Blazars in the Fermi Era: The OVRO 40-m Telescope Monitoring Program (1011.3111v1)

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.

• The paper introduces a monitoring program tracking 1500+ blazars at 15 GHz to capture radio variability aligned with Fermi observations.
• It employs the intrinsic modulation index and likelihood methods to quantify variability, distinguishing real fluctuations from measurement noise with 3- to 7-sigma significance.
• Findings reveal that gamma-ray loud blazars and BL Lacs exhibit significantly higher variability, offering essential insights into jet dynamics and source evolution.

Overview of the OVRO 40-m Blazar Monitoring Program in the Fermi Era

The paper "Blazars in the Fermi Era: The OVRO 40-m Telescope Monitoring Program" presents a detailed account of a comprehensive radio monitoring effort of blazars using the Owens Valley Radio Observatory (OVRO) 40-meter telescope, focusing on sources identified in the Candidate Gamma-ray Blazar Survey (CGRaBS). This program is designed to complement the observations made by the Fermi Gamma-ray Space Telescope's Large Area Telescope (LAT) by providing high-cadence, long-term radio light curves at 15 GHz for over 1500 blazar sources.

Methodology

The monitoring program commenced in late 2007, preceding the Fermi-LAT operations by approximately a year. The initial selection included 1158 northern sky blazar candidates from CGRaBS, subsequently expanding to over 1500 sources. Each source is observed twice weekly, yielding a typical uncertainty of 3%. The light curves from 2008-2009 were utilized to analyze the variability properties of these sources.

A notable aspect of the methodology is the employment of the intrinsic modulation index, which quantifies variability amplitude while accounting for observational uncertainties. The analysis utilizes a likelihood approach to assess whether the observed variability could be ascribed to intrinsic source properties or artifacts of measurement noise.

Key Results

The paper presents several significant findings concerning the variability amplitude of blazars:

• Gamma-ray vs. Non-Gamma-ray Loud Blazars: A marked difference (7-$\sigma$ significance) in variability amplitudes between gamma-ray-loud and quiet blazars was observed, with the former showing nearly double the variability.
• BL Lacertae Objects (BL Lacs) vs. Flat Spectrum Radio Quasars (FSRQs): BL Lacs exhibit higher (3-$\sigma$ significance) variability amplitudes compared to FSRQs.
• Redshift Trends: Among FSRQs, lower redshift sources show greater variability than their higher redshift counterparts with 3-$\sigma$ confidence. This trend could imply intrinsic evolution in variability or selection effects due to time dilation.

Implications and Future Directions

The paper's findings have profound implications for understanding the physical processes governing blazar emission, particularly concerning jet dynamics and the mechanisms driving gamma-ray production. The observed correlation between radio variability and gamma-ray emission underscores the potential role of joint radio and gamma-ray observations in probing jet physics.

Furthermore, the identification of variability amplitude as a distinguishing feature among different blazar types could aid in refining source models. The variability trends with redshift may provide insights into the evolution of jet properties over cosmic time scales.

The authors suggest that continued monitoring and integration of additional Fermi-discovered blazars will be pivotal. Future analyses, including cross-correlation studies between radio and gamma-ray bands, could elucidate the temporal and spectral dynamics of these energetic sources.

In conclusion, the OVRO 40-m monitoring program represents a crucial component of multiwavelength studies in understanding blazar variability, providing a wealth of radio data that enhances our ability to explore the intricate nature of these complex astrophysical entities.