E. Christensen

FIRST RESULTS FROM THE CATALINA REAL-TIME TRANSIENT SURVEY

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.

Probing the Outer Galactic halo with RR Lyrae from the Catalina Surveys

We present analysis of 12,227 type-ab RR Lyraes (RRLs) found among the 200 million public light curves in Catalina Surveys Data Release 1. These stars span the largest volume of the Milky Way ever surveyed with RRLs, covering ~20,000 deg2 of the sky (0° 1500 of the RRLs. Using the accurate distances derived for the RRLs, we show the paths of the Sagittarius tidal streams crossing the sky at heliocentric distances from 20 to 60 kpc. By selecting samples of Galactic halo RRLs, we compare their velocity, metallicity, and distance with predictions from a recent detailed N-body model of the Sagittarius system. We find that there are some significant differences between the distances and structures predicted and our observations.

Evidence for a Milky Way Tidal Stream Reaching Beyond 100?kpc

We present the analysis of 1207 RR Lyrae found in photometry taken by the Catalina Surveys Mount Lemmon telescope. By combining accurate distances for these stars with measurements for ~14,000 type-ab RR Lyrae from the Catalina Schmidt telescope, we reveal an extended association that reaches Galactocentric distances beyond 100 kpc and overlaps the Sagittarius stream system. This result confirms earlier evidence for the existence of an outer halo tidal stream resulting from a disrupted stellar system. By comparing the RR Lyrae source density with that expected based on halo models, we find the detection has ~8σ significance. We investigate the distances, radial velocities, metallicities, and period-amplitude distribution of the RR Lyrae. We find that both radial velocities and distances are inconsistent with current models of the Sagittarius stream. We also find tentative evidence for a division in source metallicities for the most distant sources. Following prior analyses, we compare the locations and distances of the RR Lyrae with photometrically selected candidate horizontal branch stars and find supporting evidence that this structure spans at least 60° of the sky. We investigate the prospects of an association between the stream and the unusual globular cluster NGC 2419.

A systematic search for close supermassive black hole binaries in the Catalina Real-time Transient Survey

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, 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 discs 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 Discovery and Nature of the Optical Transient CSS100217:102913+404220

We report on the discovery and observations of the extremely luminous optical transient CSS100217:102913+404220 (CSS100217 hereafter). Spectroscopic observations showed that this transient was coincident with a galaxy at redshift z = 0.147 and reached an apparent magnitude of V ~ 16.3. After correcting for foreground Galactic extinction we determine the absolute magnitude to be M_V = –22.7 approximately 45 days after maximum light. Over a period of 287 rest-frame days, this event had an integrated bolometric luminosity of 1.3 × 10^(52) erg based on time-averaged bolometric corrections of ~15 from V- and R-band observations. Analysis of the pre-outburst Sloan Digital Sky Survey (SDSS) spectrum of the source shows features consistent with a narrow-line Seyfert 1 galaxy. High-resolution Hubble Space Telescope and Keck follow-up observations show that the event occurred within 150 pc of the nucleus of the galaxy, suggesting a possible link to the active nuclear region. However, the rapid outburst along with photometric and spectroscopic evolution are much more consistent with a luminous supernova. Line diagnostics suggest that the host galaxy is undergoing significant star formation. We use extensive follow-up of the event along with archival Catalina Sky Survey NEO search and SDSS data to investigate the three most likely sources of such an event: (1) an extremely luminous supernova, (2) the tidal disruption of a star by the massive nuclear black hole, and (3) variability of the central active galactic nucleus (AGN). We find that CSS100217 was likely an extremely luminous Type IIn supernova and occurred within the range of the narrow-line region of an AGN. We discuss how similar events may have been missed in past supernova surveys because of confusion with AGN activity.

Cataclysmic variables from the Catalina Real-time Transient Survey

We present 855 cataclysmic variable candidates detected by the Catalina Real-time Transient Survey (CRTS) of which at least 137 have been spectrosc opically confirmed and 705 are new discoveries. The sources were identified from the ana lysis of five years of data, and come from an area covering three quarters of the sky. We study the amplitude distribution of the dwarf novae CVs discovered by CRTS during outburst, and fi nd that in quiescence they are typically two magnitudes fainter compared to the spectr os opic CV sample identified by SDSS. However, almost all CRTS CVs in the SDSS footprint have ugriz photometry. We analyse the spatial distribution of the CVs and find evidence that many of the systems lie at scale heights beyond those expected for a Galactic thin disc population. We compare the outburst rates of newly discovered CRTS CVs with the previously known CV population, and find no evidence for a difference between them. However, we fin d that significant evidence for a systematic difference in orbital period distribution . We discuss the CVs found below the orbital period minimum and argue that many more are yet to be identified among the full CRTS CV sample. We cross-match the CVs with archival X-ray ca talogs and find that most of the systems are dwarf novae rather than magnetic CVs.

