BLAZARS IN THE FERMI ERA: THE OVRO 40 m TELESCOPE MONITORING PROGRAM

Abstract

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 (δ > −20 • ) sources from the Candidate Gammaray Blazar Survey (CGRaBS) and now encompasses over 1500 sources, each observed twice per week with about 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-σ), 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-σ) 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) FS-RQs are found to vary more strongly than high-redshift FSRQs, with 3-σ significance. These findings represent an important step toward understanding why some blazars emit gamma-rays while others, with apparently similar properties, remain silent.

The MASIV Survey – IV. Relationship between intra-day scintillation and intrinsic variability of radio AGNs

“…Each source was observed at a cadence of two flux measurements a week between 2008 January to 2011 December. The original sample of 1158 sources monitored by OVRO (Richards et al 2011) were selected from the Candidate Gamma-Ray Blazar Survey (CGRaBS, Healey et al 2008); these CGRaBS sources were selected to have spectral indices, radio flux densities and X-ray flux densities similar to sources with EGRET gamma-ray detections, such that they would have a high chance of being detected in gamma-rays by Fermi. After the release of the Fermi -LAT catalogue, an additional ∼ 400 gamma-ray detected sources from the First LAT AGN Catalogue (1LAC, Abdo et al 2010) were added to the OVRO monitoring sample.…”

“…After the release of the Fermi -LAT catalogue, an additional ∼ 400 gamma-ray detected sources from the First LAT AGN Catalogue (1LAC, Abdo et al 2010) were added to the OVRO monitoring sample. For this present paper, we focus only on the radio-selected CGRaBS sources from the Richards et al (2011) sample, since we are interested in understanding the variability characteristics of radio-selected AGNs. We ignore the additional gamma-ray selected 1LAC sources in their sample.…”

“…To characterize the variability amplitudes of the OVRO lightcurves, the 'intrinsic modulation index', described in Richards et al (2011), is used. This estimate uses the maximum likelihood method to determine the standard deviation of the flux densities of each source lightcurve, normalized by the mean flux density.…”

“…Since our ISS data are drawn from the MASIV Survey, we briefly summarize the observations, characterization of the variability amplitudes, and main results of the survey in Section 2. In Section 3, we discuss the relationship between MASIV ISS amplitudes and the strengths of the predominantly intrinsic 15 GHz flux variations measured over 4 years by the Owens Valley Radio Observatory (OVRO) blazar monitoring program (Richards et al 2011(Richards et al , 2014, for a subset of sources observed in both programs. Using published VLBI data (Pushkarev & Kovalev 2015), we argue that the statistically significant correlation between the observed 5 GHz and 15 GHz variability amplitudes can be attributed to their mutual dependence on the angular sizes of the sub-milliarcsecond radio core components (Section 4).…”

The gamma-ray emitting radio-loud narrow-line Seyfert 1 galaxy PKS 2004−447

“…More sophisticated variability quantities would require better sampling (see, e.g., Richards et al 2011 for an overview). We calculate V ν between 5.5 GHz and 19.0 GHz, which are the frequencies with the best coverage.…”