Coordinated Fermi/Optical Monitoring of Blazars and the Great 2009 September Gamma-ray Flare of 3C 454.3

TL;DR

This paper details a comprehensive optical spectropolarimetric monitoring program for gamma-ray-bright blazars during the Fermi mission, highlighting observations of 3C 454.3 during a major gamma-ray flare and emphasizing the importance of coordinated multi-wavelength studies.

Contribution

It introduces a detailed optical spectropolarimetric dataset for blazars during the Fermi era, including spectral and polarization measurements, and demonstrates its application through a case study of 3C 454.3.

Findings

3C 454.3 was optically bright during the gamma-ray flare.

The blazar showed surprisingly low polarization during the flare.

The dataset enables tracking of spectral index changes and polarization variations.

Abstract

We describe the optical spectropolarimetric monitoring program at Steward Observatory centered around gamma-ray-bright blazars and the LAT Monitored Source List planned for Fermi Cycles 2-4. The large number of measurements made during Cycle 1 of the Fermi mission are available to the research community and the data products are summarized (see http://james.as.arizona.edu/~psmith/Fermi). The optical data include spectropolarimetry at a resolution of ~20 A, broad-band polarization and flux measurements, and flux-calibrated spectra spanning 4000-7600 A. These data provide a comprehensive view of the optical variability of an important sample of objects during the Fermi Era. In addition to broad-band flux and linear polarization monitoring, the spectra allow for the tracking of changes to the spectral index of the synchrotron continuum, importance of non-synchrotron emission features, and how and when the polarization varies with wavelength, an important clue as to the structure of the emission region or the identification of multiple nonthermal components. As an illustration, we present observations of 3C 454.3 obtained in 2009 September during an exceptionally bright gamma-ray flare. The blazar was optically bright during the flare, but except for a few short periods, it showed surprisingly low polarization (P < 5%). Opportunities exist within the Fermi research community to coordinate with our long-term optical monitoring program toward the goal of maximum scientific value to both the Fermi and associated radio VLBI monitoring of blazars.