Blazars in the Fermi Era: The OVRO 40-m Telescope Monitoring Program

Joseph L. Richards*, Walter Max-Moerbeck, Vasiliki Pavlidou, Oliver G. King, Timothy J. Pearson, Anthony C. S. Readhead, Rodrigo Reeves, Martin C. Shepherd, Matthew A. Stevenson, Lawrence C. Weintraub, Lars Fuhrmann, Enimanouil Angelakis, J. Anton Zensus, Stephen E. Healey, Roger W. Romani, Michael S. Shaw, Keith Grainge, Mark Birkinshaw, Katy Lancaster, Diana M. WorrallGregory B. Taylor, Garret Cotter, Ricardo Bustos

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

293 Citations (Scopus)

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 (delta > -20 degrees) 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 (6 sigma), 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 (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.

Original languageEnglish
Article number29
Pages (from-to)29
Number of pages22
JournalAstrophysical Journal Supplement Series
Volume194
Issue number2
DOIs
Publication statusPublished - Jun 2011

Bibliographical note

23 pages, 24 figures. Submitted to ApJS

Keywords

  • BL Lacertae objects: general
  • galaxies: active
  • methods: statistical
  • quasars: general
  • radio continuum: galaxies
  • EXTRAGALACTIC RADIO-SOURCES
  • GAMMA-RAY EMISSION
  • LINEAR-POLARIZATION PROPERTIES
  • READHEAD SURVEY SOURCES
  • LARGE-AREA TELESCOPE
  • BL LACERTAE OBJECTS
  • BLACK-HOLES
  • RELATIVISTIC JET
  • TOTAL FLUX
  • VARIABILITY

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