Self-lensing flares from black hole binaries I: general-relativistic ray tracing of black hole binaries

TL;DR

This paper models self-lensing flares from black hole binaries using general-relativistic ray tracing, exploring how various parameters affect the light curves and potential observational signatures.

Contribution

It introduces detailed ray-tracing simulations of self-lensing flares in black hole binaries, accounting for complex effects like strong deflections and time delays.

Findings

Light curve shapes vary with binary parameters like separation and eccentricity.

Strong deflections and time delays significantly influence flare profiles.

SLFs can help identify sources and measure black hole shadow sizes.

Abstract

The self-lensing of a massive black hole binary (MBHB), which occurs when the two BHs are aligned close to the line of sight, is expected to produce periodic, short-duration flares. Here we study the shapes of self-lensing flares (SLFs) via general-relativistic ray tracing in a superimposed binary BH metric, in which the emission is generated by geometrically thin accretion flows around each component. The suite of models covers eccentric binary orbits, black hole spins, unequal mass binaries, and different emission model geometries. We explore the above parameter space, and report how the light curves change as a function of, e.g., binary separation, inclination, and eccentricity. We also compare our light curves to those in the microlensing approximation, and show how strong deflections, as well as time-delay effects, change the size and shape of the SLF. If gravitational waves (GWs) from the inspiraling MBHB are observed by LISA, SLFs can help securely identify the source and localizing it on the sky, and to constrain the graviton mass by comparing the phasing of the SLFs and the GWs. Additionally, when these systems are viewed edge-on the SLF shows a distinct dip that can be directly correlated with the BH shadow size. This opens a new way to measure BH shadow sizes in systems that are unresolvable by current VLBI facilities.