Fishing in the data net
When analysing data from the gravitational wave detectors, the scientists have to rely on particularly effective algorithms and extremely high computing power. The reason: the measurement accuracy currently achievable means that any gravitational wave signal would hardly stand out against the background noise.
In the LIGO-Virgo Science Collaboration (LVC), which also includes the German-British GEO600 detector in Ruthe near Hanover, all the detector data are collected together, archived and made available for analysis. Multiple copies of the data are currently stored at the different cluster locations. Around 500 terabytes of data are stored. When a detector is operating, one megabyte of data is added every second. The largest and most powerful computer cluster is ATLAS at the Max Planck Institute for Gravitational Physics/Albert Einstein Institute in Hanover. It has a peak computing power of 64 teraflop/s (floating-point operations per second).
The data are evaluated in several steps. The physicists begin by scanning large swathes of the sky for signals. If there is something distinctive in one direction, they investigate the surroundings with an algorithm which has a narrower mesh and thus requires more computing time. If the signal is confirmed, the scientists analyse its temporal characteristic and check whether it can be assigned to a specific pulsar period, for example. The Hanover-based scientists had modified the algorithm used to search for continuous sources of gravitational waves and used it successfully in the search for gamma pulsars in the data from the Fermi satellite.
A further option for analysing the astronomical data is the use of distributed computing: volunteer PC users all over the world make spare computing time on their home and office computers available for this purpose. With more than 300,000 participants Einstein@Home is one of the largest projects of its kind. The scientific responsibility for the project lies with the Center for Gravitation and Cosmology at the University of Wisconsin-Milwaukee and the Max Planck Institute for Gravitational Physics, with the financial support of the National Science Foundation and the Max Planck Society.
Since 2005, Einstein@Home has been searching data from the detectors within the LIGO-Virgo-Science Collaboration for gravitational waves from unknown, rapidly rotating neutron stars. Since March 2009, Einstein@Home has also turned its attention to the search for signals from radio pulsars in observations undertaken at the Arecibo Observatory in Puerto Rico and the Parkes Observatory in Australia. A new addition in August 2011 was a project to search for gamma pulsars in the data from the Fermi satellite. Since the first discovery of a radio pulsar in August 2010 with Einstein@Home, the global computer network has discovered 13 pulsars.