Long-term optical variability properties of the Palomar–Green quasars

We present results from a monitoring programme of 42 quasars from the Palomar–Green sample. The objects were observed for seven years at the Wise Observatory, as part of a long-term effort to monitor AGN of various types. This is the most extensive programme of its kind carried out to date on a well-defined, optically selected quasar sample. The typical sampling interval is ∼ 40 d. One-third of the quasars were observed at ∼ 60 epochs, and the rest at ∼ 30 epochs in two bands (B and R) with a photometric accuracy of ∼ 0.01 mag. We present light curves for all of the sources, and discuss the sample variability properties. All of the quasars in the sample varied during the campaign with intrinsic rms amplitudes of 5 <σB<34 per cent and 4 <σR<26 per cent. The rms amplitude and colour for the entire sample are σB=14 per cent, σR=12 per cent and σB-R=5 per cent. On time-scales of 100 to 1000 d the power spectra of the sources have a power-law shape, Pν≺ν−γ, with γ≈ 2.0 and a spread ≲ 0.6. At least half of the quasars, particularly those that are most variable, become bluer when they brighten, and the rest do not show this behaviour. We quantify this phenomenon, which has been observed previously mainly in Seyfert galaxies. The quasars which are most variable tend also to exhibit asymmetry in their variations, in the sense that the brightening phases last longer than the fading phases. We have searched for correlations between the measured variability properties and other parameters of the quasars, such as luminosity, redshift, radio loudness and X-ray slope. We find several new correlations, and reproduce some of the correlations reported by previous studies. Among them are an anticorrelation of variability amplitude with luminosity, a trend of the autocorrelation time-scale with luminosity, and an increase in variability amplitude with Hβ equivalent width. However, all of these trends have a large scatter despite the low observational uncertainties.

[1]  Matthias Dietrich,et al.  Structure and Kinematics of Quasar Broad Line Regions , 1999 .

[2]  B. M. Peterson,et al.  New Constraints on the Continuum Emission Mechanism of Active Galactic Nuclei: Intensive Monitoring of NGC 7469 in the X-Ray and Ultraviolet , 1998, astro-ph/9804135.

[3]  The Variability Properties of X-Ray-steep and X-Ray-flat Quasars , 1998, astro-ph/9803248.

[4]  S. Mineshige,et al.  Optical Variability in Active Galactic Nuclei: Starbursts or Disk Instabilities? , 1997, astro-ph/9712006.

[5]  M. Bershady,et al.  Variable Extended Objects in Selected Area 57 , 1997, astro-ph/9711053.

[6]  M. Elvis,et al.  Deriving the Quasar Luminosity Function from Accretion-Disk Instabilities , 1997, astro-ph/9703096.

[7]  Itziar Aretxaga,et al.  QSO variability: probing the starburst model , 1996, astro-ph/9609055.

[8]  T. Alexander,et al.  Is AGN Variability Correlated with Other AGN Properties?—ZDCF Analysis of Small Samples of Sparse Light Curves , 1997 .

[9]  Dan Maoz,et al.  Astronomical Time Series , 1997 .

[10]  John N. Bahcall,et al.  Hubble Space Telescope Images of a Sample of 20 Nearby Luminous Quasars , 1996, astro-ph/9611163.

[11]  Paul S. Smith,et al.  Measurement of the Broad-Line Region Size in Two Bright Quasars , 1996, astro-ph/9608195.

[12]  Cambridge,et al.  THE QSO VARIABILITY-LUMINOSITY-REDSHIFT RELATION , 1996, astro-ph/9608057.

[13]  G. Bao,et al.  Anticorrelation of Variability Amplitude with X-Ray Luminosity for Active Galactic Nuclei , 1996 .

[14]  P. Hewett,et al.  The Radio Properties of Optically Selected Quasars. III. Comparison Between Optical and X-Ray Selected Samples , 1996, astro-ph/9606158.

[15]  J. Baldwin,et al.  Optical monitoring of luminous AGN - I. Radio-loud quasars , 1996 .

[16]  M. Hawkins.  Dark Matter from Quasar Microlensing , 1995 .

[17]  T. Alexander.  The signature of microlensing in QSO variability-redshift correlations , 1995, astro-ph/9502027.

