A pulsar timing array (PTA) is a set of
pulsar
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
s which is analysed to search for correlated signatures in the pulse arrival times. There are many applications for pulsar timing arrays. The best known is the use of an array of
millisecond pulsar
A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio, X-ray, and gamma ray portions of the electromagnetic spectrum. The leading theory for the origin of ...
s to detect and analyse
gravitational wave
Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...
s. Such a detection would result from a detailed investigation of the correlation between arrival times of pulses emitted by the millisecond pulsars as a function of the pulsars' angular separations.
Overview
Millisecond pulsars are used because they are not prone to the
starquakes and accretion events which can affect the period of classical pulsars.
One influence on these propagation properties are low-frequency gravitational waves, with a frequency of 10
ā9 to 10
ā6 hertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is sā1, meaning that on ...
; 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
ā8 Hz, one cycle per year, one would find that pulses arrive 20 ns early in July and 20 ns late in January). This is a 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 the observer 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 accurate sky locations for the sources by this method, as analysing timings for twenty pulsars would produce a region of uncertainty of 100 square degreesa patch of sky about the size of the constellation
Scutum
The ''scutum'' (; plural ''scuta'') was a type of shield used among Italic peoples in antiquity, most notably by the army of ancient Rome starting about the fourth century BC.
The Romans adopted it when they switched from the military formati ...
which would contain at least thousands of merging galaxies.
The main goal of PTAs is measuring the amplitude of background gravitational waves caused by a history of supermassive black hole mergers. The amplitudes can describe the history of how galaxies were formed. The bound on the amplitude of the background waves is called an upper limit. The amplitude of the gravitational wave background is less than the upper limit.
Some supermassive black holes binaries may form a stable binary and only merge after many times the current age of the universe. This is called the ''
final parsec problem.'' It is unclear how supermassive black holes approach each other at this distance.
While supermassive black hole binaries are the main source of very low frequency gravitational waves, other sources could generate the waves, such as
cosmic string
Cosmic strings are hypothetical 1-dimensional topological defects which may have formed during a symmetry-breaking phase transition in the early universe when the topology of the vacuum manifold associated to this symmetry breaking was not simp ...
s, which may have formed early in the history of the universe. When cosmic strings interact, they can form loops that decay by radiating gravitational waves.
[
]
Active and proposed PTAs
Globally there are four active pulsar timing array projects. The first three projects (PPTA, EPTA, and NANOGrav) have begun collaborating under the title of the
International Pulsar Timing Array The International Pulsar Timing Array (IPTA) is a multi-institutional, multi-telescope collaboration comprising the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), the Parkes Pulsa ...
project, later InPTA also became an active part.
# Th
Parkes Pulsar Timing Array(PPTA) at the
Parkes radio-telescope has been collecting data since March 2005.
# The
European Pulsar Timing Array
The European Pulsar Timing Array (EPTA) is a European collaboration to combine five 100-m class radio-telescopes to observe an array of pulsars with the specific goal of detecting gravitational waves. It is one of three pulsar timing array project ...
(EPTA) uses data from the four largest radio telescopes in Europe:
#*
Lovell Telescope
The Lovell Telescope is a radio telescope at Jodrell Bank Observatory, near Goostrey, Cheshire in the north-west of England. When construction was finished in 1957, the telescope was the largest steerable dish radio telescope in the world at 76 ...
#*
Westerbork Synthesis Radio Telescope
The Westerbork Synthesis Radio Telescope (WSRT) is an aperture synthesis interferometer built on the site of the former World War II Nazi detention and transit camp Westerbork, north of the village of Westerbork, Midden-Drenthe, in the northea ...
#*
Effelsberg Telescope
#*
Nancay Radio Telescope.
#* Upon completion the
Sardinia Radio Telescope
The Sardinia Radio Telescope (SRT) is 64-metre fully steerable radio telescope near San Basilio, Sardinia, San Basilio, Province of Cagliari, Sardinia, Italy. Completed in 2011, it is a collaboration between the Istituto di Radioastronomia di Bolo ...
will be added to the EPTA also.
# The
North American Nanohertz Observatory for Gravitational Waves (NANOGrav) uses data collected by the
Arecibo
Arecibo (; ) is a city and municipality on the northern coast of Puerto Rico, on the shores of the Atlantic Ocean, located north of Utuado and Ciales; east of Hatillo; and west of Barceloneta and Florida. It is about west of San Juan, the ...
and
Green Bank radio telescope
The Robert C. Byrd Green Bank Telescope (GBT) in Green Bank, West Virginia, US is the world's largest fully steerable radio telescope, surpassing the Effelsberg 100-m Radio Telescope in Germany. The Green Bank site was part of the National Radi ...
s.
# Th
Indian Pulsar Timing Array(InPTA) uses the upgraded
Giant Metrewave Radio Telescope
The Giant Metrewave Radio Telescope (GMRT), located near Pune, Junnar, near Narayangaon at khodad in India, is an array of thirty fully steerable parabolic radio telescopes of 45 metre diameter, observing at metre wavelengths. It is operated by t ...
.
References
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External links
*Pulsar timing array simulato
SimPTAEuropean Pulsar Timing ArrayNorth American Nanohertz Gravitational Wave Observatory (NANOGrav)
{{Gravitational wave observatories
Gravitational-wave astronomy