Black holes as telescopes: Discovering supermassive binaries through quasi-periodic lensed starlight

Abstract

Supermassive black hole (SMBH) binary systems are unavoidable outcomes of galaxy mergers. Their dynamics encode information about their formation and growth, the composition of their host galactic nuclei, the evolution of galaxies, and the nature of gravity. Many SMBH binaries with separations pc-kpc have been found, but closer (sub-parsec) binaries remain to be confirmed. Identifying these systems may elucidate how binaries evolve past the ``final parsec'' until gravitational radiation drives them to coalescence. Here we show that SMBH binaries in non-active galactic nuclei can be identified and characterized by the gravitational lensing of individual bright stars, located behind them in the host galaxy. The rotation of `caustics' -- regions where sources are hugely magnified due to the SMBH binary's orbit and inspiral -- leads to Quasi-Periodic Lensing of Starlight (QPLS). The extreme lensing magnification of individual bright stars produces a significant variation in the host galaxies' luminosity; their lightcurve traces the orbit of the SMBH binary and its evolution. QPLS probes the population of sources observable by pulsar timing arrays and space detectors (LISA, TianQin), offering advance warning triggers for merging SMBHs for coincident or follow-up GW detections. SMBH population models predict $1-50[190-5,000]\left(n_{\star }/{\mathrm{pc}}^{-3}\right)$ QPLS binaries with period less than $10[40]$ yr with comparable masses and $z<0.3$, where $n_{\star }$ is the stellar number density. Additionally, stellar and orbital motion will lead to frequent instances of single/double flares caused by SMBHBs with longer periods. This novel signature can be searched for in a wealth of existing and upcoming time-domain photometric data: identifying quasi-periodic variability in galactic lightcurves will reveal an ensemble of binary systems and illuminate outstanding questions around them.

arXiv comment26 pages, 10 figures, accepted for publication in PRL

References in corpus (63)

• Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies
• The Host Galaxies of AGN
• LSST: from Science Drivers to Reference Design and Anticipated Data Products
• The Zwicky Transient Facility: System Overview, Performance, and First Results
• TianQin: a space-borne gravitational wave detector
• The NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background
• Search for an isotropic gravitational-wave background with the Parkes Pulsar Timing Array
• Searching for the nano-Hertz stochastic gravitational wave background with the Chinese Pulsar Timing Array Data Release I
• The Hyper Suprime-Cam SSP Survey: Overview and Survey Design
• The second data release from the European Pulsar Timing Array III. Search for gravitational wave signals
• The Assembly and Merging History of Supermassive Black Holes in Hierarchical Models of Galaxy Formation
• The Zwicky Transient Facility: Science Objectives
• Formation of Supermassive Black Holes
• Black holes, gravitational waves and fundamental physics: a roadmap
• Massive black hole binary mergers within sub-pc scale gas discs
• Eccentricity Evolution for Planets in Gaseous Disks
• Gravitational Lensing by Black Holes
• Cosmology with the Laser Interferometer Space Antenna
• The Population of Viscosity- and Gravitational Wave-Driven Supermassive Black Hole Binaries Among Luminous AGN
• Long Term Evolution of Massive Black Hole Binaries
• The Quest for Dual and Binary Supermassive Black Holes: A Multi-Messenger View
• The Role of Gas in the Merging of Massive Black Holes in Galactic Nuclei. I. Black Hole Merging in a Spherical Gas Cloud
• Understanding caustic crossings in giant arcs: characteristic scales, event rates, and constraints on compact dark matter
• Constraining the Mass and Radius of Neutron Stars in Globular Clusters
• Mass Deficits, Stalling Radii, and the Merger Histories of Elliptical Galaxies
• Testing modified gravity at cosmological distances with LISA standard sirens
• Limiting eccentricity of sub-parsec massive black hole binaries surrounded by self-gravitating gas discs
• Relativistic boost as the cause of periodicity in a massive black-hole binary candidate
• A Maximum Stellar Surface Density in Dense Stellar Systems
• Optimization of the Observing Cadence for the Rubin Observatory Legacy Survey of Space and Time: a pioneering process of community-focused experimental design
• Extended-Source Effect and Chromaticity in Two-Point-Mass Microlensing
• Microlensing with advanced contour integration algorithm: Green's theorem to third order, error control, optimal sampling and limb darkening
• Dark matter under the microscope: Constraining compact dark matter with caustic crossing events
• Electromagnetic Counterparts to Massive Black Hole Mergers
• Periodic self-lensing from accreting massive black hole binaries
• Interaction of massive black hole binaries with their stellar environment: III. Scattering of bound stars
• Pre-Merger Localization of Gravitational-Wave Standard Sirens With LISA: Triggered Search for an Electromagnetic Counterpart
• Binary Black Hole Merger in Galactic Nuclei: Post-Newtonian Simulations
• Gravitational Waves from SMBH Binaries in Light of the NANOGrav 15-Year Data
• Microlensing of Extremely Magnified Stars near Caustics of Galaxy Clusters
• The Universe at extreme magnification
• Multi-messenger Time-domain Signatures Of Supermassive Black Hole Binaries
• Eccentric evolution of SMBH binaries
• Eccentricity evolution of massive black hole binaries from formation to coalescence
• LEGWORK: A python package for computing the evolution and detectability of stellar-origin gravitational-wave sources with space-based detectors
• Gravitational Self-Lensing in Populations of Massive Black Hole Binaries
• The NANOGrav 11yr Data Set: Limits on Supermassive Black Hole Binaries in Galaxies within 500Mpc
• Self-lensing flares from black hole binaries II: observing black hole shadows via light-curve tomography
• On the eccentricity evolution of massive black hole binaries in stellar backgrounds
• Disc galaxies formed from major mergers in Illustris
• Sensitivity of present and future detectors across the black-hole binary gravitational wave spectrum
• Implications of the pulsar timing array detections for massive black hole mergers in the LISA band
• Extremely efficient mergers of intermediate mass black hole binaries in nucleated dwarf galaxies
• A public code for astrometric microlensing with contour integration
• Probing the inner kpc of massive galaxies with strong gravitational lensing
• Rapidly Rotating Lenses: Repeating features in the lightcurves of short period binary microlenses
• The Search for Cosmological Black Holes: A Surface Brightness Variability Test
• Clustering clusters: unsupervised machine learning on globular cluster structural parameters
• Coincident Multimessenger Bursts from Eccentric Supermassive Binary Black Holes
• Effects of supermassive binary black holes on gravitational lenses
• Seeing Star Formation Regions with Gravitational Microlensing
• Possible Gravitational Microlensing Events in the Optical Lightcurve of Active Galaxy S5 0716+714
• Dancing with black holes