Considerations for the Observability of Kinematically Offset Binary AGN
  The gravitational waves from Massive black-hole (MBH) binaries are expected
to be detected by pulsar timing arrays in the next few years. While they are a
promising source for multimessenger observations as binary AGN, few convincing
candidates have been identified in electromagnetic surveys. One approach to
identifying candidates has been through spectroscopic surveys searching for
offsets or time-dependent offsets of broad emission lines (BLs), which may be
characteristic of Doppler shifts from binary orbital motion. In this study, we
predict the parameter space of MBH binaries that should be kinematically
detectable. There is a delicate trade-off between requiring binary separations
to be large enough for BL regions to remain attached to one of the AGN, but
also small enough such that their orbital velocity is detectable. We find that
kinematic signatures are only observable for the lower-mass secondary AGN, for
binaries with total-masses above about $10^8 M_\odot$, and separations between
0.1 and 1 pc. We motivate our usage of a kinematic-offset sensitivity of $10^3$
km/s, and a sensitivity to changing offsets of $10^2$ km/s. With these
parameters, and an Eddington ratio of 0.1, we find that 0.5% of binaries have
detectable offsets, and only 0.03% have detectable velocity changes. Overall,
kinematic binary signatures should be expected in fewer than one in $10^4$ AGN.
Better characterizing the intrinsic variability of BLs is crucial to
understanding and vetting MBH binary candidates. This requires multi-epoch
spectroscopy of large populations of AGN over a variety of timescales.
