Chelsea L. MacLeod

MODELING THE TIME VARIABILITY OF SDSS STRIPE 82 QUASARS AS A DAMPED RANDOM WALK

We model the time variability of ~9000 spectroscopically confirmed quasars in SDSS Stripe 82 as a damped random walk (DRW). Using 2.7 million photometric measurements collected over 10 yr, we confirm the results of Kelly et al. and Kozlowski et al. that this model can explain quasar light curves at an impressive fidelity level (0.01-0.02 mag). The DRW model provides a simple, fast (O(N) for N data points), and powerful statistical description of quasar light curves by a characteristic timescale (τ) and an asymptotic rms variability on long timescales (SF∞). We searched for correlations between these two variability parameters and physical parameters such as luminosity and black hole mass, and rest-frame wavelength. Our analysis shows SF∞ to increase with decreasing luminosity and rest-frame wavelength as observed previously, and without a correlation with redshift. We find a correlation between SF∞ and black hole mass with a power-law index of 0.18 ± 0.03, independent of the anti-correlation with luminosity. We find that τ increases with increasing wavelength with a power-law index of 0.17, remains nearly constant with redshift and luminosity, and increases with increasing black hole mass with a power-law index of 0.21 ± 0.07. The amplitude of variability is anti-correlated with the Eddington ratio, which suggests a scenario where optical fluctuations are tied to variations in the accretion rate. However, we find an additional dependence on luminosity and/or black hole mass that cannot be explained by the trend with Eddington ratio. The radio-loudest quasars have systematically larger variability amplitudes by about 30%, when corrected for the other observed trends, while the distribution of their characteristic timescale is indistinguishable from that of the full sample. We do not detect any statistically robust differences in the characteristic timescale and variability amplitude between the full sample and the small subsample of quasars detected by ROSAT. Our results provide a simple quantitative framework for generating mock quasar light curves, such as currently used in LSST image simulations.

The Sloan Digital Sky Survey quasar catalog: ninth data release

We present the Data Release 9 Quasar (DR9Q) catalog from the Baryon Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey III. The catalog includes all BOSS objects that were targeted as quasar candidates during the survey, are spectrocopically confirmed as quasars via visual inspection, have luminosities Mi[z = 2] 2.15 (61 931) is ~2.8 times larger than the number of z > 2.15 quasars previously known. Redshifts and FWHMs are provided for the strongest emission lines (C iv, C iii], Mg ii). The catalog identifies 7533 broad absorption line quasars and gives their characteristics. For each object the catalog presents five-band (u, g, r, i, z) CCD-based photometry with typical accuracy of 0.03 mag, and information on the morphology and selection method. The catalog also contains X-ray, ultraviolet, near-infrared, and radio emission properties of the quasars, when available, from other large-area surveys. The calibrated digital spectra cover the wavelength region 3600−10 500 A at a spectral resolution in the range 1300 < R < 2500; the spectra can be retrieved from the SDSS Catalog Archive Server. We also provide a supplemental list of an additional 949 quasars that have been identified, among galaxy targets of the BOSS or among quasar targets after DR9 was frozen.

A DESCRIPTION OF QUASAR VARIABILITY MEASURED USING REPEATED SDSS AND POSS IMAGING

We provide a quantitative description and statistical interpretation of the optical continuum variability of quasars. The Sloan Digital Sky Survey (SDSS) has obtained repeated imaging in five UV-to-IR photometric bands for 33,881 spectroscopically confirmed quasars. About 10,000 quasars have an average of 60 observations in each band obtained over a decade along Stripe 82 (S82), whereas the remaining ~25,000 have 2-3 observations due to scan overlaps. The observed time lags span the range from a day to almost 10 years, and constrain quasar variability at rest-frame time lags of up to 4 years, and at rest-frame wavelengths from 1000 A to 6000 A. We publicly release a user-friendly catalog of quasars from the SDSS Data Release 7 that have been observed at least twice in SDSS or once in both SDSS and the Palomar Observatory Sky Survey, and we use it to analyze the ensemble properties of quasar variability. Based on a damped random walk (DRW) model defined by a characteristic timescale and an asymptotic variability amplitude that scale with the luminosity, black hole mass, and rest wavelength for individual quasars calibrated in S82, we can fully explain the ensemble variability statistics of the non-S82 quasars such as the exponential distribution of large magnitude changes. All available data are consistent with the DRW model as a viable description of the optical continuum variability of quasars on timescales of ~5-2000 days in the rest frame. We use these models to predict the incidence of quasar contamination in transient surveys such as those from the Palomar Transient Factory and Large Synoptic Survey Telescope.

