Evidence for a receding dust sublimation region around a supermassive
  black hole
  The near-IR emission in Type 1 AGNs is thought to be dominated by the thermal
radiation from dust grains that are heated by the central engine in the
UV/optical and are almost at the sublimation temperature. A brightening of the
central source can thus further sublimate the innermost dust, leading to an
increase in the radius of the near-IR emitting region. Such changes in radius
have been indirectly probed by the measurements of the changes in the time lag
between the near-IR and UV/optical light variation. Here we report direct
evidence for such a receding sublimation region through the near-IR
interferometry of the brightest Type 1 AGN in NGC4151. The increase in radius
follows a significant brightening of the central engine with a delay of at
least a few years, which is thus the implied destruction timescale of the
innermost dust distribution. Compiling historic flux variations and radius
measurements, we also infer the reformation timescale for the inner dust
distribution to be several years in this galactic nucleus. More specifically
and quantitatively, we find that the radius at a given time seems to be
correlated with a long-term average of the flux over the previous several (~6)
years, instead of the instantaneous flux. Finally, we also report measurements
of three more Type 1 AGNs newly observed with the Keck interferometer, as well
as the second epoch measurements for three other AGNs.
