The Unanticipated Phenomenology of the Blazar PKS~2131$-$021: A Unique Super-Massive Black Hole Binary Candidate

Abstract

Most large galaxies host supermassive black holes in their nuclei and are subject to mergers, which can produce a supermassive black hole binary (SMBHB), and hence periodic signatures due to orbital motion. We report unique periodic radio flux density variations in the blazar PKS~2131$-$021, which strongly suggest an SMBHB with an orbital separation of $\sim 0.001-0.01$ pc. Our 45.1-year radio light curve shows two epochs of strong sinusoidal variation with the same period and phase to within $<2\%$ and $\sim 10\%$, respectively, straddling a 20-year period when this variation was absent. Our simulated light curves accurately reproduce the ``red noise'' of this object, and Lomb-Scargle, weighted wavelet Z-transform, and least-squares sine wave analyses demonstrate conclusively, at the $4.6\sigma$ significance level, that the periodicity in this object is not due to random fluctuations in flux density. The observed period translates to $2.082\pm 0.003$ years in the rest frame at the $z=1.285$ redshift of PKS~2131$-$021. The periodic variation in PKS~2131$-$021 is remarkably sinusoidal. We present a model in which orbital motion, combined with the strong Doppler boosting of the approaching relativistic jet, produces a sine-wave modulation in the flux density which easily fits the observations. Given the rapidly-developing field of gravitational wave experiments with pulsar timing arrays, closer counterparts to PKS~2131$-$021 and searches using the techniques we have developed are strongly motivated. These results constitute a compelling demonstration that the phenomenology, not the theory, must provide the lead in this field.

24 pages, 13 figure, 3 Tables, accepted for publication in APJL

References in corpus (23)

• Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation
• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• The Zwicky Transient Facility: Data Processing, Products, and Archive
• The Zwicky Transient Facility: Science Objectives
• A Compact Supermassive Binary Black Hole System
• A possible close supermassive black-hole binary in a quasar with optical periodicity
• The Population of Viscosity- and Gravitational Wave-Driven Supermassive Black Hole Binaries Among Luminous AGN
• MOJAVE: XV. VLBA 15 GHz Total Intensity and Polarization Maps of 437 Parsec-Scale AGN Jets From 1996-2017
• MOJAVE. XVII. Jet Kinematics and Parent Population Properties of Relativistically Beamed Radio-Loud Blazars
• MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VIII. Faraday rotation in parsec-scale AGN jets
• Fully-Automated Reduction of Longslit Spectroscopy with the Low Resolution Imaging Spectrometer at Keck Observatory
• A method for the estimation of the significance of cross-correlations in unevenly sampled red-noise time series
• A model for periodic blazars
• MOJAVE XIX: Brightness Temperatures and Intrinsic Properties of Blazar Jets
• Constraining the Orbit of Supermassive Black Hole Binary 0402+379
• ESO-VLT optical spectroscopy of BL Lac objects: II. New redshifts, featureless objects and classification assessments
• Jet precession driven by a supermassive black hole binary system in the BL Lac object PG 1553+113
• A hidden population of high-redshift double quasars unveiled by astrometry
• Explaining temporal variations in the jet position angle of the blazar OJ 287 using its binary black hole central engine model
• The NANOGrav 11yr Data Set: Limits on Supermassive Black Hole Binaries in Galaxies within 500Mpc
• Detection of a possible high-confidence radio quasi-periodic oscillation in the BL Lac PKS J2134-0153
• Bimodal radio variability in OVRO-40m-monitored blazars
• Multimessenger pulsar timing array constraints on supermassive black hole binaries traced by periodic light curves

Cited by in corpus (34)

• Electromagnetic Signatures from Supermassive Binary Black Holes Approaching Merger
• Appreciating mergers for understanding the non-linear $M_{\mathrm{bh}}$-$M_{\ast ,\mathrm{spheroid}}$ and $M_{\mathrm{bh}}$-$M_{\ast ,\mathrm{galaxy}}$ relations, updated herein, and the implications for the (reduced) role of AGN feedback
• Prospects for Future Binary Black Hole GW Studies in Light of PTA Measurements
• The Lunar Gravitational-wave Antenna: Mission Studies and Science Case
• Multiwavelength periodicity search in a sample of $\gamma$-ray bright blazars
• Multi-messenger characterization of Mrk 501 during historically low X-ray and $\gamma$-ray activity
• The demographics of obscured AGN from X-ray spectroscopy guided by multiwavelength information
• Nanohertz Gravitational Wave Astronomy during the SKA Era: An InPTA perspective
• Refining the 2022 OJ 287 impact flare arrival epoch
• Periodic Gamma-ray Modulation of the blazar PG 1553+113 Confirmed by Fermi-LAT and Multi-wavelength Observations
• Accretion properties of X-ray AGN: Evidence for radiation-regulated obscuration with redshift-dependent host galaxy contribution
• Evidence for a milliparsec-separation Supermassive Binary Black Hole with quasar microlensing
• Singular Spectrum Analysis of Fermi-LAT Blazar Light Curves: A Systematic Search for Periodicity and Trends in the Time Domain
• Final stage of merging binaries of supermassive black holes: observational signatures
• The variability patterns of the TeV blazar PG 1553+113 from a decade of MAGIC and multi-band observations
• The Host Galaxy of the Recoiling Black Hole Candidate in 3C 186: An Old Major Merger Remnant at the Center of a z=1 Cluster
• Revisiting radio variability of the blazar 3C 454.3
• Observational Evidence of a Centi-parsec Supermassive Black Hole Binary Existing in the Nearby Galaxy M81
• Coincident Multimessenger Bursts from Eccentric Supermassive Binary Black Holes
• A catalogue of candidate milli-parsec separation massive black hole binaries from long term optical photometric monitoring
• Investigating Optical Variability of the Blazar S5 0716+714 On Diverse Time-scales
• Hot, cold, and multi-component accretion flows around supermassive black hole binaries
• The detection of possible $\gamma$-ray quasi-periodic modulation with $\sim$600 days from the blazar S2 0109+22
• Effects of eccentricity on accreting binary black holes: MHD simulations in full GR reveal novel periodicities in jet power and synchrotron spectra
• Multiwavelength Variability Analysis of the Blazar PKS 0727-11: A $\sim$168 Days Quasi-periodic Oscillation in Gamma-ray
• Searching for supermassive black holes binaries within SRG/eROSITA-De I: Properties of the X-ray selected candidates
• About 300 days optical quasi-periodic oscillations in the long-term light curves of the blazar PKS 2155-304
• Atacama Cosmology Telescope: Observations of supermassive black hole binary candidates. Strong sinusoidal variations at 95, 147 and 225 GHz in PKS 2131$-$021 and PKS J0805$-$0111
• Binary black holes gone MAD: Magnetically arrested minidisks around nonspinning black holes
• Milliarcsecond astrometric oscillations in active galactic nuclei as a precursor of multi-messenger gravitational wave events
• The possible long-term periodic variability of the extremely luminous quasar WISE J090924.01+000211.1
• Description and Application of the Surfing Effect
• Multi-Band Optical Variability of Blazar 1ES 2344+514 on Diverse Time-Scales
• Simulations of linear polarization of precessing AGN jets at parsec scales