Pulsar timing array
Encyclopedia
A pulsar timing array is a set of millisecond pulsar
s that can be used to detect and analyse gravitational waves. Such a detection would result from a detailed investigation of the arrival times of pulses emitted by these millisecond pulsars.
Millisecond pulsars are used because they appear not to be prone to the starquakes and accretion events which can affect the period of classical pulsars.
The most interesting influence on these propagation properties is low-frequency gravitational waves, with a frequency of 10−9 to 10−6 Hertz
; the expected astrophysical sources of such gravitational waves are massive black hole binaries in the centres of merging galaxies, where tens of millions of solar masses are in orbit with a period between months and a few years.
The gravitational waves cause the time of arrival of the pulses to vary by a few tens of nanoseconds over their wavelength (so, for a frequency of 3 x 10 −8 Hz, one cycle per year, you would find that pulses arrive 20 ns early in July and 20 ns late in January). This is an extremely delicate experiment, although millisecond pulsars are stable enough clocks that the time of arrival of the pulses can be predicted to the required accuracy; the experiments use collections of 20 to 50 pulsars to account for dispersion effects in the atmosphere and in the space between us and the pulsar. It is necessary to monitor each pulsar roughly once a week; a higher cadence of observation would allow the detection of higher-frequency gravitational waves, but it is unclear whether there would be loud enough astrophysical sources at such frequencies.
It is not possible to get particularly accurate sky locations for the sources by this method - analysing timings for twenty pulsars would produce a region of uncertainty of 100 square degrees, a patch of sky about the size of the constellation Scutum
which would contain at least thousands of merging galaxies.
has been collecting data since March 2005. The European Pulsar Timing Array uses data from the four largest telescopes in Europe: the Lovell Telescope
, the Westerbork Synthesis Radio Telescope
, the Effelsberg Telescope and the Nancay Radio Telescope
. Upon completion the Sardinia Radio Telescope will be added to the EPTA also. The North American Nanohertz Gravitational Wave Observatory uses data collected by the Arecibo and Green Bank Radio telescopes. These three projects have begun collaborating under the title of the International Pulsar Timing Array
project.
Millisecond pulsar
A millisecond pulsar is a pulsar with a rotational period in the range of about 1-10 milliseconds. Millisecond pulsars have been detected in the radio, X-ray, and gamma ray portions of the electromagnetic spectrum. The origin of millisecond pulsars is still unknown...
s that can be used to detect and analyse gravitational waves. Such a detection would result from a detailed investigation of the arrival times of pulses emitted by these millisecond pulsars.
Millisecond pulsars are used because they appear not to be prone to the starquakes and accretion events which can affect the period of classical pulsars.
The most interesting influence on these propagation properties is low-frequency gravitational waves, with a frequency of 10−9 to 10−6 Hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
; the expected astrophysical sources of such gravitational waves are massive black hole binaries in the centres of merging galaxies, where tens of millions of solar masses are in orbit with a period between months and a few years.
The gravitational waves cause the time of arrival of the pulses to vary by a few tens of nanoseconds over their wavelength (so, for a frequency of 3 x 10 −8 Hz, one cycle per year, you would find that pulses arrive 20 ns early in July and 20 ns late in January). This is an extremely delicate experiment, although millisecond pulsars are stable enough clocks that the time of arrival of the pulses can be predicted to the required accuracy; the experiments use collections of 20 to 50 pulsars to account for dispersion effects in the atmosphere and in the space between us and the pulsar. It is necessary to monitor each pulsar roughly once a week; a higher cadence of observation would allow the detection of higher-frequency gravitational waves, but it is unclear whether there would be loud enough astrophysical sources at such frequencies.
It is not possible to get particularly accurate sky locations for the sources by this method - analysing timings for twenty pulsars would produce a region of uncertainty of 100 square degrees, a patch of sky about the size of the constellation Scutum
Scutum
Scutum is a small constellation introduced in the seventeenth century. Its name is Latin for shield.-History:Scutum is the only constellation that owes its name to a non-classical historical figure...
which would contain at least thousands of merging galaxies.
Active and proposed PTAs
Globally there are three active pulsar timing array projects. The Parkes Pulsar Timing Array at the Parkes radio-telescopeParkes Observatory
The Parkes Observatory is a radio telescope observatory, 20 kilometres north of the town of Parkes, New South Wales, Australia. It was one of several radio antennas used to receive live, televised images of the Apollo 11 moon landing on 20 July 1969....
has been collecting data since March 2005. The European Pulsar Timing Array uses data from the four largest telescopes in Europe: the Lovell Telescope
Lovell Telescope
The Lovell Telescope is a radio telescope at Jodrell Bank Observatory, near Goostrey, Cheshire in the north-west of England. When it was constructed in 1955, the telescope was the largest steerable dish radio telescope in the world at 76.2 m in diameter;it is now the third largest, after the...
, the Westerbork Synthesis Radio Telescope
Westerbork Synthesis Radio Telescope
The Westerbork Synthesis Radio Telescope is an aperture synthesis interferometer near camp Westerbork, north of the village of Westerbork, Midden-Drenthe, in the northeastern Netherlands. It consists of a linear array of 14 antennas with a diameter of 25 metres arranged on a 2.7 km East-West line...
, the Effelsberg Telescope and the Nancay Radio Telescope
Nançay Radio Telescope
The Nançay Decimetric Radio Telescope is located in the small commune of Nançay, two hours' drive south of Paris, France. The radio telescope saw first light in 1965, after an inauguration by the then French president, Charles de Gaulle. It is one of the largest radio telescopes in the world...
. Upon completion the Sardinia Radio Telescope will be added to the EPTA also. The North American Nanohertz Gravitational Wave Observatory uses data collected by the Arecibo and Green Bank Radio telescopes. These three projects have begun collaborating under the title of the International Pulsar Timing Array
International Pulsar Timing Array
The International Pulsar Timing Array is a consortium of consortia ,comprising the European Pulsar Timing Array , the North American Nanohertz Observatory for Gravitational Waves , and the Parkes Pulsar...
project.