Searching for Binary Supermassive Black Holes via Variable Broad Emission Line Shifts: Low Binary Fraction

Abstract

Supermassive black hole binaries (SMBHs) are expected to result from galaxy mergers, and thus are natural byproducts (and probes) of hierarchical structure formation in the Universe. They are also the primary expected source of low-frequency gravitational wave emission. We search for binary BHs using time-variable velocity shifts in broad Mg ii emission lines of quasars with multi-epoch observations. First, we inspect velocity shifts of the binary SMBH candidates identified in Ju et al. (2013), using SDSS spectra with an additional epoch of data that lengthens the typical baseline to ∼ 10 yr. We find variations in the line-of-sight velocity shifts over 10 years that are comparable to the shifts observed over 1-2 years, ruling out the binary model for the bulk of our candidates. We then analyze 1438 objects with 8 yr median time baselines, from which we would expect to see velocity shifts > 1000 km s −1 from sub-pc binaries. We find only one object with an outlying velocity of 448 km s −1 , indicating, based on our modeling, that 1 per cent (the value varies with different assumptions) of SMBHs that are active as quasars reside in binaries with ∼ 0.1 pc separations. Binaries either sweep through these small separations rapidly or stall at larger radii.

Constraining sub-parsec binary supermassive black holes in quasars with multi-epoch spectroscopy – III. Candidates from continued radial velocity tests

“…This is broadly consistent with the previous result of Runnoe et al (2017) within uncertainties, which were based on the spectroscopic monitoring of quasars with the largest singleepoch broad-line velocity offsets. Taken at face value, the fraction is higher than the result suggested by Wang et al (2017) in a similar study but based on the analysis of Mg II, which may be at least partly due to the difference between broad Hβ and Mg II.…”

“…Unlike previous work, we focus on the general quasar population (Shen et al 2013, hereafter Paper I; see also Ju et al 2013;Wang et al 2017) and those with single-peaked offset broad emission lines (Liu et al 2014b, hereafter Paper II; see also Tsalmantza et al 2011;Eracleous et al 2012;Decarli et al 2013;Runnoe et al 2017). We have studied the temporal broad-line velocity shifts using the largest sample of quasars with multi-epoch spectroscopy (Papers I & II) based on the SDSS DR7 spectroscopic quasar catalog (Schneider et al 2010;Shen et al 2011).…”

The Time-domain Spectroscopic Survey: Target Selection for Repeat Spectroscopy

“…The program is designed to obtain a third-epoch spectrum for all candidates, with highest priority given to the seven robust candidates, in order to search for monotonic velocity shifts relative to first epoch. The first results from this program were reported in Wang et al (2017), in which the authors rule out a binary model for the bulk of candidates by comparing the variations in the velocity shifts over 1-2years and 10years. They also find that 1% of active SMBHs reside in binaries with ∼0.1pc separations observed in the TDSS.…”

“…They also find that 1% of active SMBHs reside in binaries with ∼0.1pc separations observed in the TDSS. The example shown in the bottom panel of Figure 9 is a candidate from Wang et al (2017) with a prominent line shift.…”

VLBA 24 and 43 GHz observations of massive binary black hole candidate PKS 1155 + 251

“…A similar fraction of dual AGNs at the kpc scale is also found from AGNs selected in hard X-ray (Koss et al 2012) or radio images (Fu et al 2015). However, due to resolution limitations, finding pc-scale BBHs in AGNs remains difficult when using optical and X-ray images, although the line-of-sight radial velocity shifts from long-term spectroscopic monitoring of broad emission lines may also be useful to find the subpc BBHs in AGNs (Wang et al 2017;Runnoe et al 2017). Multi-wavelength approaches should be used to search of dual AGNs and massive BBHs; see Komossa, Backer & Liu (2016) for a recent review.…”