The International Pulsar Timing Array (IPTA) is a multi-institutional, multi-telescope collaboration[1] comprising the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), the Parkes Pulsar Timing Array (PPTA) in Australia, and the Indian Pulsar Timing Array Project (InPTA[2][3]). The goal of the IPTA is to detect ultra-low-frequency gravitational waves, such as from mergers of supermassive black holes, using an array of approximately 30 pulsars. This goal is shared by each of the participating institutions, but they have all recognized that their goal will be achieved more quickly by combining their respective efforts and resources.
There are also affiliated observers from other timing arrays that plan eventually to join.
The basic experiment exploits the predictability of the times of arrival (TOAs) of pulses from millisecond pulsars (MSPs) and uses them as a system of galactic clocks. Disturbances in the clocks will be measurable at the Earth. A disturbance from a passing gravitational wave will have a particular signature across the ensemble of pulsars, and will thus be detected.
The experiment is analogous to ground-based interferometric detectors such as LIGO and VIRGO, where the time of flight of a laser beam is measured along a particular path and compared to the time of flight along an orthogonally oriented path. Instead of the time of flight of a laser beam, the IPTA is measuring the time of flight of an electromagnetic pulse from the pulsar. Instead of 4 km arms, as in the case of LIGO, the 'arms' of the IPTA are thousands of light-years - the distance between the pulsars and the Earth. Each of the PTAs times approximately 20 MSPs each month. With extensive overlap between the collaborations, the total number of MSPs timed by the IPTA, and thus the number of 'arms' in the detector, is approximately 30.
These differences between the IPTA and the ground-based interferometers allow them to probe a completely different range of gravitational-wave frequencies and thus a different category of sources. Whereas ground-based detectors are sensitive to between tens and thousands of Hz, the IPTA is sensitive to between tens and hundreds of microHertz. The primary source of gravitational waves in this range is expected to be binary mergers of supermassive black holes with billions of solar masses, thought to be abundant in the universe at the centers of galaxies, resulting from previous mergers of those galaxies.
The resources of the IPTA are substantial. The EPTA uses large quantities of time on Europe's five 100-meter class telescopes: the Lovell Telescope in England, the Effelsberg 100-m Radio Telescope in Germany, the Sardinia Radio Telescope in Italy, the Westerbork Synthesis Radio Telescope in the Netherlands, and the Nançay Radio Telescope in France. Together these five telescopes make up the Large European Array for Pulsars (LEAP), in which they operate together as a single 300-meter class telescope. NANOGrav uses about 1 day per month of time at the 100 m Green Bank Telescope, and prior to its collapse, 0.5 days per month at the 300 m Arecibo Observatory in Puerto Rico. The PPTA uses several days per month at the 64 m Parkes Radio Telescope in Australia.
Pulsar timing was tied for top ranking in the "medium size" category for priorities from the Particle Astrophysics and Gravitational Panel of the Astro2010 Decadal Review sponsored by the U.S. National Academy of Sciences.[4]
The IPTA is coordinated and advised by the IPTA Steering Committee, a seven-member committee with two representatives from each of the three IPTA consortium members plus the immediate past chair. Currently on the committee are Richard Manchester (current chair; CSIRO Astronomy and Space Science; PPTA), Willem van Straten (Swinburne University of Technology; PPTA), Scott Ransom (National Radio Astronomy Observatory; NANOGrav), Ingrid Stairs (University of British Columbia; NANOGrav), Ben Stappers (Jodrell Bank Centre for Astrophysics; EPTA), Gilles Theureau (University of Orléans; EPTA), and Andrea Lommen (past chair; Franklin & Marshall College). Each of the three consortium members are also members of the Gravitational Wave International Committee, an advisory council consisting of the leaders of gravitational wave experiments worldwide.