from pole to pole… to pole?

Scientistjenluc_6There’s been quite a bit of buzz in the blogosphere about Sunday’s exciting news: a 3D image "mapping" the dark matter. (Links to some excellent explanations with even more links can be found here, here, here, and here.) Sadly, I missed the press conference on the subject due to prior commitments, but I did finally make it down to the Washington Convention Center today to take in a few presentations at the American Astronomical Society meeting. In addition to the enormous stack of press releases awaiting my perusal, I received a spiffy Press badge that attaches via a magnetic strip. So much better than the usual safety pin variety (which can ruin a fine silk blouse), or those awkward shoe-lace type things one hangs around one’s neck — definitely not designed for a woman’s anatomy, shall we say.

Chalk up yet another innovative application of the humble magnet: meeting badges. What will they think of next? If "they" means astronomers and researchers working at the Arecibo Observatory in Puerto Rico, they’ve thought of something far more ingenious to account for some unusual observations in the radio emission signatures produced by a pulsar in the famous Crab Nebula, located in the constellation Taurus some 6300 light years away. Here’s the sound bite version of the story, with more details further down:

The researchers expected the radio emission spectra to be identical for both the main pulse and the interpulse, since each is associated with a magnetic pole (north and south). Prior theoretical models of pulsars predicted this would be the case, but the experimental data showed otherwise. The two signatures were wildly different, and among the possible explanations is the existence of a third "magnetic pole" located elsewhere in the star. It’s a classic case of the left hand not knowing what the right hand is doing — just substitute "north pole" and "south pole" for right and left. And according to theorist Jean Eilek of New Mexico Tech, "It knocks  just about every existing theory of pulsar radio emission for a loop." This makes the Crab Nebula (henceforth dubbed "Crabby") very special indeed, since other pulsars fall pretty much in line with existing theoretical models.

The story of the Crab Nebula goes back 1000 years. Or, if you want to be more poetical about it: A long time ago, in a galaxy far, far away, a massive star died a violent, explosive death, and then collapsed into a spinning neutron star. In 1054 AD, the light from that supernova explosion (Supernova 1054) finally reached Earth, shining brightly enough to be seen in daylight for 23 days, and remaining visible in the night sky for a whopping 653 days. It was such a rare event that there is mention of it in the records kept by Japanese, Arab, and Chinese astronomers, as well as circumstantial evidence indicating that the Anasazi also observed the supernova. The Crab Nebula is the cloudy remnant from that spectacular death — a gassy sort of memorial to a once-magnificent star.

Thus was Crabby born. It took awhile for him to show his face to astronomers, however. Fast forward some 700 years to 1731, when an English physician and amateur astronomer named John Bevis recorded his observation of  Crabby’s pretty blue glow, which derives from the whirling electrons
whirling at the speed of light around the star’s magnetic field lines. He included it is his own star atlas, Uranographia Britannica, which was published posthumously in 1786, even though he completed it in 1750. (The publisher went bankrupt. Bummer.) Twenty-seven years later, Charles Messier would independently "rediscover" the Crab Nebula.)Bevis_m1

But there was more to Crabby than simply met the eye: he turned out to have a pulsar as a "heart." Fast forward another 250 years or so, and a young female graduate student working at the Mullard Radio Astronomy Observatory near Cambridge. In the summer of 1967, Jocelyn Bell was manning the new telescope she’d helped her advisor, Anthony Hewish, cobble together, and analyzing the resulting data (some 100 feet of paper every day). She soon noticed an anomaly in the data: a "bit of scruff" that turned out to be a regular signal, consistently coming from the same patch of sky. Could it be the long-awaited signal from an extraterrestrial civilization? Bell and Hewish mischievously dubbed the source "LGM-1" for "Little Green Men," although it turned out to be a pulsar — an exciting discovery that earned Hewish a Nobel Prize. Bell was famously overlooked for the honor, even though she was the one who technically "discovered" the original anomaly in the data.

In essence, a pulsar is a rotating neutron star.  Just like a lighthouse, it emits twin beams of radiation that appear to pulse 30 times per second, producing a main pulse and an interpulse. That rotational energy is converted into electromagnetic energy, giving rise to the short, powerful burst of radio emissions observed by astronomers. However, no one is quite sure what the actual physical mechanism is that causes the energy conversion in the first place, hence, the continued interest of astronomers in studying these funny little objects. The current thinking is that it results from a collapsing soliton, and that collapse abruptly converts energy into electromagnetic radiation. What causes the collapse is still a bit of a mystery.

Pulsars generate simply enormous magnetic and electric fields, thanks to plasma clouds (patches of electron/positron gas) in its atmosphere, from whence the emission blasts emerge. Eilek’s colleague, Tim Hankins, estimates the plasma clouds to be smaller than a soccer ball and slightly larger than baseball, and the blasts of radiation — which can be as powerful as 10% the total power of the sun — occur over very short time frames: on the order of four-tenths of a nanosecond. In the most recent work, Hankins and his colleagues took the average
profiles of Crabby’s main pulses and interpulses, across a wide range in the electromagnetic spectrum (radio
to infrared all the way up to X-ray emissions). Instead of the two being identical, the main pulse shows no sign of the strange frequency structure in the main pulses, while those very short powerful blasts never show up in the interpulse signature. Even more unusual is that the band "spacings" found in the interpulse spectrum aren’t equal, but seem to change in proportion to the frequency.

