Self-lensing flares from black hole binaries III: general-relativistic ray tracing of circumbinary accretion simulations

TL;DR

This paper models self-lensing flares in black hole binaries using general-relativistic ray tracing on hydrodynamical simulations, revealing characteristic flux amplification patterns and potential observational signatures for upcoming gravitational wave detectors.

Contribution

It introduces a detailed general-relativistic ray-tracing approach applied to hydrodynamical simulations of circumbinary disks, enhancing understanding of electromagnetic signatures of black hole binaries.

Findings

Significant periodic flux amplification with sharp flare shapes.

Confirmation that transits and shadows produce characteristic flares.

Shocks from mass-trading can mimic self-lensing flares at lower inclinations.

Abstract

Self-lensing flares (SLFs) are expected to be produced once or twice per orbit by an accreting massive black hole binary (MBHB), if the eclipsing MBHBs are observed close to edge-on. SLFs can provide valuable electromagnetic (EM) signatures to accompany the gravitational waves (GWs) detectable by the upcoming Laser Interferometer Space Antenna (LISA). EM follow-ups are crucial for, e.g., sky-localization, and constraining the Hubble constant and the graviton mass. We use high-resolution two-dimensional viscous hydrodynamical simulations of a circumbinary disk (CBD) embedding a MBHB. We then use very high-cadence output of these hydrodynamical simulation inputs for a general-relativistic ray-tracing code to produce synthetic spectra and phase-folded light curves. Our main results show a significant periodic amplification of the flux with the characteristic shape of a sharp flare with a central dip, as the foreground black hole (BH) transits across the minidisk and shadow of the background BH, respectively. These corroborate previous conclusions based on the microlensing approximation and analytical toy models of the emission geometry. We also find that at lower inclinations, without some occlusion of the minidisk emission by the CBD, shocks from quasi-periodic mass-trading between the minidisks can produce bright flares which can mimic SLFs and could hinder their identification.