Binary-lens amplification of starlight: from magnification flares to quasi-periodic tracking of orbital evolution

Abstract

Supermassive black hole binaries (SMBHB) are expected at the center of galaxies as a result of galaxy mergers. The dynamics and evolution of SMBHBs shed light on the evolution and formation of galaxies, the gravitational wave background and the nature of gravity. Many methods have been proposed to identify sub-parsec SMBHBs via the detection of gravitational waves or the modulation of the emission from the surrounding gaseous environment. In a previous study, we proposed a new method of identifying SMBHBs in non-active galactic nuclei via gravitational lensing. The caustics, i.e. the source positions with a divergent magnification, exhibit a complex extended geometry which rotates with the binary period. Each caustic-crossing produces an extreme magnification by up to a factor of $10^{6}$. In this work, we generalize the previous study and describe the rich phenomenology of binary-lens amplification of starlight (BLAST). We explore the electromagnetic transients produced when accounting for the relative proper motion of the source, either a single or a binary star, the eccentricity and precession of the SMBHB, and the relative inclination. Systems where a star rapidly drifts across the caustic produce a single- or double-peaked flare. In the opposite limit, a star with no relative proper motion produces quasi-periodic lensed starlight (QPLS), i.e., repeated peaks that track the system's orbital evolution. We determine the expected magnification and peak duration in different circumstances, and calculate the magnification curves and their Fourier transforms. This study may help construct search pipelines to identify SMBHBs in electromagnetic time-domain surveys.