The first spectroscopic dust reverberation programme on active galactic
  nuclei: the torus in NGC 5548
  We have recently initiated the first spectroscopic dust reverberation
programme on active galactic nuclei (AGN) in the near-infrared. Spectroscopy
enables measurement of dust properties, such as flux, temperature and covering
factor, with higher precision than photometry. In particular, it enables
measurement of both luminosity-based dust radii and dust response times. Here
we report results from a one-year campaign on NGC 5548. The hot dust responds
to changes in the irradiating flux with a lag time of ~70 light-days, similar
to what was previously found in photometric reverberation campaigns. The mean
and rms spectra are similar, implying that the same dust component dominates
both the emission and the variations. The dust lag time is consistent with the
luminosity-based dust radius only if we assume a wavelength-independent dust
emissivity-law, i.e. a blackbody, which is appropriate for grains of large
sizes (of a few microns). For such grains the dust temperature is ~1450 K.
Therefore, silicate grains have most likely evaporated and carbon is the main
chemical component. But the hot dust is not close to its sublimation
temperature, contrary to popular belief. This is further supported by our
observation of temperature variations largely consistent with a heating/cooling
process. Therefore, the inner dust-free region is enlarged and the dusty torus
rather a "dusty wall", whose inner radius is expected to be
luminosity-invariant. The dust-destruction mechanism that enlarges the
dust-free region seems to partly affect also the dusty region. We observe a
cyclical decrease in dust mass with implied dust reformation times of ~5-6
months.
