Relativistic Effects Play Major Role In Neutron Star Mergers

"General relativity predicts that a pair of neutron stars orbiting one another will radiate energy in the form of gravitational waves," said Alan Calder, a researcher at the National Center for Supercomputing Applications (NCSA) at the U. of I. "This loss of energy will cause the stars to move closer and closer together, until they eventually collide."

The gravitational waves produced in such events are expected to be observed by highly specialized detectors -- such as the Laser Interferometric Gravitational-Wave Observatory -- that are being built. Because gravitational waves are extremely weak, however, theoretical templates of the anticipated waveforms will be necessary to extract the signal from the noisy background.

"A neutron star is the small but dense stellar core that remains after a supernova explosion," Calder said. "By using computer simulations and scientific visualization to study the merger of two neutron stars, we can make predictions in anticipation of detectors coming on line to actually measure the waveforms."

To run the simulation, Calder and his colleagues -- Douglas Swesty, a visiting research professor at the U. of I. and a professor of physics and astronomy at the State University of New York at Stony Brook; Edward Wang, a graduate student in the department of physics and astronomy at SUNY-Stony Brook; and NCSA visualization expert David Bock -- used the SGI/Cray Origin2000 supercomputer at the National Computational Science Alliance.

The researchers initially ran the simulation with only Newtonian hydrodynamics; then they added a post-Newtonian "correction" in the form of a relativistic radiation reaction.

"The radiation reaction dramatically altered the dynamics of the merger," said Calder, who demonstrated the simulation at the American Astronomical Society meeting in Chicago on June 3. "The reaction caused the stars to coalesce much faster, and led to very different gravitational waveforms. We also found that the final coalesced objects differed both in structure and in total angular momentum."

One particularly striking feature seen in the simulation is the formation of tidal arms during the merger that transport a substantial amount of material into a rapidly rotating disk surrounding the merger. Most of the energy that is being radiated in the form of gravitational waves comes from these tidally distorted regions, not from the most massive or most dense parts of the stars.

Although post-Newtonian methods "are extremely useful for predicting gravitational waveforms during the early stages of the inspiral," Calder said, "predicting the later stages -- where tidal effects and neutron star structure become significant -- will require a fully relativistic simulation."