Part-time Pulsars: Another Milky Way Mystery

If you’re a Physics Buzz regular, you’ve read about radio pulsars before (most recently here). A pulsar forms when a massive star explodes and its outer layers are blown away. The inside core contracts, resulting in an extremely dense, rapidly rotating neutron star. Pulsars have the strongest known magnetic fields in the universe, and beams of charged particles spew out from their magnetic poles. Like a lighthouse signal sweeping across the water, we detect pulsars by very regular radio pulses sweeping across the Earth. Currently, astronomers have detected about 2500 radio pulsars in our galaxy.

The PALFA survey’s emblem; a pulsar (light blue) is shown emitting beams of electromagnetic
radiation (yellow), overlaid on a sketch of the receivers at Arecibo Observatory.
Image Credit: Slavko Bodganov (CC BY-SA 3.0)

The PALFA collaboration uses the Arecibo Observatory in Puerto Rico to survey the sky for radio pulsars. Earlier this week, they introduced the world to two strange, “part-time” pulsars at the American Astronomical Society meeting* and in a paper published in The Astrophysical Journal. By almost every indication these pulsars are just like every other pulsar discovered in our galaxy, but there is one strange difference. Follow-up observations by radio astronomers at Jodrell Bank Observatory in the United Kingdom, led by Andrew Lyne of the University of Manchester, show that these two pulsars intermittently turn completely off. In fact, they spend most of their time in the off mode.

A part-time pulsar might pulse regularly, exactly as expected, for 20 minutes and then go dark. Weeks or months later it comes back, pulsing just as brightly and regularly as before. “They switch instantaneously between the states,” said Lyne in a statement. “They’re on and then they’re gone, disappearing without any apparent warning.” Only three other part-time pulsars like this have been discovered.

Several years of study show that one of these pulsars is on only 30% of the time. The other was on less than 1% of the time for the first three years it was studied, now it is on 16% of the time. Why the change? We don’t know. Why are they turning on and off in the first place? We don’t know. Are there more of these? It’s highly likely. A pulsar that is off most of the time is really easy to miss when you’re surveying an area of the sky only once.

The existence of these pulsars is strong evidence that whatever mechanism causes a radio pulsar to pulse can be turned on and off. Although we don’t know what this mechanism is, PALFA astronomer Victoria Kaspi from McGill University says that it is likely related to the magnetic structure of the neutron star.

There’s more. Pulsars rotate rapidly, often many times per second, but the rate at which they rotate slows down gradually and predictably over time. Astronomers call this the spin-down rate. Data collected from the two pulsars show that spin-down rate is correlated to radio emission. When a part-time pulsar is on, its spin-down rate is much higher than when it is off. This is surprising, since pulsars emit just a tiny fraction of their overall energy in a pulse. Whether a pulsar is pulsing shouldn’t have such a large impact. Just another fascinating reminder that we have lots to learn about our galaxy!

Kendra Redmond

* If you want to know what’s going on in the universe, the place to be this week is Grapevine, Texas. Located not far from Dallas and Fort Worth, the city is home to wineries, amenities for visitors traveling via DFW airport, GameStop headquarters, and this week, the center of the astronomical universe. Some 2,000 astronomers are assembled in Grapevine for the “Super Bowl of Astronomy,” the annual January meeting of the American Astronomical Society. Over the next few weeks we’ll bring you some of the meeting highlights.

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