There is a running joke in astronomy that pretty much any talk can be thwarted by the simple question: What are the effects of a magnetic field?
I’ve known that joke my whole career, but I thought I had a good enough understanding to get the feel for things until today. My mental line drawings didn’t leave room for the concept of black holes somehow reversing their magnetic fields, and now, somehow, a team led by Sibasish Laha of NASA is suggesting this is exactly what some supermassive black holes in active galaxies are doing.
Let’s back up.
Here on Earth, we know from geological records that our magnetic poles generally wander about a bit, and every 200,000 years they flip. Our magnetic field is driven, in the most simplified terms, by convecting metals setting up loop after loop after loop of essentially electromagnetic fields and together aligning to create a single, averaged-out, planetary magnetic field.
It is possible for these loops to get out of alignment, and in the ensuing chaos, for their orientations to rearrange so that the magnetic field overall stays aligned – sorta – with the Earth’s rotation, but the specific north versus south magnetic pole flips. In my personal mental model, I always imagine one of those wild Ferris wheels where each car is free to rotate, and the entire wheel is also rotating. Small factors can add up to influence the cars to mostly go round and round one way or the other, but both rotations are possible within the mechanics of the overall rotation.
I also just like to think about riding roller coasters.
Anyway, that’s my mental model and stands up for thinking about the Earth and the Sun, but as we get out toward the kinds of magnetic fields that are associated with discs of material orbiting compact objects like neutron stars and black holes, my gut instinct, which is a lying liar at times, said we should have one orientation, and it should stay put. And from looking at the literature, there isn’t exactly a thriving community of folks modeling what would happen if a black hole’s magnetic field flipped like the Sun’s magnetic field.
But there are papers asking, “What if?” Not many, but some.
And there is also a weird thing sometimes observed in active galaxies with feeding black holes. Occasionally, these systems will brighten by nearly a factor of one hundred and then fade back to normal over just a few months. Called “Changing Look AGN,” because they change how they look and astronomers are terrible at naming things, a new paper led by Laha describes the brightening as a symptom of the magnetic field reversal at the central supermassive black hole.
This work is accepted to appear in The Astrophysical Journal.
Using computer models and observations that spanned from long radio wavelengths through to the super short X-ray colors of light, this team could test different theories against the data. They ruled out the idea, which I liked, of the black hole eating a star – the light profiles and timing just didn’t match. Instead, what they say, according to Jason Decter was: The field initially weakens at the outskirts of the accretion disk, leading to greater heating and brightening in visible and UV light.
They also saw the X-ray light turn off and then turn back on. This detail matches the specifics of a magnetic field reversal, where, to quote the NASA release on this work: As the flip progresses, the field becomes so weak that it can no longer support the corona – the X-ray emission vanishes. The magnetic field then gradually strengthens in its new orientation.
Why do some black holes flip their magnetic field? No idea. What causes this misbehavior? No idea? Do we need to worry about our supermassive black hole reversing its magnetic field? Probably not, but that is a gut feeling, and again, my gut is a lying liar.
In every astronomer’s life, there comes a day when the full weight of how little we know about magnetic fields suddenly weighs heavy.
For me, today was the day.
The universe is weird. I love to watch it, and I am grateful there are people working to get at the maths behind all the wonder we see — people who are not me.
CU Boulder press release
NASA press release
“Magnetic flux inversion in a peculiar changing look AGN,” Nicolas Scepi, Mitchell C Begelman, and Jason Dexter, 2021 January 9, Monthly Notices of the Royal Astronomical Society
“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,” Sibasish Laha et al., to be published in The Astrophysical Journal (preprint on arxiv.org)