Quasar Selection Based on Photometric Variability

We develop a method for separating quasars from other variable point sources using Sloan Digital Sky Survey (SDSS) Stripe 82 light-curve data for ~ 10,000 variable objects. To statistically describe quasar variability, we use a damped random walk model parametrized by a damping timescale, τ, and an asymptotic amplitude (structure function), SF∞. With the aid of an SDSS spectroscopically confirmed quasar sample, we demonstrate that variability selection in typical extragalactic fields with low stellar density can deliver complete samples with reasonable purity (or efficiency, E). Compared to a selection method based solely on the slope of the structure function, the inclusion of the τ information boosts E from 60% to 75% while maintaining a highly complete sample (98%) even in the absence of color information. For a completeness of C = 90%, E is boosted from 80% to 85%. Conversely, C improves from 90% to 97% while maintaining E = 80% when imposing a lower limit on τ. With the aid of color selection, the purity can be further boosted to 96%, with C = 93%. Hence, selection methods based on variability will play an important role in the selection of quasars with data provided by upcoming large sky surveys, such as Pan-STARRS and the Large Synoptic Survey Telescope (LSST). For a typical (simulated) LSST cadence over 10 years and a photometric accuracy of 0.03 mag (achieved at i ≈ 22), C is expected to be 88% for a simple sample selection criterion of τ>100 days. In summary, given an adequate survey cadence, photometric variability provides an even better method than color selection for separating quasars from stars.

A systematic search for changing-look quasars in SDSS

CLM acknowledges support from the STFC Consolidated Grant (Ref. St/M001229/1). NPR acknowledges support from the STFC and the Ernest Rutherford Fellowship scheme. KH acknowledges support from STFC grant ST/M001296/1. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England.

A photometric redshift of z = 6.39 ± 0.12 for GRB 050904

In 2000, Lamb and Reichart predicted that gamma-ray bursts (GRBs) and their afterglows occur in sufficient numbers and at sufficient brightnesses at very high redshifts (z > 5) to eventually replace quasars as the preferred probe of element formation and reionization in the early universe and to be used to characterize the star-formation history of the early universe, perhaps back to when the first stars formed. Here we report the discovery of the afterglow of GRB 050904 and the identification of GRB 050904 as the first very high redshift GRB. We measure its redshift to be 6.39(+0.11,-0.12), which is consistent with the reported spectroscopic redshift (6.29 +/- 0.01). Furthermore, just redward of Ly-alpha the flux is suppressed by a factor of three on the first night, but returns to expected levels by the fourth night. We propose that this is due to absorption by molecular hydrogen that was excited to rovibrational states by the GRBs prompt emission, but was then overtaken by the jet. Now that very high redshift GRBs have been shown to exist, and at least in this case the afterglow was very bright, observing programs that are designed to capitalize on this science will likely drive a new era of study of the early universe, using GRBs as probes.Gamma-ray bursts (GRBs) and their afterglows are the most brilliant transient events in the Universe. Both the bursts themselves and their afterglows have been predicted to be visible out to redshifts of z ≈ 20, and therefore to be powerful probes of the early Universe. The burst GRB 000131, at z = 4.50, was hitherto the most distant such event identified. Here we report the discovery of the bright near-infrared afterglow of GRB 050904 (ref. 4). From our measurements of the near-infrared afterglow, and our failure to detect the optical afterglow, we determine the photometric redshift of the burst to be z = 6.39 - 0.12 + 0.11 (refs 5–7). Subsequently, it was measured spectroscopically to be z = 6.29 ± 0.01, in agreement with our photometric estimate. These results demonstrate that GRBs can be used to trace the star formation, metallicity, and reionization histories of the early Universe.

DISCOVERY OF A COSMOLOGICAL, RELATIVISTIC OUTBURST VIA ITS RAPIDLY FADING OPTICAL EMISSION

We report the discovery by the Palomar Transient Factory (PTF) of the transient source PTF11agg, which is distinguished by three primary characteristics: (1) bright (Rpeak = 18.3mag), rapidly fading (ΔR = 4mag in Δt = 2 days) optical transient emission; (2) a faint (R = 26.2 ± 0.2mag), blue (g � − R = 0.17 ± 0.29mag) quiescent optical counterpart; and (3) an associated year-long, scintillating radio transient. We argue that these observed properties are inconsistent with any known class of Galactic transients (flare stars, X-ray binaries, dwarf novae), and instead suggest a cosmological origin. The detection of incoherent radio emission at such distances implies a large emitting region, from which we infer the presence of relativistic ejecta. The observed properties are allconsistentwiththepopulationoflong-durationgamma-raybursts(GRBs),markingthefirsttimesuchanoutburst has been discovered in the distant universe independent of a high-energy trigger. We searched for possible highenergy counterparts to PTF11agg, but found no evidence for associated prompt emission. We therefore consider three possible scenarios to account for a GRB-like afterglow without a high-energy counterpart: an “untriggered” GRB (lack of satellite coverage), an “orphan” afterglow (viewing-angle effects), and a “dirty fireball” (suppressed high-energy emission). The observed optical and radio light curves appear inconsistent with even the most basic predictions for off-axis afterglow models. The simplest explanation, then, is that PTF11agg is a normal, on-axis long-durationGRBforwhichtheassociatedhigh-energyemissionwassimplymissed.However,wehavecalculated the likelihood of such a serendipitous discovery by PTF and find that it is quite small (≈2.6%). While not definitive, we nonetheless speculate that PTF11agg may represent a new, more common (>4 times the on-axis GRB rate at 90% confidence) class of relativistic outbursts lacking associated high-energy emission. If so, such sources will be uncovered in large numbers by future wide-field optical and radio transient surveys.

