A Search for Gravitational Waves from Gamma-Ray Bursts
According to Einstein's theory of General Relativity, gravitational waves are ripples in the fabric of space - a stretching and squeezing of space itself. They are caused by some of the most violent phenomena in the universe, such as the supernova explosions that blast apart dying stars, and the collisions of neutron stars and black holes. Although they were first predicted by Einstein almost a century ago, gravitational waves have not yet been directly observed. In the past decade, a dedicated network of observatories has been constructed to detect them. This network includes the Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States, the British-German GEO600 detector in Germany, and the French-Italian Virgo detector in Italy. All three experiments share their data in the quest to detect gravitational waves.
Collisions of two neutron stars or a neutron star with a black hole are astrophysical phenomena which are likely to generate gravitational waves that are strong enough to be detected on Earth. (See the results of a recent LIGO and Virgo search for gravitational waves from compact binary systems.) These cataclysmic events are also thought to be the cause of some brief flashes of very energetic gamma radiation that are observed from all parts of the sky. Astronomers call these events "short gamma-ray bursts" (short GRBs), and the duration of the flash is usually shorter than two seconds. Longer-lasting gamma-ray bursts, or "long GRBs", are thought to be produced during the explosive death throes of super-massive stars, or supernovae, and may release burst-like gravitational waves.
GRBs can only be observed from space, as Earth's atmosphere blocks gamma rays from reaching the ground. Therefore, several satellites orbiting the Earth are used to observe GRBs. The new LIGO-Virgo analysis which is reported here used data from NASA's Swift and Fermi satellites, as well as from AGILE, INTEGRAL and the Third Interplanetary Network. The analysis covers 154 GRBs that were detected during the latest joint data-taking period of the LIGO-Virgo observatories, between July 2009 and October 2010. LIGO and Virgo scientists searched for gravitational-wave signals occurring at the same time as the observed GRBs, and from the same directions on the sky. Two searches have been performed. The first one searched specifically for a signal from the coalescence of two neutron stars, or a neutron star and a black hole, from short GRBs. The second search looked for a burst-like gravitational wave signal from the collapse of a massive star from all GRBs in the data sample. No gravitational-wave signals were found by either search around the times of the GRBs, and the LIGO-Virgo analysis concludes that on average the GRB events must have taken place at distances larger than 50-100 million light years.
To those hoping for the first detection of gravitational waves, this "null" result may sound disappointing. As we write, however, the LIGO and Virgo gravitational-wave detectors are being upgraded to greatly improve their sensitivity. When these new advanced detectors become operational around 2015, they will be able to see 10 times further into space. Detection of gravitational waves in coincidence with GRBs may then become a reality, opening a new era of astrophysical research.
- Freely readable preprint of the paper describing the analysis and results: "Search for Gravitational Waves associated with Gamma-Ray Bursts during LIGO Science Run 6 and Virgo Science Runs 2 and 3" by J. Abadie et al. (the LIGO Scientific Collaboration and the Virgo Collaboration).
- Web page with data used to make the plots and tables in the paper
- An introduction to gravitational waves from inspiraling binaries
- An introduction to gravitational-wave bursts
Figures from the Publication
For more information on how these figures were generated and their meaning see the publication at arXiv.org.