, the outburst of brightness from 1ES 1927+65 might not only be the first observation of a pole reversal at the black hole – it is also probably not an isolated case in the cosmos. Models have long been predicting that such magnetic field reversals occur in active black holes. The polarity reversal at 1ES 1927+65 confirms this and at the same time underscores that supermassive black holes can also undergo rapid, dynamic changes.
“If we have seen this in this one case, then we will definitely observe it once more elsewhere – now we know what to look for,” says Begelmann. (The Astrophysical Journal, accepted; arXiv:2203.07446)
Quelle: University of Colorado Boulder
