Gravitational Self-Lensing in Populations of Massive Black Hole Binaries

TL;DR

This paper evaluates the potential of gravitational self-lensing as a detection method for massive black hole binaries using all-sky survey data, predicting significant detection rates with LSST.

Contribution

It provides the first detailed estimate of self-lensing detectability in realistic MBHB populations, highlighting the influence of disk orientation and potential observational signatures.

Findings

LSST could detect tens to hundreds of self-lensing MBHBs.

Approximately 25% of detectable systems may also show Doppler boosting.

Lensing signatures are nearly achromatic if disks are aligned with orbits.

Abstract

The community may be on the verge of detecting low-frequency gravitational waves from massive black hole binaries (MBHBs), but no examples of binary active galactic nuclei (AGN) have been confirmed. Because MBHBs are intrinsically rare, the most promising detection methods utilize photometric data from all-sky surveys. Recently D'Orazio & Di Stefano 2018 (arXiv:1707.02335) suggested gravitational self-lensing as a method of detecting AGN in close separation binaries. In this study we calculate the detectability of lensing signatures in realistic populations of simulated MBHBs. Within our model assumptions, we find that VRO's LSST should be able to detect 10s to 100s of self-lensing binaries, with the rate uncertainty depending primarily on the orientation of AGN disks relative to their binary orbits. Roughly a quarter of lensing detectable systems should also show detectable Doppler boosting signatures. If AGN disks tend to be aligned with the orbit, lensing signatures are very nearly achromatic, while in misaligned configurations the bluer optical bands are lensed more than redder ones. Whether substantial obscuring material (e.g.~a dusty torus) will be present in close binaries remains uncertain, but our estimates suggest that a substantial fraction of systems would still be observable in this case.