The Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope provides an unprecedented opportunity to study gamma-ray blazars. To capitalize on this opportunity, beginning in late 2007, about a year before the start of LAT science operations, we began a large-scale, fast-cadence 15 GHz radio monitoring program with the 40-m telescope at the Owens Valley Radio Observatory (OVRO). This program began with the 1158 northern (declination>-20 deg) sources from the Candidate Gamma-ray Blazar Survey (CGRaBS) and now encompasses over 1500 sources, each observed twice per week with a ~4 mJy (minimum) and 3% (typical) uncertainty. Here, we describe this monitoring program and our methods, and present radio light curves from the first two years (2008 and 2009). As a first application, we combine these data with a novel measure of light curve variability amplitude, the intrinsic modulation index, through a likelihood analysis to examine the variability properties of subpopulations of our sample. We demonstrate that, with high significance (7-sigma), gamma-ray-loud blazars detected by the LAT during its first 11 months of operation vary with about a factor of two greater amplitude than do the gamma-ray-quiet blazars in our sample. We also find a significant (3-sigma) difference between variability amplitude in BL Lacertae objects and flat-spectrum radio quasars (FSRQs), with the former exhibiting larger variability amplitudes. Finally, low-redshift (z<1) FSRQs are found to vary more strongly than high-redshift FSRQs, with 3-sigma significance. These findings represent an important step toward understanding why some blazars emit gamma-rays while others, with apparently similar properties, remain silent.
The rotating super-massive black holes that power active galactic nuclei (AGN) somehow accomplish the remarkable feat of channeling energy derived from their rotation and accretion disks into two relativistic jets oppositely-directed along the spin axis. In spite of intensive observational efforts over the last four decades, the detailed mechanism of this process has remained elusive, and, although several processes have been suggested, we are still largely ignorant of the composition of the jets and the forces that collimate them. The first detailed collimation mechanism to be proposed was that of a "de Laval" nozzle (Blandford & Rees 1974), which is now known to be a likely cause of re-collimation on kpc scales, but not of the initial collimation, which, as revealed by Very Long Baseline Interferometry (VLBI), clearly occurs on sub-parsec scales. Other early theories, which involve magneto-hydrodynamic winds (Blandford & Znajek 1977) and/or magnetic fields threading the inner accretion disk (Blandford & Payne 1982), remain the most promising approaches to a full understanding of the phenomenon.
An observational difficulty is that, except in a few cases (e.g. M87), radio observations, which provide the most detailed images of active galaxies, only probe the relativistic jets down to the point at which the jets become optically thick at a point some light-weeks or light-months from the site of the original collimation. Higher-frequency observations are needed to probe deeper into the jets, although interstellar scintillation observations do in some cases reveal the presence of radio emission features in some AGN that are ∼ 5-50 micro-arcseconds in extent (Kedziora-Chudczer et al. 1997;Dennett-Thorpe & de Bruyn 2000;Jauncey et al. 2000;Rickett et al. 2002Rickett et al. , 2006;;Lovell et al. 2008), which can be very persistent (Macquart & de Bruyn 2007). These mysterious, very high brightness temperature features are by no means understood, and are certainly of great interest. At optical wavelengths, rapid swings in the polarization position angle have been used to tie together flux density variations at TeV energies and variations at millimeter wavelengths (Marscher et al. 2008). At very high energies of hundreds of GeV to TeV, very rapid variations down to timescales of minutes have been observed by the HESS, MAGIC and VERITAS instruments (e.g. Aharonian et al. 2007;Aharonian et al. 2009;Acciari et al. 2009;Acciari et al. 2010). Full three-dimensional (non-axisymmetric) magnetohydrodynamic relativistic simulations are now being carried out that enable detailed interpretation of the observations over the whole electromagnetic spectrum(e.g. McKinney & Blandford 2009;Penna et al. 2010).
Relativistic beaming introduces complications in observational studies of relativistic jets. The continuum emission is strongly beamed along the jet axis, introducing strong observational selection effects. Those objects having jets that are aligned at a small angle to the line of sight are collectively known as “blazars.” Small variations in the angle between the jet axis and the line of sight result in a large range of observed properties, such as apparent luminosity, variability, and energy spectrum. Strong boosting of the continuum synchrotron emission from the jet also frequently swamps optical line emission, making it difficult or even impossible to obtain a redshift for the source. As a result, blazars are subdivided into two classes: flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs). The former class contains blazars dominated by strong broad emission lines while the latter class contains those blazars with spectra dominated by their continuum emission, and hence weak, if any, emission lines and very weak absorption lines, or no lines at all. The large variations in the energy spectrum make it difficult to study many blazars over the whole electromagnetic spectrum. As a result the study of large, carefully-selected samples is necessary to determine the physical processes and conditions of the parent population. As relativistically boosted emission can be detected even from high-redshift sources, any intrinsic scatter in jet properties and scatter due to relativistic beaming is additionally convolved with any cosmological evolution of the black holes giving rise to the jets and their environment. It is therefore not surprising that the study of the population properties of relativistic jets has, to this day, been sparse at best.
The launch of the Fermi Gamma-ray Space Telescope in June of 2008 provides an unprecedented opportunity for the systematic study of blazar jets (Atwood et al. 2009). Its Large Area Telescope (LAT) observes the sky at energies between 100 MeV and a few hundred GeV. In this energy range relativistic particles can be probed through their inverse Compton emission in the case of electron/positron jets (e.g. Dermer et al. 1992;Sikora et al. 1994;Blandford & Levinson 1995), or a c
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