Astronomy Without A Telescope – Blazar Jets

Article written: 23 Oct , 2010
Updated: 24 Dec , 2015
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Polar jets are often found around objects with spinning accretion disks – anything from newly forming stars to ageing neutron stars. And some of the most powerful polar jets arise from accretion disks around black holes, be they of stellar or supermassive size. In the latter case, jets emerging from active galaxies such as quasars, with their jets roughly orientated towards Earth, are called blazars.

The physics underlying the production of polar jets at any scale is not completely understood. It is likely that twisting magnetic lines of force, generated within a spinning accretion disk, channel plasma from the compressed centre of the accretion disk into the narrow jets we observe. But exactly what energy transfer process gives the jet material the escape velocity required to be thrown clear is still subject to debate.

In the extreme cases of black hole accretion disks, jet material acquires escape velocities close to the speed of light – which is needed if the material is to escape from the vicinity of a black hole. Polar jets thrown out at such speeds are usually called relativistic jets.

Relativistic jets from blazars broadcast energetically across the electromagnetic spectrum – where ground based radio telescopes can pick up their low frequency radiation, while space-based telescopes, like Fermi or Chandra, can pick up high frequency radiation. As you can see from the lead image of this story, Hubble can pick up optical light from one of M87‘s jets – although ground-based optical observations of a ‘curious straight ray’ from M87 were recorded as early as 1918.

Polar jets are thought to be shaped (collimated) by twisting magnetic lines of force. The driving force that pushes the jets out may be magnetic and/or intense radiation pressure, but no-one is really sure at this stage. Credit: NASA.

A recent review of high resolution data obtained from Very Long Baseline Interferometry (VLBI) – involving integrating data inputs from geographically distant radio telescope dishes into a giant virtual telescope array – is providing a bit more insight (although only a bit) into the structure and dynamics of jets from active galaxies.

The radiation from such jets is largely non-thermal (i.e. not a direct result of the temperature of the jet material). Radio emission probably results from synchrotron effects – where electrons spun rapidly within a magnetic field emit radiation across the whole electromagnetic spectrum, but generally with a peak in radio wavelengths. The inverse Compton effect, where a photon collision with a rapidly moving particle imparts more energy and hence a higher frequency to that photon, may also contribute to the higher frequency radiation.

Anyhow, VLBI observations suggest that blazar jets form within a distance of between 10 or 100 times the radius of the supermassive black hole – and whatever forces work to accelerate them to relativistic velocities may only operate over the distance of 1000 times that radius. The jets may then beam out over light year distances, as a result of that initial momentum push.

Shock fronts can be found near the base of the jets, which may represent points at which magnetically driven flow (Poynting flux) fades to kinetic mass flow – although magnetohydrodynamic forces continue operating to keep the jet collimated (i.e. contained within a narrow beam) over light year distances.

Left: A Xray/radio/optical composite photo of Centaurus A - also not technically a blazar because its jets don't align with the Earth. Credit: X-ray: NASA/CXC/CfA/R.Kraft et al.; Submillimeter: MPIfR/ESO/APEX/A.Weiss et al.; Optical: ESO/WFI. Right: A composite image showing the radio glow from Centaurus A compared with that of the full Moon. The foreground antennas are CSIRO's Australia Telescope Compact Array, which gathered the data for this image.

That was about as much as I managed to glean from this interesting, though at times jargon-dense, paper.

Further reading: Lobanov, A. Physical properties of blazar jets from VLBI observations.

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21 Responses

  1. sol88 says

    The physics underlying the production of polar jets at any scale is not completely understood. It is likely that twisting magnetic lines of force, generated within a spinning accretion disk, channel plasma from the compressed centre of the accretion disk into the narrow jets we observe. But exactly what energy transfer process gives the jet material the escape velocity required to be thrown clear is still subject to debate

    STILL???

  2. Member

    @ SOL88

    Umm… yes. What’s your point?

  3. The exact energy transfer process: There are many competing possibilities. An interesting one is what may be called “photon surfing”. This was proposed for acceleration to v = (0.26 or 0.27)c of SS433 (stellar or “microquasar”) jet via a resonant photon absorption process akin to masing or lasing. Prof G Srinivasan (then at Raman Research Institute, Bengaluru, India) who assessed my 1984/5 PhD thesis entitled “Evolution of Extragalactic Radio Sources” was intrigued by the peak at a similar speed in the distribution of hotspot advance speeds from arm asymmetry of samples of double jets, and suggested investigating physical processes (including frame-dragging & so on near the putative spinning black hole postulated as the driving engine for such astrophysical jets). A rather feeble go at this after about a decade is recorded in a section named “photon surfing” in 1998 Phys Rep 303 81-182 Bilateral symmetry in Active Galaxies. Anyone interested may contact [[email protected]] and will receive more definite information and references. References &/or information about this topic is also most welcome at this email address.

