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Register a new userAssessment of stochastic and deterministic models of 6304 quasar lightcurves from SDSS Stripe 82
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The optical light curves of many quasars show variations of tenths of a magnitude or more on time scales of months to years. This variation often cannot be described well by a simple deterministic model. We perform a Bayesian comparison of over 20 deterministic and stochastic models on 6304 QSO light curves in SDSS Stripe 82. We include the damped random walk (or Ornstein-Uhlenbeck [OU] process), a particular type of stochastic model which recent studies have focused on. Further models we consider are single and double sinusoids, multiple OU processes, higher order continuous autoregressive processes, and composite models. We find that only 29 out of 6304 QSO lightcurves are described significantly better by a deterministic model than a stochastic one. The OU process is an adequate description of the vast majority of cases (6023). Indeed, the OU process is the best single model for 3462 light curves, with the composite OU process/sinusoid model being the best in 1706 cases. The latter model is the dominant one for brighter/bluer QSOs. Furthermore, a non-negligible fraction of QSO lightcurves show evidence that not only the mean is stochastic but the variance is stochastic, too. Our results confirm earlier work that QSO light curves can be described with a stochastic model, but place this on a firmer footing, and further show that the OU process is preferred over several other stochastic and deterministic models. Of course, there may well exist yet better (deterministic or stochastic) models which have not been considered here.
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We present first results from our study of the properties of ~400 low redshift (z < 0.5) quasars, based on a large homogeneous dataset derived from the Stripe 82 area of the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7). For this sky region, deep (r~22.4) u,g,r,i,z images are available, up to ~2 mag deeper than standard SDSS images, allowing us to study both the host galaxies and the Mpc-scale environments of the quasars. This sample greatly outnumbers previous studies of low redshift quasar hosts, from the ground or from space. Here we report the preliminary results for the quasar host galaxies. We are able to resolve the host galaxy in ~80 % of the quasars. The quasar hosts are luminous and large, the majority of them in the range between M*-1 and M*-2, and with ~10 kpc galaxy scale-lengths. Almost half of the host galaxies are best fit with an exponential disk, while the rest are spheroid-dominated. There is a reasonable relation between the central black hole mass and the host galaxy luminosity.
We report on a blind survey for extragalactic radio variability that was carried out by comparing two epochs of data from the FIRST survey with a third epoch from a new 1.4 GHz survey of SDSS Stripe 82. The three epochs are spaced seven years apart and have an overlapping area of 60 deg^2. We uncover 89 variable sources down to the millijansky level, 75 of which are newly-identified, and we find no evidence for transient phenomena. This new sample of variable sources allows us to infer an upper limit to the mean characteristic timescale of AGN radio variability of 14 years. We find that only 1% of extragalactic sources have fractional variability f_var >3, while 44% of Galactic sources vary by this much. The variable sample contains a larger fraction of quasars than a comparable non-variable control sample, though the majority of the variable sources appear to be extended galaxies in the optical. This implies that either quasars are not the dominant contributor to the variability of the sample, or that the deep optical data allow us to detect the host galaxies of some low-z quasars. We use the new, higher resolution data to report on the morphology of the variable sources. Finally, we show that the fraction of sources that are variable remains constant or increases at low flux densities. This may imply that next generation radio surveys with telescopes like the Australian Square Kilometer Array Pathfinder and MeerKAT will see a constant or even increasing fraction of variable sources down into the submillijansky regime.
We present details of the construction and characterization of the coaddition of the Sloan Digital Sky Survey Stripe 82 ugriz imaging data. This survey consists of 275 deg$^2$ of repeated scanning by the SDSS camera of $2.5arcdeg$ of $delta$ over $-50arcdeg le alpha le 60arcdeg$ centered on the Celestial Equator. Each piece of sky has $sim 20$ runs contributing and thus reaches $sim2$ magnitudes fainter than the SDSS single pass data, i.e. to $rsim 23.5$ for galaxies. We discuss the image processing of the coaddition, the modeling of the PSF, the calibration, and the production of standard SDSS catalogs. The data have $r$-band median seeing of 1.1arcsec, and are calibrated to $le 1%$. Star color-color, number counts, and psf size vs modelled size plots show the modelling of the PSF is good enough for precision 5-band photometry. Structure in the psf-model vs magnitude plot show minor psf mis-modelling that leads to a region where stars are being mis-classified as galaxies, and this is verified using VVDS spectroscopy. As this is a wide area deep survey there are a variety of uses for the data, including galactic structure, photometric redshift computation, cluster finding and cross wavelength measurements, weak lensing cluster mass calibrations, and cosmic shear measurements.
We have analyzed the {it XMM-Newton} and {it Chandra} data overlapping $sim$16.5 deg$^2$ of Sloan Digital Sky Survey Stripe 82, including $sim$4.6 deg$^2$ of proprietary {it XMM-Newton} data that we present here. In total, 3362 unique X-ray sources are detected at high significance. We derive the {it XMM-Newton} number counts and compare them with our previously reported {it Chandra} Log$N$-Log$S$ relations and other X-ray surveys. The Stripe 82 X-ray source lists have been matched to multi-wavelength catalogs using a maximum likelihood estimator algorithm. We discovered the highest redshift ($z=5.86$) quasar yet identified in an X-ray survey. We find 2.5 times more high luminosity (L$_x geq 10^{45}$ erg s$^{-1}$) AGN than the smaller area {it Chandra} and {it XMM-Newton} survey of COSMOS and 1.3 times as many identified by XBootes. Comparing the high luminosity AGN we have identified with those predicted by population synthesis models, our results suggest that this AGN population is a more important component of cosmic black hole growth than previously appreciated. Approximately a third of the X-ray sources not detected in the optical are identified in the infrared, making them candidates for the elusive population of obscured high luminosity AGN in the early universe.
The SDSS-III BOSS Quasar survey will attempt to observe z>2.15 quasars at a density of at least 15 per square degree to yield the first measurement of the Baryon Acoustic Oscillations in the Ly-alpha forest. To help reaching this goal, we have developed a method to identify quasars based on their variability in the u g r i z optical bands. The method has been applied to the selection of quasar targets in the SDSS region known as Stripe 82 (the Southern equatorial stripe), where numerous photometric observations are available over a 10-year baseline. This area was observed by BOSS during September and October 2010. Only 8% of the objects selected via variability are not quasars, while 90% of the previously identified high-redshift quasar population is recovered. The method allows for a significant increase in the z>2.15 quasar density over previous strategies based on optical (ugriz) colors, achieving a density of 24.0 deg^{-2} on average down to g~22 over the 220 deg^2 area of Stripe 82. We applied this method to simulated data from the Palomar Transient Factory and from Pan-STARRS, and showed that even with data that have sparser time sampling than what is available in Stripe 82, including variability in future quasar selection strategies would lead to increased target selection efficiency in the z>2.15 redshift range. We also found that Broad Absorption Line quasars are preferentially present in a variability than in a color selection.
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