ABSTRACT
This paper reports an update to the QUEST1 (QUasar Equatorial Survey Team, Phase 1) Variability Survey (QVS) light curve catalog, which links QVS instrumental magnitude light curves to Sloan Digital Sky Survey (SDSS) objects and photometry. In the time since the original QVS catalog release, the overlap between publicly available SDSS data and QVS data has increased by 8% in sky coverage and 16,728 in number of matched objects. The astrometric matching and the treatment of SDSS masks have been refined for the updated catalog. We report on these improvements and present multiple bandpass light curves, global variability information, and matched SDSS photometry for 214,941 QUEST1 objects.
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1. INTRODUCTION
Existent and upcoming large-scale surveys are tapping into time-domain astronomy with unprecedented coverage areas and scan frequencies, facilitating fundamental and significant gains in research by changing our static view of the universe into a dynamic view. Whereas, for example, the Sloan Digital Sky Survey (SDSS) scanned a much larger portion of the sky with fewer repeat scans, The QUasar Equatorial Survey Team, Phase 1 (QUEST1) concentrated on a smaller region and obtained more scans.
The QUEST Variability Survey (QVS) is a rich data set that has proven valuable in several areas of the astrophysical community. In the time since its publication, this catalog has been utilized in studies which correlate optical quasar variability with flux other wavelength regimes (Rengstorf et al. 2006), variability with black hole mass (Wold et al. 2007), and Eddington ratio with optical variability (Wilhite et al. 2008), searches for transients in galaxy clusters (Sand et al. 2008), and Lyman-alpha flux correlations of QSO pairs (Marble et al. 2008).
This updated catalog will benefit the astronomical community by providing a larger sample of light curves over a wider region of the sky. The main improvements guiding this update to the QVS light curve catalog are (1) an increased coverage in the publicly released SDSS data, (2) a more sophisticated treatment of SDSS mask information, and (3) an improved (i.e., decreased) astrometric tolerance in the QVS–SDSS matching process
1.1. The Data Set
QUEST1 constructed a 16-chip CCD camera (Baltay et al. 2002) to be used on the 1 m Schmidt telescope at the Llano del Hato National Astronomical Observatory in Venezuela. Each CCD in the 4 × 4 array is front illuminated and has 2048 × 2048, 15 μm pixels. Normally operated in the drift-scan mode, an equatorial scan will expose any point on the sky for 143 s and result in limiting magnitudes of B = 18.5, V = 19.2, and R = 19.5 at S/N = 10. The plate scale of the system is 1
3 pixel−1 and, considering the gaps between chip rows, the camera has a total sky coverage of 5.53 deg2.
The QVS is a set of observations optimized to study multiple variability-driven projects, including Type 1a supernovae, RR Lyrae stars (Vivas et al. 2004), and quasar variability (Rengstorf et al. 2004a). These data were collected between 1999 February and 2001 April. The QVS operated in the drift-scan mode and covers a strip of high-Galactic-latitude, equatorial sky 2
4 wide in declination, centered at δ = −1°, between R.A. = 10h and 15h30m. The low-R.A. limit was set by astronomical twilight and the high-R.A. limit by proximity to the Galactic plane. Data were collected in three distinct observing seasons—1999 February–March, 2000 February–March, and 2001 March–April—resulting in several observations per lunation and multiple lunations per year for three years. Over all scans, seeing averaged 28 ± 04 and dropped as low as 17 under optimal conditions. The QVS used an RBRV filter set across the four rows of camera chips and achieved a limiting magnitude of r = 19.07 at 90% completeness when compared to SDSS photometry. The entire data reduction and analysis process for all QUEST drift-scan data is detailed in Rengstorf et al. (2004b).
