We report on the results from the first six months of the Catalina Real-Time Transient Survey (CRTS). In order to search for optical transients (OTs) with timescales of minutes to years, the CRTS analyses data from the Catalina Sky Survey which repeatedly covers 26,000 of square degrees on the sky. The CRTS provides a public stream of transients that are bright enough to be followed up using small telescopes. Since the beginning of the survey, all CRTS transients have been made available to astronomers around the world in real time using HTML tables,RSS feeds, and VOEvents. As part of our public outreach program, the detections are now also available in Keyhole Markup Language through Google Sky. The initial discoveries include over 350 unique OTs rising more than 2 mag from past measurements. Sixty two of these are classified as supernovae (SNe), based on light curves, prior deep imaging and spectroscopic data. Seventy seven are due to cataclysmic variables (CVs; only 13 previously known), while an additional 100 transients were too infrequently sampled to distinguish between faint CVs and SNe. The remaining OTs include active galactic nucleus, blazars, high-proper-motions stars, highly variable stars (such as UV Ceti stars), and transients of an unknown nature. Our results suggest that there is a large population of SNe missed by many current SN surveys because of selection biases. These objects appear to be associated with faint host galaxies. We also discuss the unexpected discovery of white dwarf binary systems through dramatic eclipses.

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...

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.