These cosmic beacons are compact neutron stars, born in supernova explosions, which rotate rapidly and steadily about their axis. Their intense magnetic field causes them to emit radio waves and gamma radiation. Their rotation sweeps the emission regions through space like the beam from a lighthouse. If the neutron star points towards Earth, it is visible as a pulsar. Not all pulsars show up in multiple spectral ranges. In some cases, the scientists measure only the flashing as a radio pulsar; in other cases, only the periodic arrival times of gamma photons can be registered. The latter type of pulsar is called a gamma-ray-only pulsar. The most likely cause for the different pulsar types are the different orientations of the emission regions in the extremely strong magnetic field of the neutron star.
But pulsars are even more mysterious: when they are young, their steady rotation is irregular, and disturbed by sudden, jerky accelerations known as glitches. Only about five percent of the pulsars exhibit this behaviour. In such a glitch, the neutron star suddenly rotates faster, then slowly decelerates again and returns to the old rotation period a few weeks later. Astronomers do not yet know why this happens, but accurate measurements of these glitches provide insights into the structure of the compact celestial bodies.
To date, astronomers have found most pulsars in the radio wave range, but thanks to NASA’s Fermi satellite they are finding more and more of these celestial bodies via their high-energy gamma radiation. Fermi has been observing the universe with its Large Area Telescope (LAT) in the gamma-rays since 2008. It has discovered hundreds of new sources, many of which are probably undiscovered pulsars.
When analysing data from gravitational wave detectors, scientists have to rely on very effective algorithms and high computing power. This is necessary, because a possible gravitational wave signal would be scarcely stronger than the background noise at the current measurement accuracy.
Within the LIGO-Virgo Science Collaboration (LVC), which also includes the German-British GEO600 detector in Ruthe near Hanover, all detector data are collected jointly, archived and made available for analysis. Several copies of around 500 Terabytes of data are stored at different computer cluster locations. When the detector network is running, one megabyte of data is generated every second. The largest and most powerful computing cluster is ATLAS at the AEI in Hanover. It has a peak computing power of 64 TFLOP/s (floating-point operations per second).
The data is analysed in several steps. First, the astrophysicists scan large areas of the sky for signals. If there is a conspicuous signal in one direction, they investigate the vicinity with an algorithm which has a narrower search grid and thus requires more computing time. If the signal is confirmed, the scientists analyse its temporal characteristic and examine whether it can be assigned to a specific pulsar period, for example. The Hanover scientists have modified the algorithm to search for continuous sources of gravitational waves and used it successfully to search for gamma-ray pulsars in Fermi data.
This project for distributed volunteer computing connects PC users from all over the world, who voluntarily donate spare computing time on their home and office computers. It has more than 320,000 participants and is therefore one of the largest projects of this kind. Scientific supporters are the Center for Gravitation and Cosmology at the University of Wisconsin-Milwaukee and the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, Hanover) with financial support from the National Science Foundation and the Max Planck Society. Since 2005, Einstein@Home has examined data from the gravitational wave detectors within the LIGO-Virgo-Science Collaboration (LVC) for gravitational waves from unknown, rapidly rotating neutron stars.
As of March 2009, Einstein@Home has also been involved in the search for signals from radio pulsars in observational data from the Arecibo Observatory in Puerto Rico and the Parkes Observatory in Australia. Since the first discovery of a radio pulsar by Einstein@Home in August 2010, the global computer network has discovered more than 40 new radio pulsars. A new search for gamma-ray pulsars in data of the Fermi satellite was added in August 2011; the project is looking for, among other things, the first millisecond pulsar, visible only in the gamma-ray range.
KNI / HOR