Chandra X-ray Observations of Quasars with Velocity-Offset Broad Lines: Assessing the Binary Supermassive Black Hole Hypothesis

TL;DR

This study uses deep Chandra X-ray observations of quasars with velocity-offset broad lines to investigate the binary supermassive black hole hypothesis, comparing mass estimates and spectral properties to broader quasar populations.

Contribution

It provides detailed X-ray analysis of velocity-offset quasars, constraining SMBH masses and spectral characteristics to evaluate binary SMBH scenarios.

Findings

Systematic differences in SMBH mass estimates from different methods.

Quasars show systematically harder X-ray photon indices.

No conclusive evidence for binary SMBHs based on current data.

Abstract

During the final stages of a galaxy merger, dynamical friction acting on the supermassive black holes (SMBHs) in the post-merger remnant can lead to the formation of a gravitationally bound binary SMBH. In the event that at least one of these SMBHs is actively accreting, the system can appear phenomenologically as an active galactic nucleus (AGN) with a broad line region (BLR) kinematically offset from the host galaxy rest frame. Such velocity offsets have been interpreted as signatures of binary SMBHs, recoiling SMBHs, or BLR gas dynamics within a single-SMBH system. We present deep Chandra X-ray observations of five nearby (0.1 < z < 0.2) Sloan Digital Sky Survey quasars whose broad emission lines are Doppler-shifted relative to their host galaxies' systemic velocities, along with archival Chandra observations of 11 additional sources from the same sample. Using our Chandra data, we constrain SMBH masses with multiple independent techniques. We find systematic, method-dependent differences among black hole mass estimates, with masses inferred from the fundamental plane of black hole activity generally lower and single-epoch virial masses typically higher than those obtained using other methods. We also compare the X-ray photon indices and optical-to-X-ray spectral indices of our quasars to the broader quasar population. While we find no strong differences in optical-to-X-ray spectral indices, we do find systematically harder X-ray photon indices than typically observed in comparable quasars. These results constrain competing physical models but do not provide conclusive evidence for or against a binary SMBH origin of the velocity-offset BLRs.