Ensemble Variability of Near-Infrared-Selected Active Galactic Nuclei

We present the properties of the ensemble variability $V$ for nearly 5000 near-infrared (NIR) AGNs selected from the catalog of Quasars and Active Galactic Nuclei (13th Ed.) and the SDSS-DR7 quasar catalog. From 2MASS, DENIS, and UKIDSS/LAS point source catalogs, we extract 2MASS-DENIS and 2MASS-UKIDSS counterparts for cataloged AGNs by catalog cross-identification. We further select variable AGNs based on an optimal criterion for selecting the variable sources. The sample objects are divided into subsets according to whether NIR light originates by optical or NIR emission in the rest frame; and we examine the correlations of the ensemble variability with the rest-frame wavelength, redshift, luminosity, and rest-frame time lag. In addition, we also examine the correlations of variability amplitude with optical variability, radio intensity, and radio-to-optical flux ratio. The rest-frame optical variability of our samples shows known negative correlations with luminosity and positive correlations with rest-frame time lag (i.e., the structure function, SF). However, no well-known negative correlation exists between the rest wavelength and optical variability. This inconsistency might be due to a biased sampling of high-redshift AGNs. NIR variability in the rest frame is anticorrelated with the rest wavelength, which is consistent with previous suggestions. However, correlations of NIR variability with luminosity and rest-frame time lag are the opposite of these correlations of the optical variability; that is, the NIR variability is positively correlated with luminosity but negatively correlated with the rest-frame time lag. Because these trends are qualitatively consistent with the properties of radio-loud quasars reported by some previous studies, most of our sample objects are probably radio-loud quasars. Finally, we also discuss the negative correlations seen in the NIR SFs.Comment: 13 pages, 10 figures, Accepted for publication in Ap

Discovery of the Optical Transient of GRB 990308

The optical transient of the faint gamma-ray burst GRB 990308 was detected by the QUEST camera on the Venezuelan 1 m Schmidt telescope starting 3.28 hr after the burst. Our photometry gives V= 18.32 ± 0.07, R = 18.14 ± 0. 06, B =18.65, and R=18.22 ± 0.05 for times ranging from 3.28 to 3.47 hr after the burst. The colors correspond to a spectral slope of close to f ∝ v^1/3 . Within the standard synchrotron fireball model, this rquires that the external medium be less dense than 10^4 cm^-3, the electrons contain more than 20% of the shock energy, and the magnetic field energy be less than 24% of the energy in the electrons for normal interstellar or circumstellar densities. We also report upper limits of V \u3e 12.0 at 132 s (with LOTIS), V \u3e 13.4 from 132 to 1029 s (with LOTIS), V \u3e 15.3 at 28.2 minutes (with Super-LOTIS), and a 8.5 GHz flux of less than 114 μJy at 110 days (with the Very Large Array). Wisconsin-Indiana-Yale-NOAO 3.5 m and Keck 10 m telescopes reveal this location to be empty of any host galaxy to R \u3e 25.7and K \u3e 123.3. The lack of a host galaxy likely implies that it is either substantially subluminous or more distant than a redshift of ∼1.2

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 1000A to 6000A. 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 time scale 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 time scales 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 PTF and LSST.Comment: 33 pages, 19 figures, replaced with accepted version. Catalog is available at http://www.astro.washington.edu/users/ivezic/macleod/qso_dr7

Measuring Lensing Magnification of Quasars by Large Scale Structure using the Variability-Luminosity Relation

We introduce a technique to measure gravitational lensing magnification using the variability of type I quasars. Quasars' variability amplitudes and luminosities are tightly correlated, on average. Magnification due to gravitational lensing increases the quasars' apparent luminosity, while leaving the variability amplitude unchanged. Therefore, the mean magnification of an ensemble of quasars can be measured through the mean shift in the variability-luminosity relation. As a proof of principle, we use this technique to measure the magnification of quasars spectroscopically identified in the Sloan Digital Sky Survey, due to gravitational lensing by galaxy clusters in the SDSS MaxBCG catalog. The Palomar-QUEST Variability Survey, reduced using the DeepSky pipeline, provides variability data for the sources. We measure the average quasar magnification as a function of scaled distance (r/R200) from the nearest cluster; our measurements are consistent with expectations assuming NFW cluster profiles, particularly after accounting for the known uncertainty in the clusters' centers. Variability-based lensing measurements are a valuable complement to shape-based techniques because their systematic errors are very different, and also because the variability measurements are amenable to photometric errors of a few percent and to depths seen in current wide-field surveys. Given the data volume expected from current and upcoming surveys, this new technique has the potential to be competitive with weak lensing shear measurements of large scale structure.Comment: Accepted for publication in Ap

