Periodic self-lensing from accreting massive black hole binaries

TL;DR

This paper proposes that self-lensing in accreting massive black hole binaries can produce observable periodic flares, offering a new method to detect and study these elusive systems.

Contribution

It introduces the concept of binary self-lensing as a signature of MBHBs and estimates the probability and characteristics of such events for various binary parameters.

Findings

Strong lensing events occur in 1-10% of monitored MBHBs.

Lensing event durations range from days to a year depending on mass and period.

Approximately 10 known MBHB candidates may exhibit detectable lensing flares.

Abstract

Nearly 150 massive black hole binary (MBHB) candidates at sub-pc orbital separations have been reported in recent literature. Nevertheless, the definitive detection of even a single such object remains elusive. If at least one of the black holes is accreting, the light emitted from its accretion disc will be lensed by the other black hole for binary orbital inclinations near to the line of sight. This binary self-lensing could provide a unique signature of compact MBHB systems. We show that, for MBHBs with masses in the range 10^6 - 10^10 solar masses and with orbital periods less than ~10 yr, strong lensing events should occur in one to 10s of percent of MBHB systems that are monitored for an entire orbit. Lensing events will last from days for the less massive, shorter period MBHBs up to a year for the most massive ~10 year orbital period MBHBs. At small inclinations of the binary orbit to the line of sight, lensing must occur and will be accompanied by periodicity due to the relativistic Doppler boost. Flares at the same phase as the otherwise average flux of the Doppler modulation would be a smoking gun signature of self-lensing and can be used to constrain binary parameters. For MBHBs with separation >100 Schwarzschild radii, we show that finite-sized source effects could serve as a probe of MBH accretion disc structure. Finally, we stress that our lensing probability estimate implies that ~10 of the known MBHB candidates identified through quasar periodicity should exhibit strong lensing flares.