A systematic search for close supermassive black hole binaries in the Catalina Real-Time Transient Survey (1507.07603v1)

Abstract: Hierarchical assembly models predict a population of supermassive black hole (SMBH) binaries. These are not resolvable by direct imaging but may be detectable via periodic variability (or nanohertz frequency gravitational waves). Following our detection of a 5.2 year periodic signal in the quasar PG 1302-102 (Graham et al. 2015), we present a novel analysis of the optical variability of 243,500 known spectroscopically confirmed quasars using data from the Catalina Real-time Transient Survey (CRTS) to look for close (< 0.1 pc) SMBH systems. Looking for a strong Keplerian periodic signal with at least 1.5 cycles over a baseline of nine years, we find a sample of 111 candidate objects. This is in conservative agreement with theoretical predictions from models of binary SMBH populations. Simulated data sets, assuming stochastic variability, also produce no equivalent candidates implying a low likelihood of spurious detections. The periodicity seen is likely attributable to either jet precession, warped accretion disks or periodic accretion associated with a close SMBH binary system. We also consider how other SMBH binary candidates in the literature appear in CRTS data and show that none of these are equivalent to the identified objects. Finally, the distribution of objects found is consistent with that expected from a gravitational wave-driven population. This implies that circumbinary gas is present at small orbital radii and is being perturbed by the black holes. None of the sources is expected to merge within at least the next century. This study opens a new unique window to study a population of close SMBH binaries that must exist according to our current understanding of galaxy and SMBH evolution.

• The paper introduces a novel wavelet and autocorrelation approach to identify periodic signals in 243,500 quasar light curves, revealing 111 SMBH binary candidates.
• The methodology confirms robust periodic behavior over a nine-year baseline, effectively distinguishing binary-induced variations from stochastic noise.
• These findings bolster galaxy formation models and highlight promising targets for gravitational wave observatories investigating SMBH mergers.

A Systematic Search for Close Supermassive Black Hole Binaries in the Catalina Real-Time Transient Survey

The paper conducted by Graham et al. focuses on the detection of periodic variability in quasars that may be indicative of close supermassive black hole (SMBH) binaries. The research utilizes data from the Catalina Real-time Transient Survey (CRTS), analyzing the optical variability of 243,500 spectroscopically confirmed quasars. This work is built on the hierarchical assembly models of galaxy formations that predict a population of supermassive black hole binaries. Such binaries could play a significant role as sources of nanohertz frequency gravitational waves, potentially detectable by periodic variability in their photometric signals.

Methodology

Graham et al. employ a novel combination of wavelet and autocorrelation function (ACF) analyses to effectively identify quasars exhibiting periodic behavior. This approach is particularly suited to the noisy and irregularly sampled CRTS time series data. The analysis criteria include:

• A strong Keplerian periodic signal with at least 1.5 cycles over a baseline of nine years to identify candidate SMBH binary systems.
• A significant deviation from expected characteristic timescales from a continuous autoregressive (CAR) model indicating periodicity not attributable to stochastic variability.

From the studied sample, 111 candidate quasars with significant periodic signals were identified, showing periods typically spread over a decade. The robustness of these detections was confirmed by simulations using known periodic objects and mock CAR(1) model-based light curves, emphasizing the authenticity of the identified periodic behavior.

Results and Implications

The periodic signals identified could stem from several potential mechanisms related to SMBH binaries:

• Jet Precession: A varying viewing angle due to jet precession caused by a binary system could account for the observed periodic light variations.
• Warped Accretion Disks or Periodic Accretion: Gravitational interactions in a binary system can lead to either warped disk geometries or periodic changes in accretion rates, causing observable periodicity.
• Alternative Explanations: These include the presence of a circumbinary accretion disk with lopsided emission patterns, or relativistic beaming effects from one SMBH’s disk moving relative to the observer's line of sight.

The distribution of observed periods supports theoretical predictions for the number of such binaries expected in a synoptic survey of this scale. Moreover, the observed systems are likely influenced by gravitational waves, supporting the presence of gas perturbations at small orbital radii. This suggests that circumbinary disks might persist at these scales, ultimately facilitating SMBH coalescence.

Theoretical and Practical Prospects

The paper opens a new observational window into the dynamics of SMBH binaries, with both practical and theoretical implications. Practically, identifying and monitoring these candidates could provide targets for gravitational wave observatories, enhancing our understanding of galaxy formation and evolution processes. Theoretically, improving our knowledge of such systems can refine models of gravitational wave emission and SMBH mass assembly over cosmic time.

Future prospects involve continued observational monitoring to confirm periodic behavior over extended baselines, possibly detecting changes in periods due to relativistic effects. Spectroscopic follow-up could verify deviations in emission line properties that are consistent with binary models. The data and methodologies developed by Graham et al. furnish a promising framework for further explorations in the dynamic and transformative field of supermassive black hole binaries.