SIMBAD references

Pulsar timing measurements can be used to detect gravitational radiation from massive black hole binaries. The ∼106 day quasi-periodic flux variations in Sagittarius A* (Sgr A*) at radio wavelengths reported by Zhao, Bower, & Goss may be due to binarity of the massive black hole that is presumed to be responsible for the radio emission. A 106 day equal-mass binary black hole is unlikely based on its short inspiral lifetime and other arguments. Nevertheless the reported quasi-periodicity has led us to consider whether the long-wavelength gravitational waves from a conjectured binary might be detected in present or future precision timing of millisecond pulsars. While present timing cannot reach the level expected for an equal-mass binary, we estimate that future efforts could. This inquiry has led us to further consider the detection of binarity in the massive black holes now being found in nearby galaxies. For orbital periods of ∼2000 day where the pulsar timing measurements are most precise, we place upper limits on the mass ratio of binaries as small as 0.06.