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We present multiple Chandra and XMM-Newton observations of the type 1.8Seyfert Galaxy NGC 1365, which shows the most dramatic X-ray spectral changesobserved so far in an AGN: the source switched from reflection dominated totransmission dominated and back in just 6 weeks. During this time the softthermal component, arising from a ~1 kpc region around the center, remainedconstant. The reflection component is constant at all timescales, and its highflux relative to the primary component implies the presence of thick gascovering a large fraction of the solid angle. The presence of this gas, and thefast variability time scale, suggest that the Compton-thick to Compton thinchange is due to variation in the line-of-sight absorber, rather than toextreme intrinsic emission variability. We discuss a structure of thecircumnuclear absorber/reflector which can explain the observed X-ray spectraland temporal properties.Comment: 12 pages, 2 figures, accepted for publication in ApJ Letter

Rapid Compton-thick/Compton-thin Transitions in the Seyfert 2 Galaxy NGC 1365