The QUEST RR Lyrae Survey: Confirmation of the Clump at 50 kpc and Other Over-Densities in the Outer Halo

We have measured the periods and light curves of 148 RR Lyrae variables from V=13.5 to 19.7 from the first 100 sq. degrees of the QUEST RR Lyrae survey. Approximately 55% of these stars belong to the clump of stars detected earlier by the Sloan Digital Sky Survey. According to our measurements, this feature has ~10 times the background density of halo stars, spans at least 37.5 deg by 3.5 deg in right ascension and declination (>=30 by >=3 kpc), lies ~50 kpc from the Sun, and has a depth along the line of sight of ~5 kpc (1 sigma). These properties are consistent with the recent models that suggest it is a tidal stream from the Sgr dSph galaxy. The mean period of the type ab variables, 0.58 d, is also consistent. In addition, we have found two smaller over-densities in the halo, one of which may be related to the globular cluster Pal 5.Comment: 12 pages (including 4 figures). Accepted for publication in the ApJ Letter

Spectral variability of quasars from multi-epoch photometric data in the Sloan Digital Sky Survey Stripe 82

We present a new approach to analysing the dependence of quasar variability on rest-frame wavelengths. We exploited the spectral archive of the Sloan Digital Sky Survey (SDSS) to create a sample of more than 9000 quasars in the Stripe 82. The quasar catalogue was matched with the Light Motion Curve Catalogue for SDSS Stripe 82 and individual first-order structure functions were computed. The structure functions are used to create a variability indicator that is related to the same intrinsic timescales for all quasars (1 to 2 yr in the rest frame). We study the variability ratios for adjacent SDSS filter bands as a function of redshift. While variability is almost always stronger in the bluer passband compared to the redder, the variability ratio depends on whether strong emission lines contribute to either one band or the other. The variability ratio-redshift relations resemble the corresponding colour index-redshift relations. From the comparison with Monte Carlo simulations of variable quasar spectra we find that the observed variability ratio-redshift relations are closely fitted assuming that (a) the r.m.s. fluctuation of the quasar continuum follows a power law-dependence on the intrinsic wavelength with an exponent -2 (i.e., bluer when brighter) and (b) the variability of the emission line flux is only about 10% of that of the underlying continuum. These results, based upon the photometry of more than 8000 quasars, confirm the previous findings by Wilhite et al. (2005) from 315 quasars with repeated SDSS spectroscopy. Finally, we find that quasars with unusual spectra and weak emission lines tend to have less variability than conventional quasars. This trend is opposite to what is expected from the dilution effect of variability due to line emission and may be indicative of high Eddington ratios in these unconventinal quasars.Comment: Accepted for publication in Astronomy and Astrophysic

Discovery of the Optical Transient of the Gamma Ray Burst 990308

The optical transient of the faint Gamma Ray Burst 990308 was detected by the QUEST camera on the Venezuelan 1-m Schmidt telescope starting 3.28 hours after the burst. Our photometry gives $V = 18.32 \pm 0.07$, $R = 18.14 \pm 0.06$, $B = 18.65 \pm 0.23$, and $R = 18.22 \pm 0.05$ for times ranging from 3.28 to 3.47 hours after the burst. The colors correspond to a spectral slope of close to $f_{\nu} \propto \nu^{1/3}$. Within the standard synchrotron fireball model, this requires that the external medium be less dense than $10^{4} cm^{-3}$, the electrons contain $> 20$ of the shock energy, and the magnetic field energy must be less than 24% of the energy in the electrons for normal interstellar or circumstellar densities. We also report upper limits of $V > 12.0$ at 132 s (with LOTIS), $V > 13.4$ from 132-1029s (with LOTIS), $V > 15.3$ at 28.2 min (with Super-LOTIS), and a 8.5 GHz flux of $< 114 \mu Jy$ at 110 days (with the Very Large Array). WIYN 3.5-m and Keck 10-m telescopes reveal this location to be empty of any host galaxy to $R > 25.7$ and $K > 23.3$. The lack of a host galaxy likely implies that it is either substantially subluminous or more distant than a red shift of $\sim 1.2$.Comment: ApJ Lett submitted, 5 pages, 2 figures, no space for 12 coauthor

