Mid-IR Dust Reverberation Mapping of AGN

The circumnuclear dusty torus is an important component in AGN unification, as it explains the difference in the features we see in Type 1 and Type 2 AGN. It is therefore important to better understand its size and structure. Reverberation mapping is a technique we can use to constrain the size of the torus, since it is too small to be imaged except by interferometry of the nearest AGN. With this technique, the optical light curves (emitted from the accretion disk) are cross-correlated with the infrared light curves (originating from the torus), and a time lag between them can be measured. The time lag can be used to constrain the size of the torus. Here, we present results from a 2.5 year campaign with the Spitzer Space Telescope during which 12 AGN were monitored at the wavelengths of 3.6 and 4.5 microns. We found that the light curves for these AGN follow one of three types of behaviors: Strong optical variations with a strong IR response, strong optical variability with no IR response (or a slow secular trend), or small optical variability with strong IR variations. The expected trend is that of strong optical variations followed by a strong IR response. The second variability type (strong optical variability with IR response of a slow secular trend) occurs in objects with large bolometric luminosities, which leads to a large dust sublimation radius, hence to a large optical-IR time lag. The third variability type occurs in low luminosity objects, and is probably due to a large contribution of starlight to the optical emission, which dilutes the optical light curve. We present cross-correlation results and measure time lags for selected AGN in our sample.