Principles of Gravitational-Wave Detection with Pulsar Timing Arrays

Michele Maiorano,Francesco De Paolis,Achille Nucita

doi:10.3390/sym13122418

Abstract

Pulsar timing uses the highly stable pulsar spin period to investigate many astrophysical topics. In particular, pulsar timing arrays make use of a set of extremely well-timed pulsars and their time correlations as a challenging detector of gravitational waves. It turns out that pulsar timing arrays are particularly sensitive to ultra-low-frequency gravitational waves, which makes them complementary to other gravitational-wave detectors. Here, we summarize the basics, focusing especially on supermassive black-hole binaries and cosmic strings, which have the potential to form a stochastic gravitational-wave background in the pulsar timing array detection band, and the scientific goals on this challenging topic. We also briefly outline the recent interesting results of the main pulsar timing array collaborations, which have found strong evidence of a common-spectrum process compatible with a stochastic gravitational-wave background and mention some new perspectives that are particularly interesting in view of the forthcoming radio observatories such as the Five hundred-meter Aperture Spherical Telescope, the MeerKAT telescope, and the Square Kilometer Array.

Highlights

Gravitational waves (GWs), predicted by Albert Einstein’s theory of General Relativity (GR) [1], are ripples in space-time propagating at the speed of light in a vacuum, emitted by compact object systems characterized by a quadrupole moment with a non-null second time derivative
Detecting such GWs is possible, through pulsar timing arrays (PTA), which exploit the telescopes generally used for radio astronomy to measure the very tiny variations in the times of arrival (ToA) of the pulses emitted by millisecond pulsars (MSP), induced by GWs
It is worth remarking that PTAs and LISA can work synergically as they are both sensitive to the GW emissions from supermassive black-hole binaries (SMBHBs)

Summary

Introduction

Gravitational waves (GWs), predicted by Albert Einstein’s theory of General Relativity (GR) [1], are ripples in space-time propagating at the speed of light in a vacuum, emitted by compact object systems characterized by a quadrupole moment with a non-null second time derivative. To observe compact object systems in earlier phases, alongside other types of low-frequency (i.e., in the frequency range [10−5, 1] Hz) GW sources, such as extreme mass-ratio inspirals (EMRI) and merging supermassive black-hole binaries (SMBHBs), space-based laser interferometers that bypass the problems due to the Earth seismic noise are needed. These GWs are expected to be generated by many sources of cosmological interest, such as inspiralling SMBHBs [7] or cosmic strings [8] Detecting such GWs is possible, through pulsar timing arrays (PTA), which exploit the telescopes generally used for radio astronomy to measure the very tiny variations in the times of arrival (ToA) of the pulses emitted by millisecond pulsars (MSP), induced by GWs. Eventually, it is worth remarking that PTAs and LISA can work synergically as they are both sensitive to the GW emissions from SMBHBs. PTAs can detect the GW emissions from an SMBHB in the earlier inspiral phases, while LISA can detect the GW emissions from the same source but in the later merger and ring-down phases. For an overview of the expected range of amplitude and frequency of GW emissions for different types of GW sources, the reader is referred to Refs. [9,10]

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Results

Conclusion