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

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.

Advancing the Landscape of Multimessenger Science in the Next Decade

“…Most of the models for the BL Lac binary candidates involve the jet launching from a more massive primary black hole perturbed by a less massive secondary black hole. The non-jet binary candidates showing periodic phenomena can be explained a number of ways, including modulated accretion from an orbiting overdense feature in the circumbinary disk called the "lump" [304-307, 310, 322, 334-339], or Doppler modulation from the orbital motion of the binary that may be augmented by strong lensing from the black holes passing near the line of sight [340][341][342][343]. Extensive surveys have turned up few reliable sources [295][296][297], though more are expected with the Vera Rubin observatory.…”