Ultra-short Period Binaries from the Catalina Surveys

We investigate the properties of 367 ultra-short period binary candidates selected from 31,000 sources recently identified from Catalina Surveys data. Based on light curve morphology, along with WISE, SDSS and GALEX multi-colour photometry, we identify two distinct groups of binaries with periods below the 0.22 day contact binary minimum. In contrast to most recent work, we spectroscopically confirm the existence of M-dwarf+M-dwarf contact binary systems. By measuring the radial velocity variations for five of the shortest-period systems, we find examples of rare cool-white dwarf+M-dwarf binaries. Only a few such systems are currently known. Unlike warmer white dwarf systems, their UV flux and their optical colours and spectra are dominated by the M-dwarf companion. We contrast our discoveries with previous photometrically-selected ultra-short period contact binary candidates, and highlight the ongoing need for confirmation using spectra and associated radial velocity measurements. Overall, our analysis increases the number of ultra-short period contact binary candidates by more than an order of magnitude.

A SERENDIPITOUS ALL SKY SURVEY FOR BRIGHT OBJECTS IN THE OUTER SOLAR SYSTEM

We use seven yearʼs worth of observations from the Catalina Sky Survey and the Siding Spring Survey covering most of the northern and southern hemisphere at galactic latitudes higher than 20° to search for serendipitously imaged moving objects in the outer solar system. These slowly moving objects would appear as stationary transients in these fast cadence asteroids surveys, so we develop methods to discover objects in the outer solar system using individual observations spaced by months, rather than spaced by hours, as is typically done. While we independently discover eight known bright objects in the outer solar system, the faintest having V = 19.8 ± 0.1, no new objects are discovered. We find that the survey is nearly 100% efficient at detecting objects beyond 25 AU for V ≾ 19.1 V ≾ 18.6 in the southern hemisphere) and that the probability that there is one or more remaining outer solar system object of this brightness left to be discovered in the unsurveyed regions of the galactic plane is approximately 32%.

Supernovae 2016bdu and 2005gl, and their link with SN 2009ip-like transients: another piece of the puzzle

PRIN-INAF; MIUR PRIN; Royal Society - Science Foundation Ireland University Research Fellowship; European Union through ERC [320360]; US National Science Foundation (NSF) [AST-1515876, AST-1515927]; NSFC [11573003]; Chinese Academy of Sciences [XDB09000000]; National Aeronautics and Space Administration (NASA) - Space Telescope Science Institute (STScI) [HST-HF-51348.001]; NASA [NAS 5-26555, NN12AR55G, NNX08AR22G, NNX12AR65G, NNX14AM74G]; NSF [AST-1515927, AST-1211916, PHY-1607611, AST-1313422, AST-1413600]; TABASGO Foundation; Christopher R. Redlich Fund; Miller Institute for Basic Research in Science (U.C. Berkeley); STScI [HST-AR-14295]; DOE [DE-FG02-97ER25308]; FONDECYT [1151445]; Ministry of Economy, Development, and Tourisms Millennium Science Initiative [IC120009]; ERCgrant [291222]; Emil Aaltonen Foundation; Finnish Cultural Foundation; Vilho, Yrjo and Kalle Vaisala Foundation of the Finnish Academy of Science and Letters; EU via ERCgrant [307260]; Quantum Universe I-Core program by Israeli Committee for Planning and Budgeting; ISF; Kimmel award; Israel Science Foundation, Minerva; I-CORE Program of the Planning and Budgeting Committee; Israel Science Foundation; Danish Agency for Science and Technology and Innovation through Sapere Aude; Villum foundation; Instrument Center for Danish Astrophysics (IDA); Gordon and Betty Moore Foundation [GBMF5490]; Center for Cosmology and Astro-Particle Physics (CCAPP) at OSU; Chinese Academy of Sciences South America Center for Astronomy (CASSACA); Mt Cuba Astronomical Foundation; US NSF [AST-1238877]; UK Science and Technology Facilities Council; NASA; NSF

Understanding extreme quasar optical variability with CRTS – I. Major AGN flares

NSF [AST-1413600, AST-1518308]; Alfred P. Sloan Foundation; National Science Foundation; U.S. Department of Energy Office of Science; NASA