A Consistent Picture Emerges: A Compact X-ray Continuum Emission Region in the Gravitationally Lensed Quasar SDSS J0924+0219

We analyze the optical, UV, and X-ray microlensing variability of the lensed quasar SDSS J0924+ 0219 using six epochs of Chandra data in two energy bands (spanning 0.4-8.0 keV, or 1-20 keV in the quasar rest frame), 10 epochs of F275W (rest-frame 1089 angstrom) Hubble Space Telescope data, and high-cadence R-band (rest-frame 2770 angstrom) monitoring spanning 11 years. Our joint analysis provides robust constraints on the extent of the X-ray continuum emission region and the projected area of the accretion disk. The best-fit half-light radius of the soft X-ray continuum emission region is between 5 x 10(13) and 10(15) cm, and we find an upper limit of 10(15) cm for the hard X-rays. The best-fit soft-band size is about 13 times smaller than the optical size, and roughly 7GM(BH)/c(2) for a 2.8 x 10(8) M-circle dot black hole, similar to the results for other systems. We find that the UV emitting region falls in between the optical and X-ray emitting regions at 10(14) cm < r(1/2,UV) <3x10(15) cm. Finally, the optical size is significantly larger, by 1.5 sigma, than the theoretical thin-disk estimate based on the observed, magnification-corrected I-band flux, suggesting a shallower temperature profile than expected for a standard disk.

MULTIWAVELENGTH OBSERVATIONS OF RADIO-QUIET QUASARS WITH WEAK EMISSION LINES

We present radio and X-ray observations, as well as optical light curves, for a subset of 26 BL Lac candidates from the Sloan Digital Sky Survey (SDSS) lacking strong radio emission and with z < 2.2. Half of these 26 objects are shown to be stars, galaxies, or absorbed quasars. We conclude that the other 13 objects are active galactic nuclei (AGNs) with abnormally weak emission features; 10 of those 13 are definitively radio quiet, and, for those with available optical light curves, their level of optical flux variability is consistent with radio-quiet quasars. We cannot exclude the possibility that some of these 13 AGNs lie on the extremely radio-faint tail of the BL Lac distribution, but our study generally supports the notion that all BL Lac objects are radio-loud. These radio-quiet AGNs appear to have intrinsically weak or absent broad emission line regions (BELRs), and, based on their X-ray properties, we argue that some are low-redshift analogs to weak line quasars (WLQs). SDSS BL Lac searches are so far the only systematic surveys of the SDSS database capable of recovering such exotic low-redshift WLQs. There are 71 more z < 2.2 radio-quiet BL Lac candidates already identified in the SDSS, but not considered here, and many of those might be best unified with WLQs as well. Future studies combining low- and high-redshift WLQ samples will yield new insight on our understanding of the structure and formation of AGN BELRs.

Time Delay and Accretion Disk Size Measurements in the Lensed Quasar SBS?0909+532 from Multiwavelength Microlensing Analysis

We present three complete seasons and two half-seasons of Sloan Digital Sky Survey (SDSS) r-band photometry of the gravitationally lensed quasar SBS?0909+532 from the U.S. Naval Observatory, as well as two seasons each of SDSS g-band and r-band monitoring from the Liverpool Robotic Telescope. Using Monte Carlo simulations to simultaneously measure the systems time delay and model the r-band microlensing variability, we confirm and significantly refine the precision of the systems time delay to , where the stated uncertainties represent the bounds of the formal 1? confidence interval. There may be a conflict between the time delay measurement and a lens consisting of a single galaxy. While models based on the Hubble Space Telescope astrometry and a relatively compact stellar distribution can reproduce the observed delay, the models have somewhat less dark matter than we would typically expect. We also carry out a joint analysis of the microlensing variability in the r and g bands to constrain the size of the quasars continuum source at these wavelengths, obtaining log {(r s, r /cm)[cos i/0.5]1/2} = 15.3 ? 0.3 and log {(r s, g /cm)[cos i/0.5]1/2} = 14.8 ? 0.9, respectively. Our current results do not formally constrain the temperature profile of the accretion disk but are consistent with the expectations of standard thin disk theory.