Using Pulsars As A Gravitational Wave Observatory

David Champion

The LIGO detection of gravitational waves has begun a new era in astronomy, and initiatives and projects are being planned across the world to observe as many cosmic events as possible. Now the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has suggested a way to detect the merging of supermassive black holes using existing radio telescopes.

The team believes that we can use pulsars, extremely fast-rotating stars, as a cosmic detector. By keeping an eye on them we should be able to detect the subtle changes due to low-frequency gravitational waves.

"Detecting this signal is possible if we are able to monitor a sufficiently large number of pulsars spread across the sky," Dr. Stephen Taylor, lead author of the paper published this week in The Astrophysical Journal Letters, said in a statement. "The smoking gun will be seeing the same pattern of deviations in all of them."

Gravitational waves are produced by accelerated massive objects; they are ripples through spacetime and they are extremely difficult to observe. When moving through the universe they compress and stretch spacetime, effectively creating a microscopic shift in the position of objects.

NANOGrav suggests the use of pulsars because these stars are excellent clocks. Pulsars form after a star goes supernova. They are incredibly dense neutron stars that spin several hundred times per second. Their spin is extremely regular, and we have the technology to measure them to within a ten-millionth of a second.

This system can be used to look for binary supermassive black holes. These systems are a common stage in galaxy mergers, and the NANOGrav suggestion will help us detect these huge object as they spiral around each other. Binary supermassive black holes hold many unknowns for scientists and they are extremely difficult to detect. Although they weigh millions or even billions of times the mass of the Sun, they are small, and they would cozily fit in the Solar System.

While NANOGrav has potential, to be successful it needs a large network of telescopes looking at many pulsars. "We're like a spider at the center of a web," added Michele Vallisneri, another member of the research group, in the statement. "The more strands we have in our web of pulsars, the more likely we are to sense when a gravitational wave passes by.

"NANOGrav is currently monitoring 54 pulsars, but we can only see some of the southern hemisphere. We will need to work closely with our colleagues in Europe and Australia in order to get the all-sky coverage this search requires."

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