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

TL;DR

This study presents a large-scale radio monitoring program of blazars using the OVRO 40-m telescope, revealing significant differences in variability between gamma-ray-loud and quiet blazars, as well as among different blazar subclasses.

Contribution

It introduces a comprehensive radio monitoring dataset and analyzes variability differences related to gamma-ray emission and blazar types for the first time.

Findings

Gamma-ray-loud blazars vary twice as much as gamma-ray-quiet ones.

BL Lac objects show larger variability amplitudes than FSRQs.

Low-redshift FSRQs exhibit stronger variability than high-redshift FSRQs.

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 (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.