Gravitational waves—the ghostly ripples in spacetime first predicted by Einstein and finally detected a century later by advanced observatories—have sparked a revolution in astrophysics, revealing the otherwise-hidden details of merging black holes and neutron stars. Now, scientists have used these waves to open another new window on the universe, providing new constraints on neutron stars' exact shapes. The result will aid researchers in their ongoing quest to understand the inner workings of these exotic objects.
So far, 11 gravitational-wave events have been detected by the LIGO (Laser Interferometer Gravitational-Wave Observatory) interferometers in Washington and Louisiana and the Virgo gravitational-wave observatory in Italy. Of these events, 10 came from mergers of binary black holes, and one from the merger of two neutron stars. In all cases, the form of the waves matched the predictions of Einstein's theory of general relativity.
For the binary black hole events, the passing waves lasted less than a second; for the merging neutron stars, the emissions occurred for about 100 seconds. But such rapid pulses aren't the only types of gravitational waves that could be streaming through the universe. In particular, solitary neutron stars might be emitting detectable gravitational waves as they spin—signals that could reveal important new details of the stars' topography and internal composition.
Neutron stars are stellar corpses, cinders left behind when giant stars explode as supernovae. The force of such explosions compresses the expiring star's core, transforming it into an ultradense ball of neutrons slightly more massive then our sun but squeezed into a diameter of merely 20 kilometers or so.
Both theoretical calculations and observational evidence suggest the physical extremes of neutron stars' origins make them perhaps the most ideally spherical macroscale objects in existence, exhibiting only the most minuscule deviations from perfection. Yet even those small imperfections can have major consequences—a slight “mountain,” just a few centimeters high on the surface, could form because of cracks in the star's crystalline crust or from the accretion of material at the star's magnetic poles, which are not necessarily the same as its axis of rotation. In such cases, the neutron star's deviation from sphericity would depend on its “equation of state”—a metric for the relation among the star's radius, mass and superstrong magnetic field.
But “one really expects any significant gravitational [waves] to come from a large-scale, [asymmetric] deformation of the star, not a small bump or mountain,” says Nathan Johnson-McDaniel of the University of Cambridge, a member of the LIGO Scientific Collaboration who studies neutron stars. Some slight asymmetry is also expected simply because of a neutron star's spin, akin to the Earth's equatorial bulge from the centrifugal force pushing away from the axis. (Some neutron stars spin so fast that a point on their surface could be moving at a tenth the speed of light.)
However it might arise, any wobble would constantly create gravitational waves, which physicists call “continuous gravitational waves.” They would register as about 100 times smaller than the gravitational waves seen so far from mergers, which themselves are miniscule deviations from flat spacetime of about one part in a billion trillion (10[SUPERSCRIPT -21]). In LIGO's second observing run, lasting from November 2016 to August 2017, the interferometers searched the entire sky for continuous gravitational waves. Next, LIGO team members sifted all signals from about four months' worth of detector time, filtering out false positives representing noise from terrestrial hiccups such as tiny earthquakes or even passing trucks. The data analysis took more than 30 million CPU-hours—more than three millennia in total. The results of this exhaustive search appeared in an online preprint in March.
Using improved data-sifting algorithms, the study's authors (who number more than 1,000) placed new, independent upper limits on neutron star ellipticity—a measure of how far the star deviates from a perfect sphere. The gist of their analysis is that no neutron stars within about 30,000 light-years of Earth appear to harbor deviations from perfect sphericity greater than about one part per million—any mountains that exist on these exotic objects must be exceedingly minute molehills, indeed.
But Nils Andersson, an applied mathematician at the University of Southampton who is not part of the LIGO collaboration, says the study needs to be put in context. Studies of known pulsars—fast-spinning neutron stars that emit lighthouselike beams of radiation—have already placed limits on ellipticity about 1,000 times better than those from this study, he says. Even so, “there may be a population of neutron stars that do not [detectably] radiate electromagnetically”—they would be too dim, perhaps because they have weak magnetic fields. The emission of continuous gravitational waves from such hypothetical stars, Andersson says, may be the only way astronomers will ever see them. “This is a difficult question, involving how strain develops in the star's elastic crust and how the internal magnetic field evolves …. We don't know how to address it.”
Even though this latest search failed to find any continuous gravitational waves, the knowledge gained from sifting through the second observation run's data could reduce the amount of very expensive computer time required for similar searches in future data sets. The improvement in sensitivity could then allow successful peeks into a largely unexamined territory. “I'm excited about this and other promising all-sky searches for gravitational waves,” says Charles Horowitz of Indiana University Bloomington, who is not a member of the LIGO or Virgo collaborations. “The gravitational-wave sky is largely unknown, and it could contain real surprises that might be extraordinary discoveries.”
Advanced LIGO's40 percent boost to instrumental sensitivity, allowing for searches for binary black hole mergers out to about 300 million light-years.third observing period began in April, and will be augmented again by an improved Virgo. This latest observing run will continue for one year, all with a
Besides the expectation of more binary mergers, the improvement in sensitivity and longer run time will also allow a deeper search for continuous waves. “I'm optimistic that continuous gravitational waves from rotating neutron stars will eventually be found,” Horowitz says, “and this will provide important information on both.”