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Published June 2011 | Published
Journal Article Open

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. This program began with the 1158 northern (δ > –20°) sources from the Candidate Gamma-ray Blazar Survey 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 (6σ), gamma-ray-loud blazars detected by the LAT during its first 11 months of operation vary with almost a factor of two greater amplitude than do the gamma-ray-quiet blazars in our sample. We also find a significant (3σ) 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 3σ significance. These findings represent an important step toward understanding why some blazars emit gamma-rays while others, with apparently similar properties, remain silent.

Additional Information

© 2011 American Astronomical Society. Received 2010 November 12; accepted 2011 March 22; published 2011 May 23. We are grateful to Russ Keeney for his tireless efforts in support of observations at the Owens Valley Radio observatory. The OVRO 40 m monitoring program is supported in part by NASA grants NNX08AW31G and NNG06GG1G and NSF grant AST-0808050. Support from the Max-Planck Institut für Radioastronomie for upgrading the OVRO 40 m telescope receiver is also acknowledged. O.G.K. acknowledges the support of a Keck Institute for Space Studies Fellowship. W.M. acknowledges support from the US Department of State and the Comisión Nacional de Investigación Científica y Tecnológica (CONICYT) in Chile for a Fulbright-CONICYT scholarship. V.P. acknowledges support for this work provided by NASA through Einstein Postdoctoral Fellowship grant number PF8-90060 awarded by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060, and thanks the Department of Physics at the University of Crete for their hospitality during the completion of part of this work. Facilities: OVRO:40m

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August 22, 2023
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