1.2. Results from the First-edition Catalog
The first edition of the QVS light curve catalog, dubbed the 200k catalog, is also presented in Rengstorf et al. (2004b). QVS light curves were left in instrumental magnitudes to avoid additional errors that would have been added in the magnitude transformation process. There is an overlap of approximately 135 deg2 between the QVS and the SDSS Second Data Release (DR2), ignoring additional losses due to SDSS DR2 masks. Figure 1 shows the QVS footprint with SDSS DR2 overlap. The inter-row gaps are slightly larger in the QVS than on the camera itself. This is due to slight variations in the central decl. value from one observation to the next. Small amounts were trimmed on either side to consider only the union of all scans in the QVS. With an astrometric tolerance of 20, a total of 198,213 QVS objects were matched to SDSS DR2 objects. The average astrometric offset was 045 ± 014. Full details are available in Rengstorf et al. (2004b).
Figure 1. Plot showing the QVS region as four strips of equatorial sky with the SDSS DR2 overlap shaded with dashed lines and the SDSS DR6 additional overlap shaded with solid lines.
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Standard image High-resolution image2. IMPROVEMENTS IN THE SECOND EDITION
2.1. Increased Overlap with Sloan Public Data
At the time of publication of Rengstorf et al. (2004b), SDSS DR2 had been released (Abazajian et al. 2004), presenting more than 88 million objects over 3324 deg2. Four subsequent data releases have occurred. The SDSS sixth public data release (DR6) is now available (Adelman-McCarthy et al. 2008), presenting 287 million objects over 9583 deg2. This increase in sky coverage removed most of the gaps in the QVS–SDSS overlap. Figure 1 also shows the additional overlap from SDSS DR6. The new QVS light curve catalog covers 147 deg2, an 8% increase from the 200k catalog. The entire QVS footprint is 152.5 deg2, so only 4%, again ignoring decreased coverage due to SDSS masks, of the QVS is not reported. The new region of overlap contains 17,475 objects. This indicates that a small number of objects (747) were effectively removed from the first-edition catalog due to improvements and refinements to the SDSS-mask treatment.
2.2. Mask Treatment
The first-edition catalog utilized a somewhat crude mask treatment. Mask data for SDSS DR2 holes only were used to make maximal-area, rectangular masks along the R.A.–decl. axes in the QVS data. This streamlined the entire process, but slightly overestimated the total area of SDSS masks. The improved treatment for the new catalog utilized the same masking software used to produce the random catalogs in Myers et al. (2006, see Section 2.2) and Myers et al. (2007, see Appendix A) as discussed therein. For each mask, the full polygonal area is now considered, which will tend to shrink the mask area at least marginally compared to using the largest possible rectangular area given the mask vertices. However, the addition of masks for bleeding and bright star trails have caused a slight (less than 0.4%) decrease in the number of matched objects in the first-edition catalog region. As with the first-edition catalog, the masks for less-than-optimal seeing are ignored.
2.3. Astrometric Matching
The astrometric tolerance for the new catalog has decreased from 20 to 17. As with the tolerance from the first-edition catalog, this is an adopted value, determined empirically. This new value is the result of improved statistics due to the somewhat larger sample and retains the bulk of the random distribution about a systematically offset mean value. All QVS objects present in the first-edition catalog were re-matched to the same SDSS object and a statistically negligible number of objects (44) had matches with an astrometric difference greater than 17. Figure 2 shows a contour plot of all valid matches between the QVS and SDSS DR6. The mean offset of the QVS astrometry is 041 ± 015 to the NW of the SDSS astrometry (Δα = 029 ± 016; Δδ = 029 ± 014). The distribution about the mean value appears close to Gaussian (kurtosis = 2.4 in R.A. dir. and 3.3 in decl. dir.; mode of distribution offset from mean by Δα = 002 and Δδ = 003).
Figure 2. Contour plot showing offset between QVS and SDSS astrometry for 214,941 objects. Contour lines are roughly logarithmic. The large error bar shows the standard deviation about the mean offset value. The small solid diamond shows the peak of the distribution.
