Gravitational waves from an SMBH binary in M 87

Abstract

We study gravitational-wave (GW) emission from a hypothetical supermassive black-hole (SMBH) binary at the center of M 87. The existence of an SMBH other than that usually identified with the central active galactic nucleus (AGN) is a possible explanation for the observed displacement [∼O(1) pc] between the AGN and the galactic centroid, and it is reasonable to assume considering the evolution of SMBHs through galaxy mergers. Because the period of the binary and the resulting GWs is much longer than the observational time-span, we calculate the variation of the GW amplitude, rather than the amplitude itself. We investigate its dependence on the orbital elements and the second BH mass, taking the observational constraints into account. The frequency of the GWs is too low to be detected with the conventional pulsar timing array and we propose a new method for detecting such low-frequency GWs with the distribution function of pulsar spin-down rates. Although the GWs from an SMBH binary that explains the observed displacement are extremely hard to detect even by the new method, GWs are still useful for probing the M 87 center.