Gamma-ray variability and correlation properties of blazars observed with Fermi LAT

📝 Abstract

The Fermi Large Area Telescope (Fermi LAT) provides long term systematic monitoring observations of the gamma-ray emission from blazars. The variability properties and the correlation with other wavelength bands are important clues for the evaluation of blazar models. We present results from timing and multiwavelength correlation analysis and discuss differences between blazar classes.

💡 Analysis

The Fermi Large Area Telescope (Fermi LAT) provides long term systematic monitoring observations of the gamma-ray emission from blazars. The variability properties and the correlation with other wavelength bands are important clues for the evaluation of blazar models. We present results from timing and multiwavelength correlation analysis and discuss differences between blazar classes.

📄 Content

arXiv:1112.4614v1 [astro-ph.HE] 20 Dec 2011 Gamma-ray variability and correlation properties of blazars observed with Fermi LAT S. Larsson on behalf of the Fermi LAT collaboration and many multiwavelength collaborators Department of Astronomy, Stockholm University and The Oskar Klein Centre for Cosmoparticle Physics, Stockholm, Sweden E-mail: stefan@astro.su.se Abstract. The Fermi Large Area Telescope (Fermi LAT) provides long term systematic monitoring observations of the gamma-ray emission from blazars. The variability properties and the correlation with other wavelength bands are important clues for the evaluation of blazar models. We present results from timing and multiwavelength correlation analysis and discuss differences between blazar classes.

1. Introduction More than 60 AGNs, almost all blazars, were detected by EGRET on Compton GRO, which established these sources as a powerful class of gamma-ray emitters [1]. In its first year of operation Fermi LAT has increased the number of known gamma-ray blazars by a factor of
2. Even more importantly the instrument is mapping the full sky every three hours, which allow regular monitoring of these sources on time scales from hours to years. These monitoring observations now form an important part of many ongoing efforts to study the variability and multiwavelength properties of blazars. Such observations contains information about the relative origin of different spectral components and about dynamical and radiation processes in blazar jets.
3. The Fermi Large Area Telescope Fermi (the Fermi Gamma-ray Space Telescope) was launched in June 2008 from Cape Canaveral in Florida. The main instrument onboard Fermi is the LAT (Large Area Telescope) which is sensitive to gamma-rays in the energy range 30 MeV to 300 GeV [2]. With a much larger field of view and with a much improved energy resolution compared to its predecessors, Fermi is now providing unprecedented data for studies of active galactic nuclei and other gamma-ray sources. Fermi is operated primarily in a sky survey mode where the full sky is mapped every three hours. Since LAT has a very wide field of view (about 20% of the sky) the exposure on any particular sky position is large. Pointed observations are executed only to follow up GRBs or during other exceptional outbursts. The sky surveying and wide field of view of the instrument also mean that the potential for new, serendipitous discoveries is very large. The angular resolution, which increases with photon energy, is about 1 degree at 1 GeV, while source localizations are typically 0.1 degrees.
4. LBAS, 1LAC and 2LAC Based on the first 3 months of observations a LAT bright-AGN Source list (LBAS) was produced by the Fermi LAT collaboration [3]. This list contains 106 high confidence associations (58 FSRQs, 42 BL Lacs, 2 radio galaxies and 4 of unknown type). These were sources with detection test statistic, TS > 100, corresponding approximately to 10σ detections. By comparison, the First LAT AGN catalog, 1LAC [4] based on 11 months of data included already 663 high confidence associations (sources with TS > 25, or approximately 5σ ). In the recently released second AGN catalog, 2LAC, this number of sources was increased by 50%[5].
5. Blazar variability Blazar light curves are often dominated by strong flares. The origin of these flares can e.g. be the bulk injection of new particles into the jet or strong internal shocks. If the variability is made up of shot pulses of different lengths and amplitudes produced by such events, the Power Density Spectrum (PDS) is determined by the shapes and amplitudes of the pulses and by correlations of their relative distribution in time. While pulse shapes might be associated with, e.g., cooling or light travel time scales, pulse correlations will contain information on the processes that are responsible for creating the pulses, which could be episodes of strong activity near the central black hole. By calculating the PDS, the Structure Function (SF), and other statistical mesures, we can test alternative mechanisms to produce the observed variability, keeping in mind that the same PDS or SF can be produced by more than one stochastic process. The Fermi LAT data can be used to study variability on time scales of hours to years, to measure spectral variations and to catch flaring sources with high efficiency. An example of the capability to study flux and spectral variations on subday time scales is given by the analysis of the recent outburst of 3C454.3 [6]. For timing analysis the regular sampling provided by Fermi LAT is a great advantage. The initial aim of our study of Fermi blazars is a characterization the gamma-ray variability. The tools that have been used for this are, • χ2 and excess variance to quantify the presence of variability and its significance. • Duty cycle analysis • Auto correlation function (ACF) • Structure function (SF) • Power Density Spectra (PDS) • Flare shape analysis Even though the ACF, SF and PD

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