  4. jimhenson says

    the x-ray jets beam out vast distances into the web of hot diffuse gas called the warm hot intergalactic medium or WHIM. It is easy to see through and difficult to detect, with a density only 6 protons per cubic meter and 1 million degrees. A huge amount of missing visible matter is contained in large scale structures, as evidenced by an large AGN black hole jet aligned along the sculptor wall of galaxies, that absorbs x-rays by oxygen atoms.

  5. Dave Finton says

    Personally, I love these things. Not only do they make for gorgeous pictures, but they also make for a great mystery.

  6. Torbjorn Larsson OM says

    Jets, but with a twist! … um, I’ll take my coat and let myself out now.

  7. jimhenson says

    ISM and IGM magnetic field lines should be considered the flow directions that beamed charged jet particles want to follow. x-ray jets are going to be absorbed by oxygen ions in the WHIM. Blazers and quasars could be forming smaller galaxies by an accretion mechanism.

  8. damian says

    Curiously, how sure is everyone that these are “jets” rather then funnels into a source.?

    Interstellar dust being compressed into the accretion disc of a galaxy?

  9. Member

    @ Damian

    I understand we can readily observe the movement of jet material (which is moving pretty fast) over time. It’s definitely outwards, rather than inwards. For example, check out this VLBA movie:
    http://www.nrao.edu/pr/2004/ss433/

  10. DrFlimmer says

    @ Damian

    To add a little bit to Steve’s response: We also know about relativistic Doppler boosting. This means that radiation of charged particles in the jet (i.e. synchrotron radiation) is highly beamed in the direction of motion (i.e., forward direction). Since jets are extremely bright when you look straight down their axis, we know that such a boosting is at work (there are also several other reasons). Thus the particles are coming towards us and do not fly away.
    That is also the reason why we sometimes see only one jet (the one pointing towards us) and the counter-jet is missing. It is just too faint, because most of its radiation is beamed away from us.

  11. damian says

    Interesting.

    Thanks for the answers. Just leaves me with more random questions to read up about. Do we know how fast the actual quasars (+ Source) are actually traveling?

    In Centaurus A it appears that the jets Heat up interstellar gas that is ‘eventually’ subsumed back into the galaxy. It could still be a accretion mechanism. I can imagine this process creating Dwarf galaxies, these would then orbit the solar disc and eventually be pulled in by the galactic discs.

    Galaxy dinner time? 🙂

  12. Lawrence B. Crowell says

    As the article indicates thisse jets are a manifestations of MHD. Gravitation increases the kinetic energy of material which falls into the BH. As this heats up it ionizes into a plasma. Since protons and electrons have a mass ratio of 1/2000 there is a charge separation which determines the transport properties of these particles. This motion generates a large magnetic field due to the charge separation current. The charged particles then determine their own transport by the magnetic field they generate. This results in jets.

    I did a problem years ago which involved the orbit if charged particles in Earth orbit. This was due to the growing issue of orbital debris. I numerically simulated the motion of charged particles 1-100 microns in size. These particles ended up in these tubular spiral orbits around the poles of the Earth. The Lorentz force on the particle caused them to spiral towards the poles. These were not jets of course, but if one had a lot of plasma in these “tubes” and if there was a powerful driving force with the oscillation of magnetic field lines the particles in these tubes would gain kinetic energy and be accelerated outwards in a spiraling jet. This is a “phys-101” idea of how these are generated. They are very powerful, and there are images of jets from one galaxy seriously perturbing a neighboring galaxy.

    LC

  13. Member
    Aqua says

    Now THAT’s a strong magnetic field! All due to gravitation and kinetic energy, eh?

    I think its that and more as I’m still holding out for a more multi-dimensional explanation. Where all matter is the result of interdimensional vertices or confluences… and we live as shadows amongst the ashes first breathed by stars.

  14. Lawrence B. Crowell says

    It really is all just magnetohydrodynmics (MHD). There is nothing involving quantum gravity or strings. Yet plasma physics is very difficult since the equations are very nonlinear. This is particularly the case if the plasma is dense.

