Occultation Measurement of the Size of the X-Ray-emitting Region in the Active Galactic Nucleus of NGC 1365

We present an occultation of the central X-ray-emitting region in the Seyfert galaxy NGC 1365. This extreme spectral variation (from Compton-thin to reflection-dominated and back to Compton-thin in 4 days) has been caught in a 10 day Chandra monitoring campaign consisting of six short (15 ks) observations performed every 2 days. We discuss the implications of this occultation within the scenario of a Compton-thick cloud crossing the line of sight of the X-ray source. We estimate a source size R ≤ 1014 cm and a distance of the cloud from the source D ≤ 1016 cm. This direct measurement confirms the theoretical expectations of an extremely compact X-ray source and shows that the Compton-thick circumnuclear gas is located at a distance from the center on the scale of the broad-line region.