Numerical Simulation of Binary Black Hole and Neutron Star Mergers

**Principal Investigator:**

Dr. Sascha Husa

**Affiliation:**

Universitat de les Illes Balears (Spain)

**Local Project ID:**

pr86va

**HPC Platform used:**

SuperMUC of LRZ

**Date published:**

**A team of about 20 scientists working in Europe, India, South Africa and the USA have been involved in an Astrophysics simulation project calculated on LRZ system SuperMUC. The obtained results will allow the efficient detection and identification of gravitational wave events, e.g. to tell apart black holes from neutron stars.**

Centuries of observing the sky through telescopes have unravelled an exciting and dynamic universe, however our telescopes are blind to some of the most exotic phenomena, like black holes. Einstein's theory of general relativity, which describes gravity in terms of the curvature of space-time, predicts that massive bodies dynamically distort the surrounding space-time. These space-time vibrations travel at the speed of light, and carry with them information about their source. Observing these "gravitational waves" is in many ways analogous to hearing sound, rather than looking at an image from a telescope. An international network of gravitational wave antennae in the form of kilometre-size L-shaped laser interferometers is currently being upgraded, and will start operating again in 2015. The most promising sources for detections are binary systems of dead stars.

After running out of fuel, heavy stars undergo a supernova explosion, leaving behind a neutron star or black hole. Over the course of millions of years such binary systems will shrink due to losing energy by emitting gravitational waves, and the two objects will eventually coalesce in a catastrophic event, in which more than 10 % of their total mass can be converted to gravitational waves within a fraction of a second.

Searches for such events in the detector data are based on similar algorithms as used by smartphone-apps which find the title of a song you hear in a noisy bar, comparing the noisy data to a large catalogue of songs. The "waveform" of a coalescence of black holes encodes their masses and rotation rates. The challenge is to compute a catalogue of these signals - a sound track of the universe - from the Einstein equations, which relate the motion of matter to the curvature of space-time.

Calculating the waves from one binary system typically requires several hundred thousand CPU hours, and hundreds of such data points will be required to eventually completely understand the parameter space parameterized by masses and rotation rates. The specific goal of this project on SuperMUC, using 37 million CPU hours (equivalent to 4,223 CPUs running for one year) was to extend previous results on similar-mass black holes to large mass ratios of about 1:20, as required to connect the numerical results to analytical calculations for the limit of infinite mass ratios in order to obtain a complete description of the parameter space. A team of about 20 scientists working in Europe, India, South Africa and the USA have been involved in performing the simulations and analysing the data. The results will allow the efficient detection and identification of gravitational wave events, e.g. to tell apart black holes from neutron stars.

The project was made possible through the Partnership for Advanced Computing in Europe (PRACE).

**Scientific Contact:**

Dr. Sascha Husa

Universitat de les Illes Balears, Departament de Física

Cra. de Valldemossa, km 7.5.

E-Palma (Illes Balears)/Spain

e-mail: sascha.husa@uib.es