Chandra monitoring of UGC 4203: The structure of the X-ray absorber

We present a Chandra monitoring campaign of the highly variable Seyfert galaxy UGC 4203 (the ‘Phoenix Galaxy’) which revealed variations in the X-ray absorbing column density on time-scales of 2 weeks. This is the third, clear case, after NGC 1365 and NGC 7582, of dramatic NH variability on short time-scales observed in a ‘changing look’ source, i.e. an active galactic nuclei (AGN) observed in the past in both a reflection-dominated and a Compton-thin state. The inferred limits on the distance of the X-ray absorber from the centre suggest that the X-ray ‘torus’ could be one and the same with the broad emission line region. This scenario, first proposed for an ‘ad hoc’ picture for NGC 1365, may be the common structure of the circumnuclear medium in AGN.