Deep-imaging observations of a candidate of an absorbed QSO at z = 0.653, AX J131831+3341

The results of deep-imaging observations of a candidate of an absorbed QSO at z = 0.653, AX J131831+3341, are presented. AX J131831+3341 was found during the course of optical follow-up observations of the ASCA Large Sky Survey, and has an X-ray luminosity of 10⁴⁵ erg s^-1 (2-10 keV), which corresponds to those of QSOs. Its optical spectrum shows no significant broad Hβ emission line, suggesting that the object is an absorbed QSO. Deep R and V band images reveal the presence of a point-like nucleus and an asymmetric extended component. The nuclear component has a blue color, and the optical magnitude is much fainter than that expected from the observed X-ray flux for typical type-1 AGNs. These photometric properties and the presence of broad Mg II 2800 Å emission can be explained simultaneously if the observed nuclear light is dominated by scattered nuclear light, though there is a possibility that the nuclear component is a slightly absorbed nucleus if its intrinsic X-ray to optical flux ratio is the largest among X-ray selected AGNs. The size of the extended component, which is thought to be the host galaxy of the QSO, is larger than those of normal disk galaxies at z = 0-0.75, and the absolute magnitude is similar to those of the brightest host galaxies of QSOs at redshifts smaller than 0.5. The V - R and R - I colors of the component are consistent with a 1 Gyr-old stellar population model without absorption.