[18]  B. Wilkes,et al.  The Soft X-Ray Properties of a Complete Sample of Optically Selected Quasars. II. Final Results , 1994, astro-ph/9609164.

[19]  S. Mineshige,et al.  Is a Black Hole Accretion Disk in a Self-organized Critical State? , 1994 .

[20]  M. Bershady,et al.  THE ENSEMBLE VARIABILITY PROPERTIES OF FAINT QSOS , 1994, astro-ph/9407003.

[21]  Richard G. McMahon,et al.  The variability of optically selected quasars , 1994 .

[22]  Paul S. Smith,et al.  How fast do quasar emission lines vary? First results from a program to monitor the Balmer lines of the Palomar-Green quasars , 1994 .

[23]  I. M. McHardy,et al.  On the nature of rapid X-ray variability in active galactic nuclei. , 1993 .

[24]  Interpretation of the Ionizing Photon Deficit of AGN , 1993 .

[25]  A. Cimatti,et al.  Optical variability of faint quasars , 1993 .

[26]  Paul S. Smith,et al.  Variations of the ultraviolet Fe II and Balmer continuum emission in the Seyfert galaxy NGC 5548 , 1993 .

[27]  Arlo U. Landolt,et al.  UBVRI Photometric Standard Stars in the Magnitude Range 11 , 1992 .

[28]  T. Boroson,et al.  The Emission-Line Properties of Low-Redshift Quasi-stellar Objects , 1992 .

[29]  L. Miller,et al.  The high surface density of bright ultraviolet-excess quasars. , 1992 .

[30]  C. Koen.  Confidence intervals for the Lutz–Kelker correction , 1992 .

[31]  S. Rawlings,et al.  A spectrophotometric study of BQS quasars , 1992 .

[32]  E. Giallongo,et al.  Optical Variability of Quasars: Statistics and Cosmological Properties , 1991 .

[33]  C. Lawrence,et al.  Optical polarization of a complete sample of radio sources , 1991 .

[34]  Wei Zheng,et al.  Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. I, an 8 month campaign of monitoring NGC 5548 with IUE , 1991 .

[35]  G. Berriman,et al.  An Optical Polarization Survey of the Palomar-Green Bright Quasar Sample , 1990 .

[36]  J. Bregman.  Continuum radiation from active galactic nuclei , 1990 .

[37]  Maarten Schmidt,et al.  VLA observations of objects in the Palomar Bright Quasar Survey , 1989 .

[38]  J. Scargle.  Studies in astronomical time series analysis. III - Fourier transforms, autocorrelation functions, and cross-correlation functions of unevenly spaced data , 1989 .

[39]  R. Mushotzky,et al.  On the ratio of the infrared-to-ultraviolet continuum to the X-rays in quasars and active galaxies , 1989 .

[40]  R. Green,et al.  Quasar Emission Line Strengths , 1988 .

[41]  G. Kriss.  X-ray, optical, and infrared luminosity correlations in radio-quiet quasars , 1988 .

[42]  Maarten Schmidt,et al.  Continuum energy distributions of quasars in the Palomar-Green Survey , 1987 .

[43]  P. Stetson.  DAOPHOT: A COMPUTER PROGRAM FOR CROWDED-FIELD STELLAR PHOTOMETRY , 1987 .

[44]  Maarten Schmidt,et al.  X-ray observations of the bright quasar survey , 1986 .

[45]  G. Rieke,et al.  Variability and the nature of QSO optical-infrared continua , 1985 .

[46]  A. G. Smith,et al.  Optical variability, absolute luminosity, and the Hubble diagram for QSOs. , 1983 .

[47]  H. Netzer,et al.  Variability of optically selected high redshift quasars , 1983 .

[48]  Richard F. Green,et al.  Quasar evolution derived from the Palomar bright quasar survey and other complete quasar surveys. , 1983 .

[49]  K. Mitchell,et al.  Variable quasi-stellar objects. II - Photometry and completeness of faint blue objects in the Sandage-Luyten survey field 15h 10m, +24 deg , 1978 .

[50]  J. Baldwin.  Luminosity Indicators in the Spectra of Quasi-Stellar Objects , 1977 .

[51]  A. Uomoto,et al.  Image-tube photography of a complete sample of 4C radio sources , 1976 .