“Lighthouse” Analogy No Longer Works for Pulsars

Article written: 6 Jan , 2009
Updated: 24 Dec , 2015
by

NASA’s Fermi Gamma-ray Space Telescope has found 12 previously unknown gamma-ray only pulsars, as well as identifying gamma-ray emissions from 18 known or suspected radio pulsars. And what the telescope is finding is changing the way we think of these stellar cinders. The old analogy for pulsars was a lighthouse: gamma-rays were thought to pulse out in a narrow beam from the neutron star’s magnetic poles. But this new research shows that cannot be the case. A new class of gamma-ray-only pulsars shows that the gamma rays must form in a broader region than the lighthouse-like radio beam. “We used to think the gamma rays emerged near the neutron star’s surface from the polar cap, where the radio beams form,” says Alice Harding of NASA’s Goddard Space Flight Center. “The new gamma-ray-only pulsars put that idea to rest.” She and Roger Romani from Stanford University in California spoke today at the American Astronomical Society meeting.

A pulsar is a rapidly spinning and highly magnetized neutron star, the crushed core left behind when a massive sun explodes. Most were found through their pulses at radio wavelengths, and were thought to be caused by narrow, lighthouse-like beams emanating from the star’s magnetic poles.

If the magnetic poles and the star’s spin axis don’t align exactly, the spinning pulsar sweeps the beams across the sky. Radio telescopes on Earth detect a signal if one of those beams happens to swing our way. Unfortunately, any census of pulsars is automatically biased because we only see those whose beams sweep past Earth.

“That has colored our understanding of neutron stars for 40 years,” Romani says. The radio beams are easy to detect, but they represent only a few parts per million of a pulsar’s total power. Its gamma rays, on the other hand, account for 10 percent or more. “For the first time, Fermi is giving us an independent look at what heavy stars do,” he adds.

Watch an animation of the new look at these pulsars.

Pulsars are phenomenal cosmic dynamos. Through processes not fully understood, a pulsar’s intense electric and magnetic fields and rapid spin accelerate particles to speeds near that of light. Gamma rays let astronomers glimpse the particle accelerator’s heart.

Astronomers now believe the pulsed gamma rays arise far above the neutron star. Particles produce gamma rays as they accelerate along arcs of open magnetic field. For the Vela pulsar, the brightest persistent gamma-ray source in the sky, the emission region is thought to lie about 300 miles from the star, which is only 20 miles across.

Existing models place the gamma-ray emission along the boundary between open and closed magnetic field lines. One version starts at high altitudes; the other implies emission from the star’s surface all the way out. “So far, Fermi observations to date cannot distinguish which of these models is correct,” Harding says.

Because rotation powers their emissions, isolated pulsars slow as they age. The 10,000-year-old CTA 1 pulsar, which the Fermi team announced in October, slows by about a second every 87,000 years.

Fermi also picked up pulsed gamma rays from seven millisecond pulsars, so called because they spin between 100 and 1,000 times a second. Far older than pulsars like Vela and CTA 1, these seemingly paradoxical objects get to break the rules by residing in binary systems containing a normal star. Stellar matter accreted from the companion can spin up the pulsar until its surface moves at an appreciable fraction of light speed.

“We know of 1,800 pulsars, but until Fermi we saw only little wisps of energy from all but a handful of them,” said Romani. “Now, for dozens of pulsars, we’re seeing the actual power of these machines.”

Source: NASA


12 Responses

  1. Excellent article Nancy.

    Hopefully no one calls you a crackpot or crank for challenging mainstream mathematical views which are being challenged by discordant observations.

    And hopefully you won’t have to move to Germany like Halton Arp did.

  2. Clayton says

    well, the “Lighthouse” analogy is *still better* than the notorious “Outhouse” analogy, and still better yet than the “Little House on the Prairie” analogy.

    I am standing by the “Fall of the House of Usher” analogy which one day I think will be quite mainstream and accepted by all.

    I’m just sayin’.

  3. Astrofiend says

    Well, the good old lighthouse model was always going to be a bit oversimplified. Nice to see that some added observational firepower has led us a few steps closer to a more nuanced understanding of these fascinating objects…

    Still, I’d say the lighthouse model will be taught and demonstrated to undergrads for years to come yet!

