Black Hole Jets Might Be Molded by Magnetism

Article Updated: 23 Dec , 2015

Visible-light Hubble image of the jet emitted by the 3-billion-solar-mass black hole at the heart of galaxy M87 (Feb. 1998) Credit: NASA/ESA and John Biretta (STScI/JHU)

Even though black holes — by their definition and very nature — are the ultimate hoarders of the Universe, gathering and gobbling up matter and energy to the extent that not even light can escape their gravitational grip, they also often exhibit the odd behavior of flinging vast amounts of material away from them as well, in the form of jets that erupt hundreds of thousands — if not millions — of light-years out into space. These jets contain superheated plasma that didn’t make it past the black hole’s event horizon, but rather got “spun up” by its powerful gravity and intense rotation and ended up getting shot outwards as if from an enormous cosmic cannon.

The exact mechanisms of how this all works aren’t precisely known as black holes are notoriously tricky to observe, and one of the more perplexing aspects of the jetting behavior is why they always seem to be aligned with the rotational axis of the actively feeding black hole, as well as perpendicular to the accompanying accretion disk. Now, new research using advanced 3D computer models is supporting the idea that it’s the black holes’ ramped-up rotation rate combined with plasma’s magnetism that’s responsible for shaping the jets.

In a recent paper published in the journal Science, assistant professor at the University of Maryland Jonathan McKinney, Kavli Institute director Roger Blandford and Princeton University’s Alexander Tchekhovskoy report their findings made using computer simulations of the complex physics found in the vicinity of a feeding supermassive black hole. These GRMHD — which stands for General Relativistic Magnetohydrodynamic — computer sims follow the interactions of literally millions of particles under the influence of general relativity and the physics of relativistic magnetized plasmas… basically, the really super-hot stuff that’s found within a black hole’s accretion disk.

Read more: First Look at a Black Hole’s Feast

What McKinney et al. found in their simulations was that no matter how they initially oriented the black hole’s jets, they always eventually ended up aligned with the rotational axis of the black hole itself — exactly what’s been found in real-world observations. The team found that this is caused by the magnetic field lines generated by the plasma getting twisted by the intense rotation of the black hole, thus gathering the plasma into narrow, focused jets aiming away from its spin axes — often at both poles.

At farther distances the influence of the black hole’s spin weakens and thus the jets may then begin to break apart or deviate from their initial paths — again, what has been seen in many observations.

This “magneto-spin alignment” mechanism, as the team calls it, appears to be most prevalent with active supermassive black holes whose accretion disk is more thick than thin — the result of having either a very high or very low rate of in-falling matter. This is the case with the giant elliptical galaxy M87, seen above, which exhibits a brilliant jet created by a 3-billion-solar-mass black hole at its center, as well as the much less massive 4-million-solar-mass SMBH at the center of our own galaxy, Sgr A*.

Read more: Milky Way’s Black Hole Shoots Out Brightest Flare Ever

Using these findings, future predictions can be better made concerning the behavior of accelerated matter falling into the heart of our galaxy.

Read more on the Kavli Institute’s news release here.

Inset image: Snapshot of a simulated black hole system. (McKinney et al.) Source: The Kavli Institute for Particle Astrophysics and Cosmology (KIPAC)

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8 Responses

  1. The general mechanism of magnetic field lines, getting twisted up by the rotation of the object to direct (squirt) plasma out from each pole of the rotational axis is hardly breaking news. The team has effectively simulated the prevalent theoretical model based on existing observations – which is good, I guess. But, did they make any predictions of unobserved behaviour that, if found, might then really support the model? I mean, what’s the next step?

    • lcrowell says:

      Around 1990, in my orbital dynamics and spacecraft navigation days, there was this emerging problem of orbital debris. In particular there was a concern over the tiny stuff. I wrote a program that modified orbital dynamics by including the Lorentz force, where I assumed these tiny particles took up a charge. It turned out that the smallest stuff would form these orbital tubes over the poles of the earth. The paths of these charged 1 to 10 micron sized particles would be spirals that wound up and down around the magnetic field of the Earth. My comment at the time was that this had some bearing on the formation of jets. I wrote a paper on this, with some reference to jets around neutron stars and black holes.

      The physics is similar, but it involves lots of particles and far larger gravity and magnetic forces, and these particles are in a plasma. Plasma physics is horribly difficult. Exact solutions exist only for very symmetrical and artificial situations. Plasma physics is the Navier-Stokes equation, whose mathematical solution space is not known (there is a Claymath award on this) coupled to the Maxwell equations. So things are terribly complicated and numerical work tends to dominate the subject. In some ways I think it could be said that the lack of knowledge about black hole plasmas is not too different from the difficulty in getting plasma physics right with fusion energy research.


    • DrFlimmer says:

      Sometimes it’s just nice to see an idea work out right, isn’t it? 😉

      As far as I know, it is still a fundamental problem, actually, to model jets, even numerically. As lcrowell correctly stated, plasma physics is quite complicated and lots of equations need to be solved simultaneously. A further complication lies in the fact that jet modelling contains vastly different scales: The Schwarzschild radius of the black hole, the accretion disk of a few Schwarzschuld radii, and then the jet which can extent to several Mpc! This is quite difficult to simulate, and therein lies a big problem, since many results of the simulations seem to depend on the resolution. This is a frustrating situation.

      (I should note, however, that I might not be up-to-date with my knowledge, since I only have some interest in this topic, but work on another subject of jets (radiation processes of jets).)

      • Steve Rollins says:

        Luckily we don’t need to apply the same critical thinking and concerns about resolution to climate simulations. After all, the earth is a much less complex system, and who has the time with doomsday just around the corner?

      • lcrowell says:

        Planet Earth is probably one of the most complex systems in the universe. Work on climate models involves huge supercomputers. In fact as these machines sit in huge rooms the heat generated by the machines actually creates their own sort of “weather.” The numerical codes are vastly complex. The physics involves the flow of a gas, which is the Navier-Stokes equation, and one then has to include the phase transition of water and its latent heat, coupling to the oceans and … . That stuff gets really complex.


      • Torbjörn Larsson says:

        You are trolling climate science, and badly:

        Since climate science is less dependent on simulation, no. The GW physics is easy enough to analyse from first principles, as is AGW forcing.

        On the other hand climate is noisy and devilishly complex due to the many mechanisms and scales, contrary to your claim, so the devil is in the details. The effort of thousands of climate scientists have surpassed the effort on black hole jets precisely because the same critical thinking and concerns about resolution are needed and a “doomsday” threatens. (Human deaths, exorbitant cost, new climate regime, mass extinction.) Check with IPCC -07.

        That is why climate scientists started to predict AGW, and its costs, already in the 60’s you know. Check with any encyclopedia. Plenty of time…

  2. katesisco says:

    Black hole jets might be magnetic.

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