Are black holes magnetic?

Radio astronomers have made the first measurements of the magnetic field around the black hole at the centre of the Milky Way.
15 August 2013


Dominic - Now, a paper I've been looking at this week is about the first measurement of the magnetic field of a black hole. Now, there's actually quite a big black hole at the centre of our galaxy. This is the nearest example of a black hole to our own sun, and it weighs millions of times the mass of our sun.

Now, black holes are formed at the ends of the lives of stars. All the mass of that star just collapses down to this very dense object and nothing can then escape the gravity of that black hole. Now, we've got this, what we call 'super massive black hole' at the centre of our galaxy and that actually plays quite an important role in holding our galaxy together because you've got this tremendous amount of mass there.

Kate - So, it's got a positive effect then.  I imagine black holes as these giant space hoovers that suck everything in and I imagine it would be a destructive force for our galaxy rather than a positive one.

Dominic - I mean certainly, if you get too close to a black hole, that's pretty bad news. But what the galaxy needs is lots of gravitational glue to hold all of its mass together. It's whirling around remember and if there wasn't gravitational glue to hold it together, all of this materials would just fly off into space. So, you need a lot of gravity to hold that together. But the question is, do black holes have magnetic fields and that's a very difficult question to answer because you can't see magnetic fields. You've got to look for some physical phenomenon which is affected by magnetism.

Kate - Why do we think black holes might have magnetic fields?

Dominic - Well, we think stars have quite strong magnetic fields. Our own sun for example has quite a strong magnetic field and that's what create sunspots and much of the really violent structure we see on its surface.

It was seem odd to imagine that when stars like the sun collapse, that magnetic field is just going to go away, but if these black holes have magnetic fields, that's actually quite important for how they behave with their environment. Because if you've got materials spiralling in to a black hole, that material gets very hot and it enters the state that we call 'ionised material'. It starts to glow red hot. The protons and electrons are stripped apart, and then those charged particles feel magnetic forces. So, if this black hole has a strong magnetic field, that's going to affect how the charged materials is falling into it. If we want to understand how black holes suck up material from their environment, we need to understand those magnetic fields that will really produce quite a strong force on the environment.

Kate - So, how do you measure it then? Do you have to send in some sort of satellite on a suicide mission to get close enough to measure it before it gets sucked in?

Dominic - I mean, the problem is that the centre of our galaxy is tens of thousands of light years away. So, it's a very difficult environment to go and probe, but if you can find a physical phenomenon which is affected by magnetism, you can look at how that phenomenon changes in the region of the central part of the galaxy.

Now, the technique that Rolf Ito used in his paper in Nature this week, was look at stars called 'pulsars' which are formed at the ends of the lives of stars. They're very compact and they produce beams of radiation out of their magnetic poles. Those beams of radiation, it so happens, are polarised. This is like when you wear Polaroid sunglasses and light can oscilate in two different directions and you're only letting through light that oscilates in one direction. But if that light then travels through a medium where there's A) a magnetic field and B) lots of charged particles, yhis is basically the environment you have around a black hole then that plain polarisation starts to rotate, it's an effect called faraday rotation. By looking at how much it's rotated, you can tell how much magnetic field was there.

Now, until recently, we haven't been able to find any pulsars which are really close to the centre of a black hole. Probably could have so much stuff there. It's actually quite hard to see what's going on, but this team at the Max Planck Institute in Bonn have now found a pulsar and they've got very strong evidence, it seems to be very close in to the centre of the galaxy. And so, they've looked at the light of this pulsar. Looked at the faraday rotation of its light and from that, they've inferred this black hole actually has quite a strong magnetic field. And now that we've got a number on that black hole, we can refine our models much better and how materiat behaves spiralling into that black hole. And so, we can understand how black holes accrete material from their environment much better.


"Now, we've got this, what we call 'super massive black hole' at the centre of our galaxy and that actually plays quite an important role in holding our galaxy together because you've got this tremendous amount of mass there."

Sorry, this isn't right. The Milky Way's supermassive black hole's gravitational sphere of influence (where its gravity outweighs the gravity of the surrounding stars) is actually quite small compared to the size of the galaxy. The other stars in our galaxy's bulge have a far greater gravitational influence, and we haven't even gotten into the effect of dark matter in holding the galaxy together.

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