http://arxiv.org/abs/1809.04610
Quasars whose broad emission lines show temporal, bulk radial velocity (RV) shifts have been proposed as candidate sub-parsec (sub-pc), binary supermassive black holes (BSBHs). We identified a sample of 16 BSBH candidates based on two-epoch spectroscopy among 52 quasars with significant RV shifts over a few rest-frame years. The candidates showed consistent velocity shifts independently measured from two broad lines (H$\beta$ and H$\alpha$ or Mg${\rm \,II}$) without significant changes in the broad-line profiles. Here in the third paper of the series, we present further third- and fourth-epoch spectroscopy for 12 of the 16 candidates for continued RV tests, spanning $\sim$5–15 yr in the quasars’ rest frames. Cross-correlation analysis of the broad H$\beta$ calibrated against [O${\rm\,III}]\,\lambda 5007$ suggests that 5 of the 12 quasars remain valid as BSBH candidates. They show broad H$\beta$ RV curves that are consistent with binary orbital motion without significant changes in the broad line profiles. Its broad H$\alpha$ (or Mg${\rm \,II}$) displays RV shifts either consistent with or smaller than those seen in broad H$\beta$. The RV shifts can be explained by a $\sim$0.05–0.1 pc BSBH with an orbital period of $\sim$40–130 yr, assuming a mass ratio of 0.5–2 and a circular orbit. However, the parameters are not well constrained given the few epochs that sample only a small portion of the hypothesized binary orbital cycle. The apparent occurrence rate of sub-pc BSBHs is $\lesssim$13$\pm$5% among all SDSS quasars, with no significant difference in the subsets with and without single-epoch broad line velocity offsets. Dedicated long-term spectroscopic monitoring is still needed to further confirm or reject these BSBH candidates.
H. Guo, X. Liu, Y. Shen, et. al.
Fri, 14 Sep 18
32/65
Comments: 39 pages, 11 figures, MNRAS submitted; comments welcome
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