The quest RR lyrae survey. I. The first catalog

With the 1 m Schmidt telescope of the Llano del Hato Observatory and the QUEST CCD camera, 380 deg 2 of the sky have been surveyed for RR Lyrae variables in a band 2.°3 wide in declination (centered at δ = -1°) and covering right ascensions from 4. h1 to 6. h1 and from 8. h0 to 17. h0. The bright limit (due to CCD saturation) and the faint limit are V∼13.5 and ∼19.7, respectively, which correspond to ∼4 and ∼60 kpc from the Sun. We present a catalog of the positions, amplitudes, mean magnitudes, periods, and light curves of the 498 RR Lyrae variables that have been identified in this region of the sky. The majority of these stars (86%) are new discoveries. The completeness of the survey has been estimated from simulations that model the periods and light curves of real RR Lyrae variables and take into account the pattern of epochs of observation. While the completeness of the survey varies with apparent magnitude and with position, almost everywhere in the surveyed region it is quite high (\u3e80%) for the type ab RR Lyrae variables and between 30% and 90% for the low-amplitude type c variables

Discovery of the optical transient of GRB 990308

The optical transient of the faint gamma-ray burst GRB 990308 was detected by the QUEST camera on the Venezuelan 1 m Schmidt telescope starting 3.28 hr after the burst. Our photometry gives V= 18.32 ± 0.07, R = 18.14 ± 0.06, B = 18.65 ± 0.23, and R = 18.22 ± 0.05 for times ranging from 3.28 to 3.47 hr after the burst. The colors correspond to a spectral slope of close to fv ∝ v1/3. Within the standard synchrotron fireball model, this requires that the external medium be less dense than 104 cm-3, the electrons contain more than 20% of the shock energy, and the magnetic field energy be less than 24% of the energy in the electrons for normal interstellar or circumstellar densities. We also report upper limits of V \u3e 12.0 at 132 s (with LOTIS), V \u3e 13.4 from 132 to 1029 s (with LOTIS), V \u3e 15.3 at 28.2 minutes (with Super-LOTIS), and a 8.5 GHz flux of less than 114 μJy at 110 days (with the Very Large Array). Wisconsin-Indiana-Yale-NOAO 3.5 m and Keck 10 m telescopes reveal this location to be empty of any host galaxy to R \u3e 25.7 and K \u3e 23.3. The lack of a host galaxy likely implies that it is either substantially subluminous or more distant than a redshift of ∼1.2

Discovery of the bright trans-Neptunian object 2000 EB173

We describe the discovery circumstances and photometric properties of 2000 EB173, now one of the brightest trans-Neptunian objects (TNOs) with opposition magnitude mR = 18.9 and also one of the largest Plutinos, found with the drift-scanning camera of the Quasar Equatorial Survey Team, attached to the 1 m Schmidt telescope of the National Observatory of Venezuela. We measure B-V = 0.99 ± 0.14 and V-R = 0.57 ± 0.05, a red color observed for many fainter TNOs. At our magnitude limit mK = 20.1 ± 0.20, our single detection reveals a sky density of 0.015+0.034-0.012 TNOs per square degree (the error bars are 68% confidence limits), consistent with fainter surveys showing a cumulative number proportional to 100.5mR. Assuming an inclination distribution of TNOs with FWHM exceeding 30°, it is likely that 100 to several hundred objects brighter than mR = 20.1 remain to be discovered