Focus: Neutron Star Weigh-In

Published June 14, 2005  |  Phys. Rev. Focus 15, 21 (2005)  |  DOI: 10.1103/PhysRevFocus.15.21
Figure 1
M. Shibata/Univ. of Tokyo

Death Stars. If the mass of a merging pair of neutron stars is below a threshold, a “double-size” neutron star will form (bottom image) briefly before collapsing into a black hole, according to computer simulations. Gravitational waves from the merged pair should help researchers determine the threshold mass and the properties of the exotic neutron star material. (See videos below.)

Video courtesy of M. Shibata, Univ. of Tokyo.

Swan Songs. Two neutron stars, each with 1.3 times the mass of the sun, merge to form a “double-size” neutron star. Colors indicate (logarithm of) density.

Video courtesy of M. Shibata, Univ. of Tokyo.

Two neutron stars, each with 1.4 times the mass of the sun, merge and immediately form a black hole. In this simulation, this amount of mass is too great to withstand the crush to a black hole because the neutron star material is not stiff enough. Colors indicate (logarithm of) density.

Neutron stars are made of some pretty exotic stuff–pure neutrons packed together as densely as in an atomic nucleus–and researchers don’t understand it very well. There aren’t many ways to study neutron stars, but in the 27 May PRL a physicist proposes a new way to learn about neutron star material. Gravitational waves–ripples in spacetime that may soon be detected–are emitted when a neutron star pair merges into a black hole, a common occurrence in the universe. While other researchers have used computer simulations to analyze the waves emitted just before the merger, the author’s simulations show that the post-merger waves could provide a fairly direct estimate of a neutron star’s stiffness.

As two neutron stars orbit one another, they gradually lose energy, in part by emitting gravitational waves, which allows them to fall closer and closer together. Eventually they merge and produce a black hole. But lighter pairs–with total mass less than two or three times the mass of the sun–form a giant, double-size neutron star for a fraction of a second before transforming into a black hole. No one knows exactly what this “threshold” mass is, below which a giant neutron star forms, but different models of a neutron star’s material properties predict different mass thresholds. The stiffer the material, the better it can withstand gravity’s drive to collapse the object to a black hole–and the higher the mass threshold.

Masaru Shibata of the University of Tokyo proposes using the gravitational waves emitted just after the merger to pin down the mass threshold for the “prompt” formation of a black hole. Computer simulations have shown that during the earlier “inspiraling” phase, the waves produce a characteristic signal that should allow researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) or other observatories to determine the stars’ masses. But Shibata and colleagues recently ran several simulations with different choices for more realistic models of neutron star properties [1]. They showed that if the merger forms a giant neutron star within 150 million light years of Earth, its waves could be detectable after LIGO’s upgrade, scheduled for completion by 2009.

In his latest paper, Shibata lays out the procedure and details of the analysis. Researchers looking at LIGO data could first measure the total mass of an inspiraling neutron star pair. Then they could look for the brief but characteristic signal of the merged neutron star. If they see the approximately 3 kilohertz signal, lasting for perhaps a tenth of a second or less, then the threshold must be at or above the mass of the pair, which therefore sets a lower limit.

According to Shibata, the beauty of this measurement is that you don’t need many detections or terribly high quality data to get very useful information. A single detection would immediately put a lower limit on the threshold mass. With the threshold more precisely known, neutron star theories get a direct test. “We could finally know which model is correct,” says Shibata

Stuart Shapiro of the University of Illinois at Urbana-Champaign believes Shibata’s work is important but worries that the high frequency of the gravitational waves may lie outside of the sensitivity of the LIGO detector, even after the upgrade. However, “determining the correct [theory] is still controversial,” so Shapiro says the proposed test is definitely worth a try.

–Graeme Stemp

Graeme Stemp is a freelance science writer in Haliburton, Canada.


References

  1. M. Shibata, K. Taniguchi, and K. Uryu, “Merger of Binary Neutron Stars with Realistic Equations of State in Full General Relativity,” Phys. Rev. D 71, 084021 (2005).

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