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Register a new userA possible close supermassive black-hole binary in a quasar with optical periodicity
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Quasars have long been known to be variable sources at all wavelengths. Their optical variability is stochastic, can be due to a variety of physical mechanisms, and is well-described statistically in terms of a damped random walk model. The recent availability of large collections of astronomical time series of flux measurements (light curves) offers new data sets for a systematic exploration of quasar variability. Here we report on the detection of a strong, smooth periodic signal in the optical variability of the quasar PG 1302-102 with a mean observed period of 1,884 $pm$ 88 days. It was identified in a search for periodic variability in a data set of light curves for 247,000 known, spectroscopically confirmed quasars with a temporal baseline of $sim9$ years. While the interpretation of this phenomenon is still uncertain, the most plausible mechanisms involve a binary system of two supermassive black holes with a subparsec separation. Such systems are an expected consequence of galaxy mergers and can provide important constraints on models of galaxy formation and evolution.
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Galaxies are believed to evolve through merging, which should lead to multiple supermassive black holes in some. There are four known triple black hole systems, with the closest pair being 2.4 kiloparsecs apart (the third component is more distant at 3 kiloparsecs), which is far from the gravitational sphere of influence of a black hole with mass $sim$10$^9$ M$_odot$ (about 100 parsecs). Previous searches for compact black hole systems concluded that they were rare, with the tightest binary system having a separation of 7 parsecs. Here we report observations of a triple black hole system at redshift z=0.39, with the closest pair separated by $sim$140 parsecs. The presence of the tight pair is imprinted onto the properties of the large-scale radio jets, as a rotationally-symmetric helical modulation, which provides a useful way to search for other tight pairs without needing extremely high resolution observations. As we found this tight pair after searching only six galaxies, we conclude that tight pairs are more common than hitherto believed, which is an important observational constraint for low-frequency gravitational wave experiments.
Binary supermassive black holes (BSBHs) are expected to be a generic byproduct from hierarchical galaxy formation. The final coalescence of BSBHs is thought to be the loudest gravitational wave (GW) siren, yet no confirmed BSBH is known in the GW-dominated regime. While periodic quasars have been proposed as BSBH candidates, the physical origin of the periodicity has been largely uncertain. Here we report discovery of a periodicity (P=1607$pm$7 days) at 99.95% significance (with a global p-value of ~$10^{-3}$ accounting for the look elsewhere effect) in the optical light curves of a redshift 1.53 quasar, SDSS J025214.67-002813.7. Combining archival Sloan Digital Sky Survey data with new, sensitive imaging from the Dark Energy Survey, the total ~20-yr time baseline spans ~4.6 cycles of the observed 4.4-yr (restframe 1.7-yr) periodicity. The light curves are best fit by a bursty model predicted by hydrodynamic simulations of circumbinary accretion disks. The periodicity is likely caused by accretion rate modulation by a milli-parsec BSBH emitting GWs, dynamically coupled to the circumbinary accretion disk. A bursty hydrodynamic variability model is statistically preferred over a smooth, sinusoidal model expected from relativistic Doppler boost, a kinematic effect proposed for PG1302-102. Furthermore, the frequency dependence of the variability amplitudes disfavors Doppler boost, lending independent support to the circumbinary accretion variability hypothesis. Given our detection rate of one BSBH candidate from circumbinary accretion variability out of 625 quasars, it suggests that future large, sensitive synoptic surveys such as the Vera C. Rubin Observatory Legacy Survey of Space and Time may be able to detect hundreds to thousands of candidate BSBHs from circumbinary accretion with direct implications for Laser Interferometer Space Antenna.
In this paper we continue the first ever study of magnetized mini-disks coupled to circumbinary accretion in a supermassive binary black hole (SMBBH) approaching merger reported in Bowen et al. 2018. We extend this simulation from 3 to 12 binary orbital periods. We find that relativistic SMBBH accretion acts as a resonant cavity, where quasi-periodic oscillations tied to the the frequency at which the black holes orbital phase matches a non-linear $m=1$ density feature, or ``lump, in the circumbinary accretion disk permeate the system. The rate of mass accretion onto each of the mini-disks around the black holes is modulated at the beat frequency between the binary frequency and the lumps mean orbital frequency, i.e., $Omega_{rm beat} = Omega_{rm bin} - bar{Omega}_{rm lump}$, while the total mass accretion rate of this equal-mass binary is modulated at two different frequencies, $gtrsim bar{Omega}_{rm lump}$ and $approx 2 Omega_{rm beat}$. The instantaneous rotation rate of the lump itself is also modulated at two frequencies close to the modulation frequencies of the total accretion rate, $bar{Omega}_{rm lump}$ and $2 Omega_{rm beat}$. Because of the compact nature of the mini-disks in SMBBHs approaching merger, the inflow times within the mini-disks are comparable to the period on which their mass-supply varies, so that their masses---and the accretion rates they supply to their black holes---are strongly modulated at the same frequency. In essence, the azimuthal symmetry of the circumbinary disk is broken by the dynamics of orbits near a binary, and this $m=1$ asymmetry then drives quasi-periodic variation throughout the system, including both accretion and disk-feeding. In SMBBHs approaching merger, such time variability could introduce distinctive, increasingly rapid, fluctuations in their electromagnetic emission.
Supermassive binary black holes (BBHs) are unavoidable products of galaxy mergers and are expected to exist in the cores of many quasars. Great effort has been made during the past several decades to search for BBHs among quasars; however, observational evidence for BBHs remains elusive and ambiguous, which is difficult to reconcile with theoretical expectations. In this paper, we show that the distinct optical-to-UV spectrum of Mrk 231 can be well interpreted as emission from accretion flows onto a BBH, with a semimajor axis of ~590AU and an orbital period of ~1.2 year. The flat optical and UV continua are mainly emitted from a circumbinary disk and a mini-disk around the secondary black hole (BH), respectively; and the observed sharp drop off and flux deficit at wavelength lambda ~ 4000-2500 Angstrom is due to a gap (or hole) opened by the secondary BH migrating within the circumbinary disk. If confirmed by future observations, this BBH will provide a unique laboratory to study the interplay between BBHs and accretion flows onto them. Our result also demonstrates a new method to find sub-parsec scale BBHs by searching for deficits in the optical-to-UV continuum among the spectra of quasars.
In this paper we consider a scenario where the currently observed hypervelocity stars in our Galaxy have been ejected from the Galactic center as a result of dynamical interactions with an intermediate-mass black hole (IMBH) orbiting the central supermassive black hole (SMBH). By performing 3-body scattering experiments, we calculate the distribution of the ejected stars velocities given various parameters of the IMBH-SMBH binary: IMBH mass, semimajor axis and eccentricity. We also calculate the rates of change of the BH binary orbital elements due to those stellar ejections. One of our new findings is that the ejection rate depends (although mildly) on the rotation of the stellar nucleus (its total angular momentum). We also compare the ejection velocity distribution with that produced by the Hills mechanism (stellar binary disruption) and find that the latter produces faster stars on average. Also, the IMBH mechanism produces an ejection velocity distribution which is flattened towards the BH binary plane while the Hills mechanism produces a spherically symmetric one. The results of this paper will allow us in the future to model the ejection of stars by an evolving BH binary and compare both models with textit{Gaia} observations, for a wide variety of environments (galactic nuclei, globular clusters, the Large Magellanic Clouds, etc.).
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