Friday, January 25, 2013

Pulsing pulsars

How do pulsars switch behavior?

The best model we have for pulsars is that they are collapsed stars with intense magnetic fields, spinning rapidly. The magnetic poles don't have to be aligned with the axis of rotation. The magnetic fields shape the direction of the ejected plasma: it tends to shoot out at the magnetic poles. Sort of the reverse of the way the Earth's field channels low energy particles in to make auroras at the poles.

So this tiny remnant has the equivalent of two flashlights beaming into space, and if you are in more or less the direction one of them points, you'll see something blinking at you. The star keeps much of its angular momentum when it collapses, so the rotation speed can be dramatic: as much as many times per second.

The pulsars can slow down: spinning that fast, according to General Relativity, should produce gravitational waves, which carry away energy and angular momentum. And the rate of slowing seems to match the theory.

But the pulsar described in the article is weird. It regularly switches the kind of pulsing it does, changing the ratio of radio waves to X-rays, and then changing back.

Nobody has a good model for this yet. They suspect that the magnetic field is reconfiguring itself in a periodic way, but exactly how isn't obvious. The magnetic field associated with the pulsar has roughly the same order of magnitude energy as the original star's magnetic field, but compressed into a very much smaller volume, so the field intensities are astronomical (so to speak). Is something happening on the star's surface that moves the magnetic pole back and forth? Is the magnetic field interacting with something else out there? (It would have to be pretty close.) Is the field pinning inside the star changing with movements of material inside the star? (That would be pretty dramatic: pulsars are presumed to be neutron stars, and convection of "neutronium" would have to involve truly gargantuan forces.)

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