The Variability of a Large Sample of Quasars

QSO variability: probing the starburst model

“…In recent studies of QSOs much effort has been concentrated on establishing the growth of variability with time, using the structure function (Bonoli et al 1979, Simonetti, Cordes & Heeschen 1985, Cristiani, Vio & Andreani 1990, Hook et al 1994, Trevese et al 1994, Cristiani et al 1996. The structure function (SF) of a light curve running from t = 0 to τ obs is defined as…”

“…Much of our knowledge about QSO variability is based on studies of large samples of sources monitored on photographic plates over a period of one or two decades. In these kinds of studies we can place the Rosemary Hill Observatory sample (McGimsey et al 1975, Scott et al 1976, Pica & Smith 1983, Pica et al 1988, Smith et al 1993, the ESO/SERC field 287 sample (Hawkins 1983(Hawkins ,1986(Hawkins ,1993, the Selected Area 57 sample (Trevese et al 1989(Trevese et al ,1994 the Selected Area 94 sample (Cristiani, Vio & Andreani 1990, Cristiani et al 1996 and the South Galactic Pole sample (Hook et al 1994).…”

“…Previous to the comparison of theory and observations, we have to consider that our predictions for light curves are computed in the rest frame B band, while real QSOs are observed in B band but their measured light has been emitted at shorter wavelengths. The mean magnitude of each QSO was k-corrected as in the original paper that presented and analyzed the data (Hook et al 1994) in order to give the magnitudes represented in figure 5. In addition to this correction, we must consider that the variability properties of the light emitted by QSOs from B band to up to λ824 Å, for the highest redshift QSOs of the sample (z = 4.1), are different if they behave similarly to low-redshift AGN.…”

“…For each of these points a corresponding error is generated under the actual error distribution function of the QSO brightness band. The error distribution functions are described in Hook et al (1994). The process is repeated a hundred times in order to provide a set of simulated samples from which we can extract results with statistical significance for models with different tsg values.…”

“…In the last decade, several observational studies have reported trends in the variability characteristics of optically selected quasars with either luminosity or redshift (Pica & Smith 1983, Cristiani, Vio & Andreani 1990, Giallongo, Trevese & Vagnetti 1991, Smith et al 1993, Hawkins 1993, Hook et al 1994Trevese et al 1994, Paltani & Courvoisier 1994, Cristiani et al 1996, Di Clemente et al 1996. Some of these studies (see for example Pica & Smith 1983) have shown that the observed anti-correlation between optical variability and QSO luminosity is somewhat flatter than a "1/ √ N " law, a result which seems to rule out simple Poissonian models.…”