  4. Feenixx says

    Pulsars have always fascinated me. They seem to be “alive”, in a way.
    I really like the animation, for sheer visual impact, even if it meant nothing else. It’s the beauty of Nature…. much like a time lapse movie of cells dividing, an embryo growing, or a flower opening and closing, revealing graceful motion of the intriguing hidden intricacies in Nature – but on a huge cosmic scale.
    Thanks a lot for your heads-up on the conference!

  5. The assumption has been that the “beam” from a lighthouse is a straight line. But some very simple back of the envelope calculations will show that the rotation of such a beam, at astronomical distances, becomes by far the dominant feature. In that case it is surely possible to theorise that the beam will bend at its tip in the same way a whip trails behind the wrist action of the user? That the once straight beam becomes spread out behind the logical line of observation.

    Secondly, if that is the case, may it also be the case that the rotational “sideways” movement, (energy), of the beam becomes the dominant feature of your observations?

    Thirdly, turning to the actual object. Once simply a star, perhaps it also once had orbiting planets with Iron cores, and, that the magnetic forces during the formation of the pulsar, held the iron cores in orbit around the pulsar? In that case, those orbiting iron cores will inevitably have an interaction with the rotating magnetic field of the pulsar. And, in that case there may very well be another explanation for the rotating beam emanating from a pulsar….

    Something for you all to think about.

  6. trux says

    Planets do not seem to be a likely cause – they would have to be extremely close to the star and orbitting with speed close to the speed of light. Actually, the planets would have to be several magnitudes closer to the planet than their own diameter, which is impossible. At bigger distances they would already need to orbit with speeds bigger than the speed of light.

    The only possibility would be a perfectly regular chain of planets on the same orbit (a uniformly perforated ring), where the holes in the ring would cause the pulsing, but that’s of course pure nonsense for many reasons.

  7. Ignoramus says

    Any reason why the “equator” in the picture above is the dominant location for Pulsars?

  8. jerry says

    The ‘simple lighthouse model’ assumed a physical rotating energy source. What the new constraint requires is a pulse source that migrates in more than one plane, something you can’t do with a solid body.

    This is not so much the ‘mainstream’ being wrong, as new information ruling out a simple model.

    As far as identifying the source… we still lack a full understanding of what is happening, and whether or not new physics is required to explain it.

  9. Peter says

    To our ignoramous…
    Who isn’t one, if you are interested in the facts behind interesting articles and pics…
    The “horizon” is the galactic disc, as seen from here on earth. Naturally, since we are talking stars and not vast systems like galaxies and clusters, we have to be concerned with those within our own. Others in other galaxies are just too far away unless they happen to go supernova. So since most stars in the Milky Way are toward the centre and either side thereof, that’s where we find our pulsars.
    Hope that makes sense! Keep questing!

  10. Trux,

    You are assuming the planets stay in their normal orbits. What I am assuming is that the magnetic forces “capture” the iron core/s and attract and then entrain them in the magnetic field of the rotating star. In that case the core will increase its angular momentum and orbital velocity as it is attracted towards the star. If the magnetic field is strong enough, then the iron core may very well be at an orbit at the surface of the star.. The core in turn may well not retain its spherical shape, but, because this, perhaps, deformed iron mass, is orbiting around the star at its equator and at or near or even beneath the surface, it cannot become an integral constituent of the surface of the star simply because of the difference in surface speeds.

    We may be observing a homogeneous mass magnetically influencing the surface of the star, but orbiting at or near the surface at speeds encompassing the effects we are observing. Multiple cores explaining the sometimes anomalous observations of cyclic variations in or observations.

  11. Sakib says

    @ ignoramus: Pulsars are produced from the death of massive stars. Massive stars only last for a short time. These stars are born in huge clouds of gas and dust, most of these clouds are situated in the inner parts of the Milky Way, most are between 1000 and 30 000 light years from us. The Milky Way is more or less 100 000 light years wide.

  12. Ignoramus says

    If most of the Pulsars that we can see originate in the galaxy then indeed we will see them as mostly located around the “equator”
    Thanks Sakib and Peter

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