X-ray luminous galaxies.: I.: Chandra observations of IRAS 00317-2142

We present Chandra observations of the enigmatic galaxy IRAS 00317-2142, which is classified as a star-forming galaxy on the basis of the ionization level of its emission lines. However, a weak broad H wing and a high X-ray luminosity give away the presence of an active nucleus. The Chandra image reveals a nuclear point source (L(2-10 keV) similar to 6 x 10(41) ergs s(-1)), contributing over 80% of the galaxy X-ray counts in the 0.3-8 keV band. This is surrounded by some fainter nebulosity extending up to 6 kpc. The nucleus does not show evidence for short-term variability. However, we detect long-term variations between the ROSAT, ASCA, and Chandra epochs. Indeed, the source has decreased its flux by over a factor of 25 in a period of about 10 yr. The nuclear X-ray spectrum is well represented by a power law with a photon index of Gamma = 1.91(-0.15)(+0.17), while the extended emission is represented by a Raymond-Smith component with a temperature of similar to0.6 keV. We find no evidence for the presence of an Fe line. The nucleus is absorbed by an intrinsic column density of N-H similar to 8 x 10(20) cm(-2). Thus, the Chandra observations suggest that at least the X-ray emission is dominated by a type 1 active galactic nucleus (AGN). Then the observed optical spectrum can be explained by the masking of the nucleus by the powerful star-forming component. These, together with previous X-ray observations of galaxies with no clear signs of AGN activity in the optical (e.g., NGC 6240), cast doubt on the optical classification scheme and have implications for the nature of the normal galaxies detected in deep Chandra X-ray surveys.