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Standard image High-resolution image2.4. Limiting Magnitude
A limiting magnitude for this light curve catalog is obtained by examining SDSS magnitude histograms for all catalog objects. Due to criteria in the data reduction and analysis Rengstorf et al. (2004b), every object is required to be detected in both of the R filters. Once matched with SDSS DR6, it is noted that every object also has an r-band detection. The SDSS r filter has, therefore, been used to determine a limiting magnitude for the QVS. Figure 3 shows an r-mag histogram with the spline fits used to determine the limiting magnitude at 90% completeness. The distribution peaks at r = 18.94. Since SDSS DR6 reports a 95% detection repeatability down to r = 22.2 for point sources, we assume that the decrease in the histogram fainter than r = 18.94 is due entirely to the limiting magnitude of the QUEST data. The bottom plot in Figure 3 compares the fit on the bright side of the distribution peak to the fit of the full data. This gives a measure of completeness and limiting magnitude, which falls below 90% at r = 19.07. For completeness, and to give a rough estimate for magnitude limits in the other SDSS filters, Figure 4 shows histograms for each of the five SDSS filters.
Figure 3. Top plot shows a histogram of SDSS r magnitudes for every object in the light curve catalog. The black line shows spline fit to all data. The dotted line shows spline fit to data brighter than r = 18.9 with linear interpolation to fainter magnitudes. The bottom plot shows the ratio of bright-data spline to full-data spline as a measure of completeness.
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Figure 4. Histograms for all five SDSS magnitudes. Note that x-axis scales are not the same between histograms. The dotted line in r histogram shows the 90% complete limiting magnitude. Numbers in the top-left corner of each plot relate the number of objects to SDSS detections in the given filter.
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3. LIGHT CURVE CATALOG, VOLUME 2
The second QVS light curve catalog (LCCv2) has the same format as the first version of the catalog. LCCv2 is divided into five tables: one main table listing global parameters, and one file for each of the four broadband filters used in the QVS. The main table lists a unique running catalog number (Column 1), the QUEST identifier (Column 2), the QUEST1 R.A. (Column 3), and Decl. (Column 4) (J2000.0) in decimal degrees, the Global Confidence Level (GCL) for variability (Column 5; see Rengstorf et al. (2004b) and Rengstorf et al. (2006, Section 3) for a complete description of this parameter), the number of data points in each of the four light curves (Columns 6–9), the SDSS DR6 object identification (objID) string (Column 10), and the SDSS DR6 u, g, r, i, & z model magnitudes.13 Table 1 shows a portion of the main catalog table. 14
Table 1. QVS Light Curve Catalog, Version 2: Main Table
| R.A. | Decl. | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| CatNo | QUEST Identifier | (decimal degrees) | GCL | Nr1 | Nb | Nr3 | Nv | objID | u | g | r | i | z | |
| 000001 | J100000.1-011759.3 | 150.000290 | −1.299796 | 19.72 | 19 | 0 | 14 | 19 | 587729151456837855 | 21.094 | 19.070 | 18.225 | 17.897 | 17.660 |
| 000002 | J100000.1-021155.0 | 150.000473 | −2.198605 | 27.91 | 19 | 18 | 18 | 18 | 587729150383096034 | 23.637 | 16.961 | 16.376 | 16.187 | 16.124 |
| 000003 | J100000.3-000521.4 | 150.001114 | −0.089266 | 16.20 | 15 | 0 | 20 | 21 | 588848899899523174 | 19.477 | 17.742 | 17.082 | 16.836 | 16.725 |
| 000004 | J100000.3-011941.3 | 150.001129 | −1.328150 | 35.10 | 15 | 11 | 10 | 16 | 587729151456837767 | 16.508 | 15.371 | 14.960 | 14.820 | 14.769 |
| 000005 | J100000.4-015835.6 | 150.001709 | −1.976558 | 28.62 | 21 | 18 | 17 | 20 | 587725083578859525 | 18.801 | 17.766 | 17.575 | 17.535 | 17.585 |
Only a portion of this table is shown here to demonstrate its form and content. A machine-readable version of the full table is available.
