Electromagnetic signatures of supermassive black hole binaries resolved by PTAs

Abstract

Pulsar timing arrays (PTAs) may eventually be able to detect not only the stochastic gravitational-wave (GW) background of SMBH binaries, but also individual, particularly massive binaries whose signals stick out above the background. In this contribution, we discuss the possibility of identifying and studying such `resolved' binaries through their electromagnetic emission. The host galaxies of such binaries are themselves expected to be also very massive and rare, so that out to redshifts z~2 a unique massive galaxy may be identified as the host. At higher redshifts, the PTA error boxes are larger and may contain as many as several hundred massive-galaxy interlopers. In this case, the true counterpart may be identified, if it is accreting gas efficiently, as an active galactic nucleus (AGN) with a peculiar spectrum and variable emission features. Specifically, the binary's tidal torques expel the gas from the inner part of the accretion disk, making it unusually dim in X-ray and UV bands and in broad optical emission lines. The tails of the broad wings of any FeK-alpha emission line may also be `clipped' and missing. The binary's orbital motion, as well as the gas motions it induces, may trigger quasiperiodic variations. These include coherent flux variability, such as luminous, multi-wavelength flares, as well as Doppler shifts of broad emission lines and `see-saw' oscillations in the FeK-alpha line. Additional features, such as evidence for a recent major merger or dual collimated jets, could also corroborate the counterpart. These properties would make resolved PTA sources stand out among AGN with similar overall luminosities and allow their identification.