2013A&A...559A..86D

The true nature of ultraluminous X-ray sources (ULX) is still currently vigorously debated. One of the most interesting possibilities is that these sources are intermediate-mass (M∼10³-10⁵M_☉) black holes. The most luminous ULX (L_2-10 keV_∼10⁴¹erg/s) are the most suitable candidates for being genuine intermediate-mass black holes (IMBH). XMMUJ132218.3-164247 was proposed to be the most luminous ULX known so far. For this reason a firm assessment of its intrinsic nature is needed and pursued in this work. We precisely defined the position of XMMUJ132218.3-164247 using a short Chandra pointing of the field. Then, we obtained high quality optical spectra of the source with the FORS2 instrument on the VLT to define its nature and distance. We then used unpublished data obtained with XMM-Newton to investigate its nature and emission properties in more detail. Features in its optical spectrum place XMMUJ132218.3-164247 at a redshift of z∼1, implying that it is a background QSO instead of an ULX. We clearly detected some emission lines typical of a QSO, including OIII lines and a broad Mg II line that indicates that the source is a type I AGN. The X-ray spectrum of the source is well modeled by a simple power-law with absorption slightly in excess to the Galactic value. No emission feature at the energy of the FeKα is present in the data. Finally, the source has been detected at a X-ray flux level higher (by ∼40%) than previously measured and reported in its discovery work. XMMUJ132218.3-164247 is not a ULX but a type I QSO. It shows a standard X-ray spectrum and exhibits a variability pattern that is typical of QSOs. A very rough estimate of its black-hole mass yields a value of M_BH∼2x10⁷M_☉.