How neutron stars merge - first accurate calculation of the last orbits of binary systems of neutron stars

AEI scientists described for the first time new numerical methods for the calculation of gravitational wave signals.

January 11, 2010

An article describing an important progress in modelling the last orbits of binary systems of neutron stars has been recently published in Physical Review Letters.

Simulations of inspiralling neutron stars and black holes provide insights into the possible structure of gravitational wave signals. They significantly increase the probability of identifying gravitational waves in the detector data. The article focuses on the last few orbits before coalescence of binary neutron star systems and their experimental observation (which is expected in the coming years) by the network of gravitational wave detectors LIGO, GEO600 and Virgo - LIGO is in the United States, GEO600 is the German-British gravitational wave detector in Ruthe near Hannover, operated by the Albert Einstein Institute and Virgo is the French-Italian detector installed in Cascina, near Pisa.

By exploiting the computational power of supercomputers and by performing the most extended and accurate calculations of this process, a team of scientists from Germany, France, the U.S. and Japan was able to improve a simplified but analytical description of the inspiral stage, when the two stars are still separate but their orbit is shrinking because of the large emission of gravitational waves. The refinement of the analytic description via accurate numerical calculations has shown that it can be used with success also to describe objects with finite dimensions, such as neutron stars, extending its validity beyond the much simpler case of binary black holes. “We found out how to introduce corrections in simplified models of binary neutron stars, so that they can reproduce the results of complex numerical simulations”, says Professor Luciano Rezzolla, head of the numerical relativity group at the Albert Einstein Institute/AEI in Potsdam. This breakthrough opens the way to a large- scale modelling of the gravitational signal emitted by the last orbits of a system of two neutron stars with a precision adequate for detection.

An insight into neutron stars

In addition, the work provides important evidence that the detection of this type of signal will allow one to measure the ratio between the mass and radius of neutron stars. This measurement is potentially very important because it will give access to new information on the equation of state of ultra-dense nuclear matter, of which neutron stars are made, and which remains essentially unknown.

The article is the result of a collaboration between a team of numerical and analytical theorists of Relativistic gravity: Luca Baiotti (Osaka Universities, Japan), Thibault Damour (IHÉS, France), Bruno Giacomazzo (NASA Goddard, USA), Alessandro Nagar (IHÉS, France) and Luciano Rezzolla (Albert Einstein Institute, Germany).

Background information

Neutron stars

Neutron stars are the final stage of massive stars that explode as a supernova. In the explosion process, the core of the star collapses to form a compact object with roughly 1.4 solar masses that mostly consists of nuclear matter, predominantly of neutrons.

Cosmic collisions

Collisions of astronomical compact objects such as black holes and neutron stars are among the most powerful events in the universe. Scientists of the numerical relativity group (leader: professor Luciano Rezzolla) at the Max Planck Institute for Gravitational Physics simulate mergers of black holes and neutron stars, calculate the energy released in these events and comparing it with the one obtained from astronomical observations. In addition, because of their compactness and mass, black holes and neutron stars are the most promising sources of gravitational waves.

Gravitational-wave astronomy

Gravitational waves are a direct prediction of Einstein’s theory of general relativity and represent simple ripples in the fabric of space and time which are produced by large and compact masses when moving at very large velocities. Scientists from all over the world anxiously await for the first direct detection of gravitational waves. With gravitational wave astronomy we will have the possibility to know much of the still unknown 96% of the Universe. By detecting them we will effectively “listen” to the Universe, that is, observe it in a range of frequencies which is very different from the at which we routinely obtain astronomical electromagnetic information. Besides dealing with the enormous experimental difficulties related with the detection of the gravitational-wave signals, the search for gravitational waves requires detailed knowledge of the expected signals and cutting-edge methods for managing huge amounts of data.

In the ‘Astrophysical Relativity’ division at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Potsdam-Golm, lead by professor Bernard F. Schutz, both research fields are investigated in internationally leading working groups.

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