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The global confidence level (GCL) is a parameter used to determine the probability that an object detected as variable is not variable due to random fluctuations. The GCL is an average of a variability confidence level (CL) for each bandpass, weighted by the fraction of valid scans in that bandpass in which the object was detected. In each bandpass, the CL is calculated for each object using the χ2 probability function as presented in Press et al. (1992). The CL value gives the probability that an object with no intrinsic variability should have a smaller χ2 value than other nonvariable objects in the ensemble solution at that magnitude. Please see Rengstorf et al. (2004b) for a complete description.
The four light curve tables have a variable number of columns per row, depending on the number of data points in the light curve. An individual bandpass table lists the same unique running catalog number as is present in the main table (Column 1), and, for each point in the light curve, a set of three data: a truncated Julian Date (JD − 2451244.0), the light curve (instrumental) magnitude, and the error on the light curve magnitude. Considering the drift scan nature of the QVS, the Julian Date is calculated at the center of 15 bins in R.A., resulting in an overall precision of three minutes in JD (σJD = 0.002). Note that while standard R, B, and V filters were used in the QUEST1 camera, this catalog lists only instrumental, cloud-corrected magnitudes and are denoted as r1, b, r3, and v. Table 2 shows a portion of the r1 light curve catalog. See Rengstorf et al. (2004b) for a complete description of the instrumental magnitude and magnitude error calculations.
Table 2. LCCv2: r1 Light Curve Table
| ID # | jd1 | mlc1 | σlc1 | jd2 | mlc2 | σlc2 | ⋅⋅⋅ | jdN | mlcN | σlcN |
|---|---|---|---|---|---|---|---|---|---|---|
| 000001 | 8.19652 | 15.191 | 0.027 | 21.10138 | 15.166 | 0.031 | ⋅⋅⋅ | 768.02291 | 15.175 | 0.031 |
| 000002 | 8.19652 | 13.391 | 0.009 | 21.10138 | 13.368 | 0.009 | ⋅⋅⋅ | 769.02569 | 13.369 | 0.008 |
| 000003 | 8.19652 | 14.469 | 0.013 | 21.10138 | 14.49 | 0.015 | ⋅⋅⋅ | 390.02222 | 14.479 | 0.012 |
| 000004 | 8.19652 | 11.962 | 0.003 | 21.10138 | 11.978 | 0.003 | ⋅⋅⋅ | 768.02291 | 11.954 | 0.004 |
| 000005 | 8.19652 | 14.606 | 0.019 | 21.10138 | 14.620 | 0.023 | ⋅⋅⋅ | 769.02569 | 14.601 | 0.022 |
Only a portion of this table is shown here to demonstrate its form and content. The individual light curve catalogs for the r1, b, r3, and v observations are available in four separate tables within a tar.gz file.
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4. CONCLUSION
An updated light curve catalog from the QUEST1 Variability Survey is presented. Using the sixth public data release from the Sloan Digital Sky Survey, 12 deg2 in sky coverage and 16,728 matched objects have been added to the original light curve catalog, first published in 2004. These additions to the catalog are due to increased overlap between QUEST and SDSS and to improved matching and mask treatment algorithms.
The authors thank A. D. Myers for insightful discussions about the treatment of SDSS mask information. We also thank the referee for some very helpful comments and suggestions.
This work is based on observations obtained at the Llano del Hato National Astronomical Observatory, operated by the Centro de Investigaciones de Astronomía for the Ministerio de Ciencia y Tecnologia of Venezuela.
This work was funded in part by a grant from the Purdue Research Foundation.
Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web site is http://www.sdss.org/.
The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.
Footnotes
- *
Based on observations obtained at the Llano del Hato National Astronomical Observatory, operated by the Centro de Investigaciones de Astronomía for the Ministerio de Ciencia y Tecnologia of Venezuela.
- 13
SDSS model magnitudes simultaneously give an accurate magnitude for both point and extended sources, similar to PSF and Petrosian magnitudes, respectively.
- 14
All light curve tables are available electronically and by contacting the author.