Evidence for a milliparsec-separation supermassive binary black hole with quasar microlensing

Abstract

We report periodic oscillations in the 15-year-long optical light curve of the gravitationally lensed quasar Q J0158−4325 at z s = 1.29. The signal is enhanced during a high magnification microlensing event of the quasar that the fainter lensed image, B, underwent between 2003 and 2010. We measure a period of P o = 172.6 ± 0.9 days, which translates to 75.4 ± 0.4 days in the quasar frame. The oscillations have a maximum amplitude of 0.26 ± 0.02 mag and decrease concurrently with the smooth microlensing amplitude. We explore four scenarios to explain the origin of the periodicity: (1) the high magnification microlensing event is due to a binary star in the lensing galaxy, (2) Q J0158−4325 contains a supermassive binary black hole system in its final dynamical stage before merging, (3) the quasar accretion disk contains a bright inhomogeneity in Keplerian motion around the black hole, and (4) the accretion disk is in precession. Of these four scenarios, only a supermassive binary black hole can account for both the short observed period and the amplitude of the signal, through the oscillation of the accretion disk towards and away from high-magnification regions of a microlensing caustic. The short measured period implies that the semi-major axis of the orbit is ∼ 10 −3 pc and that and the coalescence timescale is t coal ∼ 1000 years, assuming that the decay of the orbit is solely powered by the emission of gravitational waves. The probability of observing a system so close to coalescence, in a sample of only 30 monitored lensed quasars, suggests either a much larger population of supermassive binary black holes than predicted or, more likely, that some other mechanism significantly increases the coalescence timescale. Three tests of the binary black hole hypothesis include: (i) the recurrence of oscillations in photometric monitoring during any future microlensing events in either image, (ii) spectroscopic detection of Doppler shifts (up to ∼ 0.01c) associated with optical emission in the vicinity of the black holes, and (iii) the detection of gravitational waves through pulsar timing array experiments, such as the Square Kilometre Array, which will have the sensitivity to detect the ∼100 nano-hertz emission.Key words. methods: data analysis -gravitational lensing: microlensing -quasars: supermassive black holes 1 To date, OJ 287 is the only confirmed close SMBBH, which was detected from the repeated pairs of outbursts every 12.2 years, interpreted as a secondary black hole crossing the accretion disk of the primary black hole (Valtonen et al. 2008).

Searching for clues of past binary supermassive black hole mergers in nuclear star clusters

“…The results summarized abo v e imply that SMBHBs should be commonly present at the centre of galaxies. Ho we ver, despite e xtensiv e surv e ys, only a few such candidates have been identified (Rodriguez et al 2006 ;Burke-Spolaor 2011 ;Tremblay et al 2016 ;Millon et al 2022 ), suggesting that SMBHBs quickly merge or escape their galactic nucleus (Merritt & Milosavljevi ć 2005 ).…”

Cosmo-tomography toward PKS 1830−211: Variability of the quasar and of its foreground molecular absorption monitored with ALMA

“…For example, the reverberation mapping technique within the broad-line region in active galactic nuclei (AGN) provides size estimates from the line variability, which, combined with spectroscopy measurements of the kinematics, yields a mass estimate for the central black hole (e.g., Bahcall et al 1972;Blandford & McKee 1982). Another example is provided by the remarkable periodicity in the optical light curve of the gravitationally lensed quasar Q J 0158−4325, which with the help of microlensing led (Millon et al 2022) to propose a model based on a pair of supermassive black holes in the coalescence stage. Yet another application is the use of temporal variability of H I, optical, or ultraviolet absorption lines to study the small-scale structures down to a few AU in the interstellar medium (ISM; see e.g., a review by Stanimirović & Zweibel 2018).…”