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The near-IR emission in Type 1 AGNs is thought to be dominated by the thermalradiation from dust grains that are heated by the central engine in theUV/optical and are almost at the sublimation temperature. A brightening of thecentral source can thus further sublimate the innermost dust, leading to anincrease in the radius of the near-IR emitting region. Such changes in radiushave been indirectly probed by the measurements of the changes in the time lagbetween the near-IR and UV/optical light variation. Here we report directevidence for such a receding sublimation region through the near-IRinterferometry of the brightest Type 1 AGN in NGC4151. The increase in radiusfollows a significant brightening of the central engine with a delay of atleast a few years, which is thus the implied destruction timescale of theinnermost dust distribution. Compiling historic flux variations and radiusmeasurements, we also infer the reformation timescale for the inner dustdistribution to be several years in this galactic nucleus. More specificallyand quantitatively, we find that the radius at a given time seems to becorrelated with a long-term average of the flux over the previous several (~6)years, instead of the instantaneous flux. Finally, we also report measurementsof three more Type 1 AGNs newly observed with the Keck interferometer, as wellas the second epoch measurements for three other AGNs.

Evidence for a receding dust sublimation region around a supermassive black hole