A possible close supermassive black-hole binary in a quasar with optical periodicity

Abstract

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 1 . The recent availability of large collections of astronomical time series of flux measurements (light curves) 2-5 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 ± 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 ∼ 9 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.Subparsec supermassive black-hole (SMBH) binary systems are not resolvable except possibly with long baseline radio interferometry. An alternative approach to their detection is through a modulated variability -caused by, for example, perturbations in their accretion disks or precession of relativistic jets, if they are present (see Fig. 1). The best known candidate, OJ 287 6 , has shown a pair of outburst peaks every 12.2 years for at least the past century: this object can be interpreted as a secondary SMBH perturbing the accretion disk of the primary SMBH at regular intervals 7 . Systematic searches for equivalent systems to date 8-10 have attempted to identify them from broad-line velocity offsets in their optical and near-infrared spectra but cannot detect the closest pairs (with 0.1 pc separation).We applied a novel joint wavelet and autocorrelation function (ACF) based technique that identifies objects exhibiting strongly periodic behavior in their light curves (M.J.G. et al, manuscript in preparation) to the largest set of quasar time series currently available. These are drawn from the Catalina Real-time Transient Survey (CRTS) [11][12][13] . PG 1302-102 (see Fig. 2) is the strongest 1 arXiv:1501.01375v1 [astro-ph.GA] 7 Jan 2015 periodic candidate out of twenty objects meeting the selection criteria: strong constant wavelet peak, strong ACF detection of periodic behaviour, sufficient temporal coverage for 1.5 or greater cycles at the detected period, and a phased light curve well-described by a sinusoid. For statistical comparison, we have also generated a simulated light curve for each known quasar based on a damped random walk (DRW) model, a standard statistical description of the optical variability of quasars 1 , using the CRTS time sampling. We find that only one object from the simulated data of 247,000 quasars satisfies the same selection criteria, showing that the number of quasars selected is statistically significant and...

Spectroscopic Indication of a Centi-Parsec Supermassive Black Hole Binary in the Galactic Center of Ngc 5548

“…One needs to carefully investigate the dynamics of accretion disks around binary black holes and the influences of the tidal torques (e.g., Liu & Shapiro 2010). On the other hand, recently D'Orazio et al (2015b) interpreted the periodicity of the light curve by relativistic Doppler boosting and successfully applied their model to the periodic light curve of PG1302-102 (Graham et al 2015b). For NGC5548, Doppler boosting is unimportant because the line-of-sight velocities of the binary are non-relativistic ( < -V c 10 ; 2 see Figure 10(b)).…”

“…Although there exists alternative explanations, a periodic variability in time series of flux measurements is widely used to search for SMBHB candidates (e.g., Graham et al 2015aGraham et al , 2015bLiu et al 2015). The best-known candidate is OJ287, which exhibits outburst peaks every ∼12 yr (Valtonen et al 2008).…”

“…There is increasing indirect evidence that supermassive black hole binaries (SMBHBs) are common in galactic centers, including the presence of cores in the central brightness distributions, which are regarded as a consequence of stars ejected by an SMBHB (Ebisuzaki et al 1991;Merritt & Milosavljević 2005;Kormendy et al 2009), and long-term periodicity in optical light curves (Valtonen et al 2008;Graham et al 2015b;Liu et al 2015;Zheng et al 2016). Dual active galactic nuclei (AGNs) with kiloparsec-scale separation, products of galaxy mergers, have been unambiguously detected in a number of cases (Komossa et al 2003;Comerford et al 2015;Fu et al 2015).…”

Gaia-like astrometry and gravitational waves

“…Recently, such systems were often discussed in the literature (see, e.g., [12,13,33,48]). It is believed that binary supermassive black holes are a relatively common product of interaction and merging of galaxies in the typical course of their evolution.…”

“…Nevertheless, it seems to be useful to give estimates of the expected strain of the gravitational waves from those sources. Substituting the corresponding parameters of the candidates [12,47,48] into the formulas above one gets strains of about h ∼ 2 × 10 −16 with a period of 6 yr for OJ287 assuming the chirp mass of 8 × 10 8 M , h < ∼ 5 × 10 −16 with a period of 2.6 yr for PG 1302-102, and h < 1.3 × 10 −12 (P/ 1 yr) −2/3 for M87 assuming equal mass components. Although we cannot identify promising sources of gravitational waves for Gaia astrometry now, it is important to note that h is proportional to M 5/3 /r so that moderate increase in the chirp mass can compensate greater distances.…”