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Those Jets from the poles of black holes

  1. Oct 17, 2011 #1
    Can anyone point me to a journal article that describes, mathematically or otherwise, how those jets shoot from the poles of some black holes with accretion discs and AGN.

    Also, if you will forgive the additional question, is there any reason why the physics that describe the ejection from black hole poles might not also explain Pulsars and Magnetars?

    By this I mean to ask if Pulsars and Magnetars might be ejecting photons from both poles, but through some process its axis has gone wobbly (like Mars does from time to time) and from earth we only see the radio signal from one pole? I understand that if an object is orbited, rather than orbiting something, it is much more likely to always orbit on its axis.

    Thank you for your help, and don't hesitate to tell me if this is simply a moronic question and to go hang out on a Star Trek fan site (and I don't like Star Trek).

    Sorry if this is a frequently asked question. I haven't searched the database yet.
     
  2. jcsd
  3. Oct 17, 2011 #2

    FlexGunship

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    Black holes accelerate ejection material by gravity, pulsars emit their own electromagnetic radiation, and magnetars acceleration ejection material by magnetism (if they do at all; usually they release stored energy in the form of a GRB).

    If you could combine the mathematics of electromagnetism and gravity you'd win a few Nobel prizes... to say the least.

    EDIT: Sorry, I guess I didn't address your question about an article from a journal.

    DOUBLE EDIT: After reading a bit more, it appears that the relativistic jets of a black hole can acquire a magnetic field of their own after ejection.
     
    Last edited: Oct 17, 2011
  4. Oct 17, 2011 #3

    jambaugh

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    The source of the energy is the incredible heat of the accretion disks. As the material falls inward, it is being compresses and undergoing collisions with its neighboring infalling material.

    The reason it is ejected through the poles has to do with the incredibly strong magnetic field of the BH. Think of it in terms of the northern lights in reverse. Solar wind impinging on the Earth's magnetic field is stopped by the fact that charged particles in a magnetic field will be accelerated in a direction perpendicular to the field lines and to the direction of motion. Only charged particle moving parallel to the field lines will not be turned back on themselves, hence the particles in the solar wind can only reach the atmosphere by sliding down the magnetic field near the poles. This causes the aurora.

    Now reverse the phenomenon, imagine trying to shoot charged particles out in all directions and they will only be able to escape along the poles. It is like the magnetic bottles they are using to create fusion reactions. The energy around the black hole in the form of very energetic charged particles is bottled up by the magnetic field except at the poles where it can "squirt" out
     
  5. Oct 17, 2011 #4
    Thanks, I thought that was what was going on, like a reverse of the northern lights.

    But a neutron star doesn't have enough gravity, or the accretion disc's energy, to create a similar energy beam on the pole of the star?
     
  6. Oct 17, 2011 #5

    jambaugh

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    The neutron star has the energy of its own formation. Recall that it is the remnant of a nova/supernova.

    Edit: Also... In so far as drawing in surrounding matter, a neutron star is no different from a BH in its use of gravity. In-falling gas and dust will release an appreciable percentage of its mass energy equivalent in falling to the surface of a neutron star.

    Remember a neutron star just under the mass to form a BH will still essentially behave exactly the same as a BH outside its surface.

    EDIT2: See http://imagine.gsfc.nasa.gov/docs/science/know_l1/pulsars.html"
     
    Last edited by a moderator: Apr 26, 2017
  7. Oct 18, 2011 #6
    Okay, thanks. So a neutron star can only accelerate mass toward its centre of gravity at well below c, whereas a black hole accelerate mass to almost c before the event horizon. This means that angular momentum increases the speed of mass even higher, as it is moved by magnetism up toward the pole, increasing the speed even more. So that much of this momentum, when it reaches the pole, is closely approaching c and then departs from near the event horizon as, basically, plasma and photons of differing energy?

    I am curious about this as one of the episodes of "The Universe" (I could chase it up if it mattered) suggested that objects the size of bowling balls were travelling at 99.99% of c from the jets of AGN (remember, Blazars cannibalise planets as well as gas giants, and probably even neutron stars). But it appears to me that even a grain of sand would be well ripped apart before being ejected from an AGN.

    (Thanks for the Pulsar link about the poles of neutron stars. I used to work at an observatory in the late '70s as a teenager, expecting to start a career in astrophysics. But the guy I worked for was so depressed after his eight year project on superclusters failed to prove his thesis, that I gave up on it as a career as well, thinking I could be destined to waste an equivalent time on something that wasn't even there [eight years seems a long time when you are a teenager]. Of course, now I know that dark matter and energy so fudged his thesis and that, if only he had thought to rethink his experiment, he might now have had his "Scientific American" feature, that he so dearly desired, were he to have thought through the implications of his failed thesis.)
     
  8. Oct 18, 2011 #7

    jambaugh

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    Why do you assume "well below c"? Again a neutron star can be just on the verge of becoming a BH and so could approach the BH's gravitational behavior.

    That scenario reads too much into it. Don't think in terms of "increasing mass", that's "old paradigm" relativity. Just think in terms of energy (some of which is rest mass). Focus then on the released potential energy of particles as they fall down a gravity well.

    There are two causes for directed radiation from a neutron star.

    One is that the infalling charged particles are directed to the magnetic poles. The "aurora" goes all the way down to the surface and the infalling particles release a great deal of energy (x-rays) by heating the spot near the magnetic pole.

    The other is similar to the BH mechanism, if substantial amount of infalling gas (say stolen from a binary partner) is being "sucked in" then it is compressed at the poles, heated and some is released in high energy jets.

    Finally note that I'm no expert on pulsars, (I've a general understanding of SR, GR, and EM) but understand the mechanisms available to them. A better read person may point out some more exotic features of their behavior or better description of them than mine.

    This is beyond my knowledge. But one thought. A grain of sand may well be ripped apart but a planet may not be completely. I can imagine its iron core being thrust outward and blasted into chunks which while evaporating are being accelerated outward. It is conceivable to me that they might get out far enough to retain say bowling ball size fragments before being evaporated. I haven't enough theory to "do the math" on this sort of speculation though.

    Imagine being hit in the face by one though! It would ruin your day (as well as that of most everyone else in the solar system). Talk about a fast ball!
     
  9. Oct 19, 2011 #8
    Here is an intro

    http://www.astronomy.ohio-state.edu/~ryden/ast825/ch13.pdf

    But one thing that you have to realize is that the answer to the question is "we aren't entirely sure". Modelling jets is an active area of research, and the last time I checked, it was in a "we have some decent guesses" mode.


    Yes. With pulsars and magnetars, you can attach a jet or magnetic field to the surface of a star. With black holes, you can't. It turns out that black hole candidates are a lot less noisy than neutron star candidates. With neutron stars, there is a surface and you can dump matter onto the surface and get big flashes of energy. With black holes, there is no surface, so you tend not to see big flashes of energy with black holes.

    No. It's a great question, and like a lot of other great questions, the answer is "we are still trying to figure out a lot of the details."
     
  10. Oct 19, 2011 #9
    Gravitationally yes. Magnetically no.
     
  11. Oct 19, 2011 #10
    Not well below C. The escape velocity at the surface of a neutron star is a few tenths of C, and that's enough to generate a lot of things that you see with black holes.

    Same thing happens with neutron stars. Although there is a lot of hand waving going on here.

    Would be interested in a citation. Of the top of my head, after getting heated to several million degrees, I'd expect everything to be plasma.

    Turning lemons into lemonade is all about what research is about. Doing research that conclusively shows that something isn't there or that something won't work is progress.
     
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