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A small fraction of quasars have long been known to show bulk velocityoffsets in the broad Balmer lines with respect to the systemic redshift of thehost galaxy. Models to explain these offsets usually invoke broad-line regiongas kinematics/asymmetry around single black holes (BHs), orbital motion ofmassive (~sub-pc) binary black holes (BBHs), or recoil BHs, but single-epochspectra are unable to distinguish between these scenarios. The line-of-sight(LOS) radial velocity (RV) shifts from long-term spectroscopic monitoring canbe used to test the BBH hypothesis. We have selected a sample of 399 quasarswith offset broad H-beta lines from the SDSS DR7 quasar catalog, and haveconducted second-epoch optical spectroscopy for 50 of them. Combined with theexisting SDSS spectra, the new observations enable us to constrain the LOS RVshifts of broad H-beta lines with a rest-frame baseline of a few years tonearly a decade. Using cross-correlation analysis, we detect significant (99%confidence) radial accelerations in the broad H-beta lines in 24 of the 50objects. We suggest that 9 of the 24 detections are sub-pc BBH candidates,which show consistent velocity shifts independently measured from a secondbroad line (either H-alpha or Mg II) without significant changes in thebroad-line profiles. Combining the results on the general quasar populationstudied in Paper I, we find a tentative anti-correlation between the velocityoffset in the first-epoch spectrum and the average acceleration between twoepochs, which could be explained by orbital phase modulation when the timeseparation between two epochs is a non-negligible fraction of the orbitalperiod of the motion causing the line displacement. We discuss the implicationsof our results for the identification of sub-pc BBH candidates in offset-linequasars and for the constraints on their frequency and orbital parameters.[Abridged]

astrophysical journal/the astrophysical journal

Constraining Sub-Parsec Binary Supermassive Black Holes in Quasars with  Multi-Epoch Spectroscopy. II. The Population with Kinematically Offset Broad  Balmer Emission Lines