Changing look: from Compton-thick to Compton-thin, or the re-birth of fossil AGN

We discuss the properties of a small sample of Seyfert 2 galaxies whose X-ray spectrum changed appearance on time scales of years, becoming reflection-dominated from Compton-thin, or viceversa. A reflection–dominated spectrum is usually taken as evidence of Compton-thick absorption, but we instead argue that such a spectrum is due to a temporary switching–off of the nuclear radiation. The observations discussed here may help explaining mismatches between optical and X-ray classifications, and provide new strong and direct evidence of the presence of more than one cold circumnuclear region in Seyfert 2 galaxies.