http://arxiv.org/abs/1404.5967
We have been using the 0.76-m Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory to optically monitor a sample of 157 blazars that are bright in $\gamma$-rays, being detected with high significance (test-statistic TS $> 100$, i.e., $\ge 10\sigma$) in one year by the Large Area Telescope (LAT) on the {\it Fermi} Gamma-ray Space Telescope. We attempt to observe each source on a 3-day cadence with KAIT, subject to weather and seasonal visibility. The $\gamma$-ray coverage is essentially continuous. KAIT observations extend over much of the 5-year Fermi mission for several objects, and most have $>$100 optical measurements spanning the last three years. These blazars (flat-spectrum radio quasars and BL Lac objects) exhibit a wide range of flaring behavior. Using the discrete correlation function (DCF), here we search for temporal relationships between optical and $\gamma$-ray light curves in the 40 brightest sources in hopes of placing constraints on blazar acceleration and emission zones. We find strong optical/$\gamma$-ray correlation in many of these sources at roughly day timescales. A stacked average DCF of the 40 sources verifies this correlation trend, with a peak above 99% significance near a lag of 0 day. These findings strongly support the widely accepted leptonic models of blazar emission. However, we also find examples of apparently uncorrelated flares (optical flares with no $\gamma$-ray counterpart and $\gamma$-ray flares with no optical counterpart) that challenge simple, one-zone models of blazar emission. Moreover, we find that FSRQs tend to have $\gamma$-rays leading the optical, while intermediate and high synchrotron peak blazars with the most significant peaks have smaller lags/leads. It is clear that long-term monitoring at high cadence is necessary to reveal the underlying physical correlation, in many cases.
D. Cohen, R. Romani, A. Filippenko, et. al.
Fri, 25 Apr 14
48/65
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