    LC

  15. DrFlimmer says

    @ Lawrence B. Crowell

    They are very powerful, and there are images of jets from one galaxy seriously perturbing a neighboring galaxy.

    I guess, you mean something like 3C321.

  16. Lawrence B. Crowell says

    I would not want to be on a planet that gets in the path of one of these close up. It is a sort of astronomical firehose. The 3C321 is clearly a violent event making havoc, with what looks like an almost Sci-Fi vision of a bright death star beam opening. One might speculate that these compress gas and dust and stimulate star formation.

    LC

  17. jimhenson says

    “EM forces can explain the jets in stars and galaxies without requiring dark matter and black holes, in the same way that they are being produced in our fusion reactors. By plasma being pinched by huge electrical currents and forming a plasmoid which then breaks down. The process then repeats as long as there is current and voltage”, says a famous brilliant scientist who I don’t wish to name of course, but am quoting as an authority. “It is possible that relativistic particles stream with the ISM and IGM magnetic fields, but we don’t have evidence to conclusively show that.”

  18. DrFlimmer says

    @ JimHenson

    As usual, I don’t get the point you intend to make. I also would like to know the “authority” you quote – otherwise there is no need to quote, if one cannot check it.
    The problem with your quote is obviously that we still lack fusion reactors (as much as I regret it!). I know that in those experiments are highly energetic particles and events involved, however I have never heard that a fusion experiment on earth produced a jet of relativistic particles.
    The only fusion experiment that works steadily in our neighbourhood so far is the sun, where the plasma is confined by gravity. But the sun does not produce jets.

    So, what’s the point?

  19. Lawrence B. Crowell says

    Black holes obviously play a role. So gravitation is in the physics here. However, it has to be pointed out that one can treat the BH with Newtonian gravity as a pretty good approximation. There is no need for frontier types of physics, such as quantum gravity, quantum black holes or AdS/CFT physics to understand galactic jets.

    LC

  20. jimhenson says

    the points are: 1) that gravity from a black hole isn’t necessary to hold a galaxy together, or to produce a jet and its associated x-rays. 2) this promotes a good discussion and a way to shoot down the messenger who disbelieves that the age of the universe is determined by when the big-bang happened. Clearly, my brilliant authority would not want me of all people to write or tell about him, especially not here where sceptics of PC run me and it down, falsely comparing it to EU. we both believe that plasma cosmology is the best cosmology that we have today!! he states that “alpha particles in the PDF are fast, but they don’t have relativistic speed. sometimes stars produce jets, but not consistently. It depends on the current flow and voltage, which are often varying and/or pulsed. Conditions are always changing. Gravity from a black hole isn’t necessary to hold a galaxy together, or to produce a jet. EM forces over vast distances can potentially slow the inside of a galaxy and speed up the outside, like a dynamo with inductance. Dark matter and black holes are not required. to understand these things, it is important to keep in mind that energy is constantly being transformed. Electric currents produce magnetic fields, which direct electrons and ions. This is true in the lab and in deep space. A galaxy is like an electric motor, with momentum, gravity, and EM fields. when lines of flux try to get bunched up together, there is resistance. when electricity becomes concentrated through strong currents in the interstellar plasma, you can have a pinch. A PINCH can CAUSE A JET. Energy can be concentrated or diffused in many different forms and paths. Nothing is permanent and slight variations can lead to very different results”.

  21. jimhenson says

    @L Crowell authoriy: ALFVEN DISCOVERED AND CREATED THE SCIENCE OF MAGNETOHYDRODYNAMICS MHD. HE also started PLASMA COSMOLOGY ONLY about 30 YEARS AGO!!! NEWTON is about 400 YEARS old teaching, and MAXWELL about 100 years ! give or take some decades or centurys who cares. WISE UP and study EM and Gravity. EM BOTH ATTRACTS AND REPELS and the force is far stronger when the distances increase then is gravity! The ionsphere is the plasma double layer DL around the earth, and the solar system ribbon is a larger scale DL. ALFVEN Says there are 28+ scalable sizes of PLASMA by Density and Temperature !!!!!!!! believe in your big-bang, but 90 percent of that model requires invisible undetected matter!!!! EM requires NO DARK MATTER and NO BLACK HOLES of which no data has been obtained to even prove that they exist! It will take 10 years until the LHC can produce any data on black holes!!! The future is PC and the kids love it !
    All the rest of you jerks all blew away even dr quack fummer shut up !

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