Gravitational wave astronomy with the SKA

TL;DR

The paper outlines how a Pulsar Timing Array (PTA) leveraging the Square Kilometre Array (SKA) will enable nanoHertz gravitational wave detection by timing a large number of millisecond pulsars over long baselines. It surveys GW sources (SMBHB networks, cosmic strings, inflation), detection strategies, and the role of VLBI in source localization, while detailing limiting noise processes (jitter, timing noise, ISM effects) and a realistic SKA-era observing plan. It additionally discusses data-management challenges and projected detection timelines, demonstrating that SKA1 could achieve a first GW detection within about five years, with SKA2 enabling detailed GW source characterization, anisotropy studies, and tests of gravity in the strong-field regime. The work emphasizes the SKA's transformative potential for GW astronomy, galaxy evolution, and fundamental physics, contingent on a broad, coordinated international PTA program and robust data infrastructure.

Abstract

On a time scale of years to decades, gravitational wave (GW) astronomy will become a reality. Low frequency (nanoHz) GWs are detectable through long-term timing observations of the most stable pulsars. Radio observatories worldwide are currently carrying out observing programmes to detect GWs, with data sets being shared through the International Pulsar Timing Array project. One of the most likely sources of low frequency GWs are supermassive black hole binaries (SMBHBs), detectable as a background due to a large number of binaries, or as continuous or burst emission from individual sources. No GW signal has yet been detected, but stringent constraints are already being placed on galaxy evolution models. The SKA will bring this research to fruition. In this chapter, we describe how timing observations using SKA1 will contribute to detecting GWs, or can confirm a detection if a first signal already has been identified when SKA1 commences observations. We describe how SKA observations will identify the source(s) of a GW signal, search for anisotropies in the background, improve models of galaxy evolution, test theories of gravity, and characterise the early inspiral phase of a SMBHB system. We describe the impact of the large number of millisecond pulsars to be discovered by the SKA; and the observing cadence, observation durations, and instrumentation required to reach the necessary sensitivity. We describe the noise processes that will influence the achievable precision with the SKA. We assume a long-term timing programme using the SKA1-MID array and consider the implications of modifications to the current design. We describe the possible benefits from observations using SKA1-LOW. Finally, we describe GW detection prospects with SKA1 and SKA2, and end with a description of the expectations of GW astronomy.

Figures (3)

• Figure 1: The gravitational wave landscape: characteristic amplitude ($h_c$), vs frequency. In the nHz frequency range a selected realisation of the expected GW signal from the cosmological population of SMBHBs is shown. Small lavender squares are individual SMBHB contributions to the signal, the dark blue triangles are loud, individually resolvable systems and the blue jagged line is the level of the unresolved background. Nominal sensitivity levels for the IPTA and SKA are also shown. In the mHz frequency range, the eLISA sensitivity curve is shown together with typical circular SMBHB inspirals at z=3 (pale blue), the overall signal from Galactic WD-WD binaries (yellow) and an example of extreme mass ratio inspiral (aquamarine, only the first 5 harmonics are shown). In the kHz range an advanced LIGO curve (based on calculations for a single interferometer) is shown together with selected compact object inspirals (purple). The brown, red and orange lines running through the whole frequency range are expected cosmological backgrounds from standard inflation and selected string models, as labeled in figure. Black dotted lines mark different levels of GW energy density content as a function of frequency ($\Omega_{gw} \propto h_c^2f^2$).
• Figure 2: Expected sky coverage of the SKA, overlaid with the positions of the current pulsars being observed as part of the IPTA project (open circles). For an array mainly at latitude $-30^\circ$ the telescope will have an approximate declination range from $-90^\circ$ to $+45^\circ$ (red dashed line). We note that 49 out of the 50 current IPTA pulsars could be observed by the SKA. The green line indicates the Galactic plane.
• Figure 3: Detection probability for four different PTAs; top left: IPTA; top right: SKA1 at an early science (50%) sensitivity level; middle left: SKA1 as defined in the baseline design document; middle right: SKA2. The top and middle panels show the predictions on the first GW detection by different PTAs as a function of time. In the IPTA case, the starting point corresponds to the present day, after 10 years of previous observations. In the SKA cases, the starting points correspond to the beginning of the SKA data-taking, disregarding previous data. The green curve gives the fraction of simulations of the Universe (which can be considered as a probability) in which a stochastic background is detected first. The red curve gives the probability of a single binary being detected first. The blue curve gives the overall probability of detecting any kind of GW signal. We assume that a GW signal is detected when it produces a signal-to-noise ratio (S/N) larger than a certain threshold. The chosen S/N threshold is 4, which may be considered realistic for single binaries, but a bit conservative for a background. A more detailed study can be found in Rosado, Sesana & Gair (in prep.). The S/N of a detection of a single binary and of a background are obtained as described in sv10 and sejr13, respectively, and are plotted as function of time in the bottom plots for SKA1 and SKA2 PTAs.