Comprehensive Monitoring of Gamma-ray Bright Blazars. I. Statistical Study of Optical, X-ray, and Gamma-ray Spectral Slopes

Abstract

We present $\gamma$-ray, X-ray, ultraviolet, optical, and near-infrared light curves of 33 $\gamma$-ray bright blazars over four years that we have been monitoring since 2008 August with multiple optical, ground-based telescopes and the Swift satellite, and augmented by data from the Fermi Gamma-ray Space Telescope and other publicly available data from Swift. The sample consists of 21 flat-spectrum radio quasars (FSRQs) and 12 BL Lac objects (BL Lacs). We identify quiescent and active states of the sources based on their $\gamma$-ray behavior. We derive $\gamma$-ray, X-ray, and optical spectral indices, ${\alpha }_{\gamma }$, ${\alpha }_X$, and ${\alpha }_o$, respectively ($F_{\nu }\propto {\nu }^{\alpha }$), and construct spectral energy distributions (SEDs) during quiescent and active states. We analyze the relationships between different spectral indices, blazar classes, and activity states. We find (i) significantly steeper $\gamma$-ray spectra of FSRQs than for BL Lacs during quiescent states, but a flattening of the spectra for FSRQs during active states while the BL Lacs show no significant change; (ii) a small difference of ${\alpha }_X$ within each class between states, with BL Lac X-ray spectra significantly steeper than in FSRQs; (iii) a highly peaked distribution of X-ray spectral slopes of FSRQs at $\sim -$0.60, but a very broad distribution of ${\alpha }_X$ of BL Lacs during active states; (iv) flattening of the optical spectra of FSRQs during quiescent states, but no statistically significant change of ${\alpha }_o$ of BL Lacs between states; and (v) a positive correlation between optical and $\gamma$-ray spectral slopes of BL Lacs, with similar values of the slopes. We discuss the findings with respect to the relative prominence of different components of high-energy and optical emission as the flux state changes.

40 pages, 28 figures, 10 Tables. Additional figures at http://www.bu.edu/blazars/VLBAproject.html. Accepted by ApJ

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