Shimmer Me This


Back in September, a fascinating paper appeared on the arXiv about a new way to search for gravitational waves.

It’s called the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and it relies on how pulsars stretch and squeeze as gravitational waves wash over them — basically, they should “shimmer” a tiny bit, and scientists should be able to detect that shimmer. This new way to search for gravitational waves is among the topics being presented at the American Astronomical Society meeting in Washington D.C. this week.

Pulsars are neutron stars that emit radio pulses, thanks to strong magnetic fields that generate beams of radio waves just like the signal from a lighthouse. Those beams sweep through space as the star rotates, and we can detect those pulses when they strike Earth.

Because they boast such clockwork precision, pulsars also offer a new means of detecting gravitational waves: scientists hope to measure tiny changes in the pulsars’ rotation as the waves pass by, which means they need a bunch of millisecond pulsars spread out across the night sky to monitor regularly.

This scheme would certainly be cheaper than the Laser Interferometer Gravitational Wave Observatory (LIGO), for example, which cost hundreds of millions of dollars just to build. NANOGrav comes in at around $66 million over 10 years. That makes budget-conscious funding agencies happy.

The problem is that you need to closely monitor rapidly-spinning millisecond pulsars, which are (a) tough to find (only 150 have been found over nearly three decades since pulsars were first discovered), and (b) not very plentiful in the part of the night sky of interest to scientists (northern hemisphere). They tend to clump together in globular star clusters, which makes them useless for detecting gravitational waves. And then you’ve got to be able to measure and characterize them with incredible accuracy to detect the minute changes that result from passing waves.

There’s good news on the “finding millisecond pulsars” front at this week’s AAS meeting. Scott Ransom of the National Radio Astronomy Observatory (NRAO) says they’ve found 17 new pulsars — in three months, no less — with the help of NASA’s Fermi Gamma-Ray Space Telescope and radio telescopes like the NSF’s Robert C. Byrd Green Bank Telescope. In 2008, the Fermi telescope found hundreds of objects emitting gamma rays throughout the Milky Way, and scientists rushed to confirm them as millisecond pulsars using radio telescopes. Ransom describes the new Fermi data as “a buried treasure map… Fermi showed us where to look” in the night sky for objects likely to be millisecond pulsars.

If scientists keep up this rate of progress, we should be catching gravitational waves with millisecond pulsars in no time. Oh, it’s on, LIGO!

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