Gravitational wave emission from binary supermassive black holes

Abstract

Massive black hole binaries (MBHBs) are unavoidable outcomes of the hierarchical structure formation process, and, according to the theory of general relativity, are expected to be the loudest gravitational wave (GW) sources in the Universe. In this paper I provide a broad overview of MBHBs as GW sources. After reviewing the basics of GW emission from binary systems and of MBHB formation, evolution and dynamics, I describe in some details the connection between binary properties and the emitted gravitational waveform. Direct GW observations will provide an unprecedented wealth of information about the physical nature and the astrophysical properties of these extreme objects, allowing to reconstruct their cosmic history, dynamics and coupling with their dense stellar and gaseous environment. In this context I describe ongoing and future efforts to make a direct detection with space based interferometry and pulsar timing arrays, highlighting the invaluable scientific payouts of such enterprises.

Limits on Anisotropy in the Nanohertz Stochastic Gravitational Wave Background

“…Introduction.-Pulsar timing arrays (PTAs) are currently being used to search for, and to eventually characterize, the nanohertz stochastic gravitational wave background (SGWB) by looking for correlated deviations in the pulse times of arrival (TOAs) of multiple radio millisecond pulsars distributed across the sky. The SGWB in the nanohertz regime is thought to be generated by the incoherent superposition of a large number of weak and unresolved GW sources, including supermassive black hole binaries (SMBHBs) [1][2][3][4][5][6][7], decaying cosmic-string networks [8][9][10][11], or primordial GWs [12,13]. Previous analyses have assumed background isotropy, which emerges as a special case from the more general anisotropy framework presented here.…”

Gravitational waves from an SMBH binary in M 87

“…Nanohertz GWs from SMBH binaries are targets of so-called pulsar timing arrays (Sesana et al 2008;Hobbs 2010;Lee et al 2011;Mingarelli et al 2013;Sesana 2013;Shannon et al 2013;Taylor & Gair 2013;Burke-Spolaor 2015;Janssen et al 2015). Because the arrival times of pulses are modulated by GWs, the time variation of timing residuals, the difference be- tween the actual arrival time and the expectation in the absence of GWs, follows the wave form of the GWs crossing the earth.…”

Constraining the solution to the last parsec problem with pulsar timing

“…We must mention that stellar scattering is not the only mechanism that could potentially influence the inspiral of SMBH binaries. Other possible effects include torquing of the binary from a cir-cumbinary disks, where the details of the effect depend sensitively on the dissipative physics of the disk, and gasdriven evolution due to massive inflows of gas that can be triggered by dynamical instabilities during the galactic merger [16,[29][30][31][38][39][40][41][42]. In general, a particular physical effect will lead to an expression for da/dt, which can be mapped to values of f b and κ in the GW spectrum.…”