TheEuropean Pulsar Timing Array (EPTA) is a European collaboration to combine five 100-m class radio-telescopes to observe an array ofpulsars with the specific goal of detectinggravitational waves. It is one of several pulsar timing array projects in operation, and one of the four projects comprising theInternational Pulsar Timing Array, the others being theParkes Pulsar Timing Array, theNorth American Nanohertz Observatory for Gravitational Waves, and theIndian Pulsar Timing Array.
Pulsars are rapidly rotating, highly magnetisedneutron stars that emit radio waves from their magnetic poles that are, due to the star's rotation, observed on Earth as a string of pulses. Due to the extremely high density of neutron stars, their rotation periods are very stable, hence the observed arrival time of the pulses are highly regular. These arrival times are called TOAs (time of arrival) and can be used to perform high-precision timing experiments.
The stability of the TOAs from most pulsars is limited due to the presence ofred noise, also called "timing noise".[1] However, there is a special class of pulsars, called millisecond pulsars (MSP), that are shown to suffer from little or no timing noise.[citation needed] Keeping track of the TOAs of different MSPs over the sky allows for a high-precision timing experiment to detectgravitational waves.
Gravitational waves (GW) are small disturbances in space-time, caused by the motion of masses, if the third time derivative of the mass quadrupole moment is non-zero. These waves are very weak, such that only the strongest waves, caused by the rapid motion of dense stars or black-holes, have a chance of being detected. A pulsar timing array (PTA) uses an array of MSPs as the endpoints of a Galaxy-scaleGW detector. It is sensitive to GWs with a frequency in the nanohertz regime, which corresponds to the regime where the stochastic GW background, caused by the coalescence of super-massive black holes in the early Universe, is predicted to exist.[citation needed] This makes PTAs complementary to other GW detectors such asLIGO,VIRGO andLISA.
The EPTA is one component of a worldwide collaboration for detecting and measuring gravitational waves, theInternational Pulsar Timing Array, which also includes theNorth American Nanohertz Observatory for Gravitational Waves (NANOGrav) and theParkes Pulsar Timing Array (PPTA).
The EPTA uses five European telescopes. These are theWesterbork Synthesis Radio Telescope, theEffelsberg Radio Telescope, theLovell Telescope, theNançay Radio Telescope and theSardinia Radio Telescope.
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Since 2009, the EPTA has made some progress thanks to a projectEuropean Research Council funded project known as the Large European Array for Pulsars (LEAP). The aim of this project is to coherently combine the five EPTA telescopes to synthesise the equivalent of a fully steerable 194-m dish.[2] This will improve the accuracy with which the pulsar TOAs can be measured by an order of magnitude, essential for the first detection ofgravitational waves within the next decade.[citation needed]