Spectroscopic dust reverberation in NGC 5548

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 {mu}m). For such grains the dust temperature is ~1450K. 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 also partly affect the dusty region. We observe a cyclical decrease in dust mass with implied dust reformation times of ~5-6 months.