That’s just plain weird, according to Hankins, because it completely rules out the previously predicted train of equally spaced bursts of power in time. Even Eilek admits to being "totally perplexed" by the unexpected results. She’s now mulling the possibility of attributing the unusual behavior to a kind of resonant cyclotron emission, similar to the "zebra bands" observed in the spectra of solar flares. Hankins isn’t entirely satisfied with that alternative, and has proposed his own explanation: we’re seeing interference fringes. That interference might be coming from a heretofore unsuspected third magnetic pole that is disturbing the magnetic fields normally generated by north and south poles. I guess we could call it the "east pole." That third pole might even have a partner, bringing the total to four. (As I.I. Rabi said when the muon was first observed, "Who ordered that?!?")

It’s clear from these results that, as far as Crabby’s
pulsar is concerned, the old bipole model just doesn’t cut it. Adapting existing models to incorporate additional poles is, to put it mildly, quite the challenge. Many of us performed the basic magnetic field experiment using iron filings on a white piece of paper; it enables you to "see" the magnetic field lines generated by the north and south poles of the magnet in the pretty patterns the filings make.Magnet250
Add two more poles, however, and the picture becomes much more complicated — so complicated that it becomes impossible to accurately predict or map the magnetic field structure.

This, my friends, is real science in action. Scientists encounter an unexpected experimental result that doesn’t match the theoretical models that have worked perfectly well to date. Rather than going into denial and clinging to the past, like whacked out Young Earth Creationists, they embrace the unknown, and start casting about for alternative models that might account for the new data.

They might not get the chance to solve the mystery if federal funding woes continue. Last November the National Science Foundation made public a report recommending decreased funding for the Arecibo Observatory. Unless other sources of funding are found, the facility will be shut down in 2011, while the radio astronomy program could be canceled as soon as this September. That would be a sad fate for a highly recognizable facility: it was used as a filming location for the 007 classic, Goldeneye, in at least one episode of The X-Files ("Little Green Men"), in the film Contact, and in the opening scenes of Species.

And it’s not the only facility in trouble, as this article in The New York Times makes clear. After narrowly averting disaster last year, Brookhaven National Laboratory is once again facing closure because of funding red tape in Congress. See Gordon Watts’ take here, and a copy of the letter circulated by Fermmilab’s director, here. The situation is so dire that the American Physical Society has issued a call to action for its members to write their Congressional representatives urging a reversal of the funding declines. Check it out, and write your own Congressman to help save the future of science. Otherwise we might one day no longer be able to take beautiful pictures like this one, featuring Crabby in all his glowing blue glory:

Crabnebula_2

9 thoughts on “from pole to pole… to pole?”

  1. PS – I notice you’ve gone to the darkside on the sidebar on the left. Must have good eyesight to read that faint (almost invisible) red print.
    Well, at least I can make out the headings in ‘bold’ – lol! laters …

  2. Nice picture of the Crab nebula!
    “In essence, a pulsar is a rotating neutron star. Just like a lighthouse, it emits twin beams of radiation that appear to pulse 30 times per second, producing a main pulse and an interpulse. That rotational energy is converted into electromagnetic energy, giving rise to the short, powerful burst of radio emissions observed by astronomers. However, no one is quite sure what the actual physical mechanism is that causes the energy conversion in the first place, hence, the continued interest of astronomers in studying these funny little objects. The current thinking is that it results from a collapsing soliton, and that collapse abruptly converts energy into electromagnetic radiation. What causes the collapse is still a bit of a mystery.”
    I did a course in this a decade ago but pulsar speculations didn’t interest me, so I can’t recall anything and don’t have the lecture notes handy. But my thinking is that conservation of energy is key. Are you sure that the slowing down of spin (ie the loss of rotational kinetic energy of the pulsar) is exactly correlates to the radiation of electromagnetic waves?
    Why isn’t it simply a matter of material (gas, dust, whatever) being attracted into the pulsar, converted into energy, and the energy being radiated away in a beam as it rotates?
    Think about the Earth! Electrically charged particles stream in from the sun, get trapped in the van Allen belts at 500-40,000 miles up (highest over the magnetic equator) and gradually trickle into the atmosphere at the magnetic poles where the magnetic field lines dip into the air and go to the poles on the ground (or below ground).
    If the rotating neutron star is like the Earth, with a magnetic field, but with the magnetic poles located well off the axis of spin (unlike the Earth where the two axes are close to one another), then electrically charged radiation (ionized matter) streaming into the rotating neutron star will drain into the magnetic poles, which will heat up, emitting a beam of radio waves.
    As it rotates, the beam will sweep around the sky like a lighthouse, as you explain, because it will emerge from the magnetic poles which don’t coincide with the poles of rotation. I can’t see any mystery!

  3. Lablem, the Crab Nebula is in a galaxy (our galaxy) and it is far far away, 6300 light-years. ” A galaxy far, far away” is a nice turn of phrase.

  4. (I’ve never commented before, but I had to share)I loved the magnetic badges too! I heard complaints, but I was tired of maming holes in my shirts. This was the first time I was actually wearing a suit to the AAS meeting and would have gotten along fine with the lapel clip, but I prefered the magnets.
    I am impressed that you are onto to NSF senior review stuff. It’s the first mention of it that I have seen in the science blogsphere. The astro-community seems to be out of touch about its effects on the field as a whole. You even reported on the findings about Arecibo better than some of the main stream press who were under the impression that the NSF was closing it down.
    As an aside, will you be at NSF when Sean gives a talk?

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