# How does an accreting black hole acquire magnetic fields?

There is actually a similar question on this site: Can magnetism escape a black hole?, however the answers do not focus on accretion, which is my main concern, so I start a new one.

In Kip Thorne's book Black Holes and Time Warps: Einstein's Outrageous Legacy, there is a paragraph in Chapter 9, describing how the central black hole of a quasar (or radio galaxy) acquires a magnetic field through accretion:

Where do these magnetic field lines come from? From the disk itself. All gas in the Universe is magnetized, at least a little bit, and the [accretion] disk's gas is no exception. As, bit by bit, the disk's gas accretes into the hole, it carries its magnetic field lines with it. Upon nearing the hole, each bit of gas slides down its magnetic field lines and through the horizon, leaving the field lines behind, sticking out of the horizon and threading it [...]. These threading field lines, firmly confined by the surrounding disk, should then extract the hole's rotational energy by the Blandford—Znajek process.

Unfortunately this description does not seem clear to me (although Thorne generally is excellent in explaining things throughout the book). To be specific, I cannot get a picture of the gas "sliding down" its magnetic field lines through the event horizon, while making the magnetic field "stick out" of the horizon at the same time. And here Thorne did not seem to point to any original research papers for further study.

So, my questions are:

(1) Do today's astronomers still believe that disk accretion is the main process by which supermassive black holes acquire a magnetic field?

(2) If the answer to (1) is "yes", then how can I build the picture (if there is such a picture) that Thorne is trying to convey in his paragraph?

(3) Is there any research paper that addresses this problem to some detail?

Any help is appreciated!

In a case like this, your best option is to go straight to the source. In this case, that would be Blandford & Znajek (1977), the originators of the proposed mechanism. they have several figures regarding different magnetospheric structures depending on different magnetic fields. However, the most important one is Figure 1, the general case:

$D$ represents the accretion disk, while $T$ is the transition zone where matter is rapidly moving radially towards the black hole. The lines represent magnetic field lines, along which charged particles. Electrons (e-) travel in the direction of the medium arrows on each field line; positrons (e+) travel in the other direction. The field lines are closer together in the disk area closer to the black hole, indicating the stronger magnetic field.

If you want a good overview of the process, this is the paper to look at. That said, it is far from an easy read.

Regarding (1), I'm not an astronomer and not really qualified to tell you what the mainstream opinion among astronomers is. However, as far as I'm aware, the Blandford-Znajek process is currently accepted as a strong possibility for generating the magnetic fields observed near most black holes (including stellar-mass black holes).

• Thanks for pointing me to that great paper. Admittedly the Blandford-Znjek paper is a seminal work on the electromagnetic extraction of black hole rotational energy, and it answers many questions regarding the energy source of quasars and radio galaxies. However, I have to say that, as for the scope of my question, this paper is only of tangential relevance since it does not address the process of material accretion to any detail. Still hoping for a more relevant answer. – Kaius Feb 18 '16 at 13:38