90
M. Burgay et al.
Pulsars can also be used as cosmic clocks in order to directly detect lowfrequency gravitational waves: this is the goal of the so-called Pulsar Timing
Arrays (PTAs) [125]. PTAs consider the Earth and particular pulsars as pairs of
test masses; the spacetime metric is perturbed by the passage of low-frequency (or
long-wavelength) gravitational waves coming from distant galaxies. This metric
perturbation causes delays in the arrival times of radio pulses from pulsars at
radio telescopes on Earth, leaving a signature in the timing residuals of pulsars.
We expect to detect a background of gravitational waves from an ensemble of
supermassive black hole binaries by studying the correlations between the timing
residuals of different Earth-pulsar pairs [126] of different angular size and forming
a PTA. PTAs are sensitive to gravitational waves in the nanohertz frequency
range, and is therefore complementary to other gravitational wave experiments
such as advanced LIGO/VIRGO and LISA which are aimed at higher gravitational
wave frequencies. The current PTA efforts include the European Pulsar Timing
Array (EPTA), which manages the LEAP project, the NANohertz Observatory for
Gravitational waves (NANOGrav) and the Parkes Pulsar Timing Array (PPTA).
These three collaborations work together as the International Pulsar Timing Array
(IPTA) [8]. No detection has been achieved so far, but limits on a background of
gravitational waves have been refined and a detection is predicted within the next
few years [127–129]. The effort in detecting gravitational wave from supermassive
black hole binaries is especially encouraged by the recent, breakthrough detection
by advanced LIGO and VIRGO of gravitational waves from coalescing stellar mass
black hole binaries (e.g. [130]) as well as a DNS binary [131]. If not detected earlier,
we expect with high confidence that the SKA or FAST will lead to a detection of
the background and help to understand these systems [132–134].
A century after Einstein’s formulation of general relativity, pulsars are crucial
to testing general relativity and alternative theories of gravity, testing fundamental
physics thanks to its constraints on the equation of state of nuclear matter, and
could soon lead to the first detection of low-frequency gravitational waves from
supermassive black hole binaries.
Acknowledgments MB, DP and AP acknowledge the collaborators at INAF-Osservatorio di
Cagliari and the international collaborators who are contributing to make the study of pulsars an
always stimulating and often surprising activity.
References
1. R.N. Manchester, G.B. Hobbs, A. Teoh, M. Hobbs, The Australia telescope national facility
pulsar catalogue. Astron. J. 129 1993 (2005)
2. A. Pellizzoni et al., Discovery of new gamma-ray pulsars with AGILE. Astrophys. J. 695,
L115–L119 (2009)
3. A.A. Abdo et al., The second Fermi Large Area Telescope catalog of gamma-ray pulsars.
Astrophys. J. Suppl. Ser. 208, 17 (2013)
4. J.M. Cordes, T.J.W. Lazio, NE2001. I. A New Model for the Galactic Distribution of Free
Electrons and its Fluctuations (2002). arXiv:astro-ph/0207156
M. Burgay et al.
Pulsars can also be used as cosmic clocks in order to directly detect lowfrequency gravitational waves: this is the goal of the so-called Pulsar Timing
Arrays (PTAs) [125]. PTAs consider the Earth and particular pulsars as pairs of
test masses; the spacetime metric is perturbed by the passage of low-frequency (or
long-wavelength) gravitational waves coming from distant galaxies. This metric
perturbation causes delays in the arrival times of radio pulses from pulsars at
radio telescopes on Earth, leaving a signature in the timing residuals of pulsars.
We expect to detect a background of gravitational waves from an ensemble of
supermassive black hole binaries by studying the correlations between the timing
residuals of different Earth-pulsar pairs [126] of different angular size and forming
a PTA. PTAs are sensitive to gravitational waves in the nanohertz frequency
range, and is therefore complementary to other gravitational wave experiments
such as advanced LIGO/VIRGO and LISA which are aimed at higher gravitational
wave frequencies. The current PTA efforts include the European Pulsar Timing
Array (EPTA), which manages the LEAP project, the NANohertz Observatory for
Gravitational waves (NANOGrav) and the Parkes Pulsar Timing Array (PPTA).
These three collaborations work together as the International Pulsar Timing Array
(IPTA) [8]. No detection has been achieved so far, but limits on a background of
gravitational waves have been refined and a detection is predicted within the next
few years [127–129]. The effort in detecting gravitational wave from supermassive
black hole binaries is especially encouraged by the recent, breakthrough detection
by advanced LIGO and VIRGO of gravitational waves from coalescing stellar mass
black hole binaries (e.g. [130]) as well as a DNS binary [131]. If not detected earlier,
we expect with high confidence that the SKA or FAST will lead to a detection of
the background and help to understand these systems [132–134].
A century after Einstein’s formulation of general relativity, pulsars are crucial
to testing general relativity and alternative theories of gravity, testing fundamental
physics thanks to its constraints on the equation of state of nuclear matter, and
could soon lead to the first detection of low-frequency gravitational waves from
supermassive black hole binaries.
Acknowledgments MB, DP and AP acknowledge the collaborators at INAF-Osservatorio di
Cagliari and the international collaborators who are contributing to make the study of pulsars an
always stimulating and often surprising activity.
References
1. R.N. Manchester, G.B. Hobbs, A. Teoh, M. Hobbs, The Australia telescope national facility
pulsar catalogue. Astron. J. 129 1993 (2005)
2. A. Pellizzoni et al., Discovery of new gamma-ray pulsars with AGILE. Astrophys. J. 695,
L115–L119 (2009)
3. A.A. Abdo et al., The second Fermi Large Area Telescope catalog of gamma-ray pulsars.
Astrophys. J. Suppl. Ser. 208, 17 (2013)
4. J.M. Cordes, T.J.W. Lazio, NE2001. I. A New Model for the Galactic Distribution of Free
Electrons and its Fluctuations (2002). arXiv:astro-ph/0207156