活动星系ESO 323-G077的低态X射线光谱揭示核区吸收的变化

We report on a Suzaku observation of the Seyfert 1 galaxy ESO323－G077 obtained in 2011, in which the source is at an historical extremely low X-ray state. A comparison with XMM observation obtained in 2006, Chandra observations in 2010, and other previous X-ray observations, indicates the source has diminished in 2 - 10 keV flux by a factor of up to ~20, the lowest X-ray flux in this object ever been seen. During the Suzaku observation, the source appears heavily absorbed, with an extremely hard X-ray spectrum at energies above 1.5 keV ( ~0.7), and a strong narrow Fe K line (EW~600 eV), typical for a Compton-thick AGN. We find dramatic changes of spectra in the energy band ~1.5-10 keV between the Suzaku, XMM and Chandra observations, which could be best interpreted by variations in the column density of the absorber along the line of sight on time scale of years ( ). If confirmed, this object would be one of the few AGNs displaying extremely transitions between Compton-thin and Compton-thick states. Both the ionized absorbers and a relativistically broadened Fe K line observed with the XMM appear to be absent in the Suzaku spectra. ESO 323-G077 is a rare AGN with almost all key X-ray spectral features detected in one single object, making it a best laboratory to further investigate the geometry and structure of the circumnuclear obscuring material and test for the unification model.