# A Large Systematic Search for Close Supermassive Binary and Rapidly Recoiling Black Holes - III. Radial Velocity Variations

## TL;DR

This study monitors 88 quasars over up to 12 years to identify supermassive black hole binaries through radial velocity variations, finding a few promising candidates and establishing mass limits to evaluate the binary hypothesis.

## Contribution

It introduces a long-term spectroscopic monitoring method and a new approach to constrain the mass and period of potential supermassive black hole binaries.

## Key findings

- Three quasars show systematic velocity changes suggesting binary motion.
- Lower mass limits for candidates range from 47,000 to 380 million solar masses.
- Most candidates do not yet rule out the binary hypothesis due to observational constraints.

## Abstract

We have been spectroscopically monitoring 88 quasars selected to have broad H$\beta$ emission lines offset from their systemic redshift by thousands of km s$^{-1}$. By analogy with single-lined spectroscopic binary stars, we consider these quasars to be candidates for hosting supermassive black hole binaries (SBHBs). In this work we present new radial velocity measurements, typically 3-4 per object over a time period of up to 12 years in the observer's frame. In 29/88 of the SBHB candidates no variability of the shape of the broad H$\beta$ profile is observed, which allows us to make reliable measurements of radial velocity changes. Among these, we identify three objects that have displayed systematic and monotonic velocity changes by several hundred km s$^{-1}$ and are prime targets for further monitoring. Because the periods of the hypothetical binaries are expected to be long, we cannot hope to observe many orbital cycles during our lifetimes. Instead, we seek to evaluate the credentials of the SBHB candidates by attempting to rule out the SBHB hypothesis. In this spirit, we present a method for placing a lower limit on the period, and thus the mass, of the SBHBs under the assumption that the velocity changes we observe are due to orbital motion. Given the duration of our monitoring campaign and the uncertainties in the radial velocities, we were able to place a lower limit on the total mass in the range $4.7\times10^4-3.8\times10^8$ $M_{\scriptscriptstyle \odot}$, which does not yet allow us to rule out the SBHB hypothesis for any candidates.