Gamma-ray Light Curves and Variability of Bright Fermi-Detected Blazars

This paper presents light curves and the first systematic characterization of variability of the 106 objects in the Fermi Large Area Telescope (LAT) Bright AGN Sample (LBAS). Weekly light curves obtained during the first 11 months of survey (August 04, 2008 - July 04, 2009), are tested for variability, and their properties are quantified through autocorrelation and structure function analysis. For the brightest sources power density spectra (PDS) and fit of the temporal structure of major flares is performed. More than 50% of the sources are variable, where high states do not exceed 1/4 of the total observation range. Variation amplitudes are larger for FSRQs and low/intermediate synchrotron peaked (LSP/ISP) BL Lac objects. Autocorrelation time scales vary from 4 to a dozen of weeks. Variable sources of the sample have 1/(f^{a}) PDS and show two modes: (1) rather constant baseline with sporadic flaring activity characterized by flatter PDS slopes resembling flickering and red-noise with occasional intermittence, and (2) - measured for a few blazars showing strong activity - complex and structured temporal profiles characterized by longer-term memory and steeper PDS slopes typical of a random-walk underlying mechanism. The average PDS slope of the brightest 22 FSRQs and the 6 brightest BL Lacs is 1.5 and 1.7 respectively. The study of temporal profiles of well resolved flares shows that they generally have symmetric profiles and that their total duration vary between 10 and 100 days.