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Science And Nature

Unusual ‘revived’ pulsars may be the ultimate gravitational wave detector

neutron star shooting beams into space

An artist’s depiction of a pulsar.(Image credit: NASA’s Goddard Space Flight Center)

Paul M. Sutter (opens in new tab)can be an astrophysicist at SUNY (opens in new tab) Stony Brook and the Flatiron Institute, host of “Ask a Spaceman (opens in new tab) and “Space Radio (opens in new tab),” and writer of “How t (opens in new tab)o Die in Space.”

Astronomers desire to use pulsars scattered round the galaxy as a huge gravitational wave detector. But why do we are in need of them, and just how do they work?

Gravitational waves, or ripples in the fabric of space-time, from a variety of sources constantly slosh through the entire universe. At this time, you’re being slightly stretched and squeezed as wave after wave passes through you. Those waves result from merging black holes, the explosions of giant stars and also the initial moments of the Big Bang.

On Earth, we’ve developed incredibly sensitive gravitational wave detectors which have been in a position to sense brief-but-loud events, such as for example black hole mergers, which last just a few seconds but generate such enormous signals that people can detect them. (“Enormous is really a relative term here; the distortion caused by the passing wave is significantly less than the width of an atomic nucleus.)

Related: The initial telescope of its kind will search for resources of gravitational waves

But ground-based detectors have a much harder time finding low-frequency gravitational waves, since those take weeks, months as well as years to feed Earth. Those forms of low-frequency waves result from mergers of giant black holes, which have a lot longer to merge than their smaller cousins do. Our detectors simply don’t possess the sensitivity to measure those small differences over such very long time spans. For that, we are in need of a much, much bigger detector.

So, rather than using instruments on the floor, we are able to use distant pulsars to greatly help us measure gravitational waves. This is actually the idea behind so-called pulsar timing arrays.

Powering up the pulsars

Pulsars already are fantastic objects, and that is particularly true for the forms of pulsars used as gravitational wave detectors.

Pulsars will be the leftover cores of giant stars and so are being among the most exotic objects ever recognized to inhabit the cosmos. They’re ultradense balls made almost purely of neutrons, with some electrons and protons thrown set for good measure. Those spinning charges switch on incredibly strong magnetic fields in some instances, probably the most powerful magnetic fields in the universe.

Those intense magnetic fields also make strong electric fields. Together, they power beams of radiation (if you are getting Death Star vibes here, you are not remote) that blast right out of the magnetic poles in each direction. Those magnetic poles don’t always fall into line with the rotational axis of the pulsar, in quite similar way Earth’s North and South magnetic poles don’t fall into line with this planet’s rotational axis.

This forces the beams of radiation to sweep out circles in the sky. When those beams cross Earth, we see them as periodic flashes of radio emission, putting the “pulse” in “pulsar.”

Related: Gravitational waves play with fast spinning stars, study suggests

Pulsars are incredibly regular. They’re so heavy, and spin so quickly, that people may use their flashes as extremely precise clocks. But most pulsars are vunerable to random starquakes (once the star’s contents shift around, disturbing the pulsar’s rotation), glitches and slowdowns that change their regularity. Which means most pulsars aren’t best for studying gravitational waves.

So instead, timing arrays depend on a subset of pulsars referred to as millisecond pulsars, which, because the name suggests, have rotational periods of several milliseconds. Astronomers think millisecond pulsars are “revived” pulsars, spun around incredible speeds after infalling material from the companion star accelerates them such as a grown-up pushing a youngster on a schoolyard merry-go-round.

Because of the ludicrous speed, millisecond pulsars can maintain fantastic precision over lengthy timescales. For instance, one pulsar, PSR B1937+21, includes a rotational amount of 1.5578064688197945 +/- 0.0000000000000004 seconds. That is the same degree of precision as our best atomic clocks.

And the ones millisecond pulsars are perfect gravitational wave detectors.

Timing the array

Here’s how it operates. First, astronomers take notice of the rotational periods of as much millisecond pulsars as you possibly can. In case a gravitational wave passes over Earth, over a pulsar as well as between us, then since it passes, it’ll change the length between Earth and the pulsar. Because the wave moves, the pulsar can look slightly closer, then slightly farther, then slightly closer, and so forth before wave has shifted.

That change in distance can look to us as changes in the rotational period. One flash from the pulsar may arrive a touch too soon; then another may arrive a touch too late. For an average gravitational wave, the shift in the timings is incredibly tiny a big change of just 10 or 20 nanoseconds every couple of months. However the measurements of the millisecond pulsars are sensitive enough that those changes could be detected at the very least in principle.

The “array” section of “pulsar timing array” originates from studying many pulsars simultaneously and searching for correlated movements: In case a gravitational wave passes over one region of space, then all of the timings from the pulsars for the reason that direction will shift together.

Many collaborations around the world purchased radio telescopes to review pulsar timing arrays for many years. Up to now, they’ve had limited success, finding shifts in timings from various pulsars but no hints of correlations. But each year, the techniques progress, and the hope is that soon, these arrays will unlock an enormous section of the gravitational wave universe.

Find out more by hearing the “Ask a Spaceman” podcast, oniTunes (opens in new tab)and askaspaceman.com. Ask your personal question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

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Paul Sutter

Paul M. Sutter can be an astrophysicist at SUNY Stony Brook and the Flatiron Institute in NEW YORK. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent 3 years at the Paris Institute of Astrophysics, accompanied by a study fellowship in Trieste, Italy, His research targets many diverse topics, from the emptiest parts of the universe to the initial moments of the Big Bang to the search for the initial stars. Being an “Agent to the Stars,” Paul has passionately engaged the general public in science outreach for quite some time. He could be the host of the favorite “Ask a Spaceman!” podcast, writer of “YOUR HOUSE in the Universe” and “How exactly to Die in Space” and he frequently appears on TV including on THE ELEMENTS Channel, that he serves as Official Space Specialist.

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