Astronomers searching for the first detection of a continuous gravitational wave say they've learned a lot by not finding it. There is little doubt the scientific rewards of detecting the first gravitational hum produced by the spinning of a slightly misshapen neutron star would be greater still, but even apparent failure has seen three papers released today.
It's only six years since the first gravitational wave was detected, caused by a collision between black holes. Finding such events is now almost routine, and was followed by the observation of the merging of two neutron stars, among the most significant scientific discoveries of the century, changing our ideas about the origins of heavy elements.
These, however, are sharp, but powerful events. Continuous gravitational waves, predicted to be produced by the spinning of very dense non-spherical objects like neutron stars, stretch their energy out over much longer periods of time. Dr Karl Wette of the Australian National University (ANU) compared the waves caused by collisions we have observed to “squawking cockatoos – loud and boisterous, they're pretty easy to spot! A continuous gravitational wave, however, is like the faint, constant buzz of a faraway bee,” in a statement. Others have described the hunt as “Like trying to capture the squeak of a mouse in the middle of a stampeding herd of elephants.”
Nevertheless, Professor Susan Scott of the ANU Research School of Physics told IFLScience, if we do detect continuous gravitational waves their size and shape would tell us a lot about the nature of neutron stars, such as how rigid matter is under the most extreme conditions outside black holes.
When neutron stars form from the remnants of supernovas, asymmetries in the explosion are thought to leave them slightly elliptical, a few millimeters longer in one direction than another, over a diameter of 10-15 kilometers. However, neutron stars' incomprehensible density (they pack more than a solar mass into that size), and the swift speed with which they spin, mean these imperfections cause them to shed energy.
“We can estimate the amount of energy a neutron star loses,” Scott told IFLScience. “If that is all in the form of gravitational waves we can calculate its amplitude at Earth.” Our detectors are now so sensitive we should be able to register the upper end of these estimates, she added.
The more we fail to find these waves, the more possibilities for neutron stars' nature we can eliminate, however. Gravitational-wave detection (or not) is very big science. Scott is one of more than a thousand authors of a paper in the Astrophysical Journal describing observations targeting 15 “young” neutron stars, hundreds or thousands of years old. “No evidence of [continuous waves] is identified,” the paper notes. This implies neutron stars are either more spherical than some models propose, or they lose much of their energy in other ways, such as the X-rays and radio waves released by pulsars.
An accompanying paper describes searching for gravitational waves from neutron stars in binary systems. Neutron stars' immense gravitational fields often allow them to draw gas off companion stars. This may create temporary unevenness as the new material settles.
A third paper narrows the search down to a single star PSR J0537−6910 which, the authors note, “Has the largest spin-down luminosity of any pulsar and exhibits frequent and strong glitches.” These features make it a particularly strong candidate for detecting continuous waves, motivating astronomers worldwide to pay it special attention. Nevertheless, nothing has been found, suggesting gravitational waves are carrying away less than 14 percent of the energy PSR J0537−6910 is losing as it rapidly slows down.