X-Ray Properties of the Quasar HE 0450-2958

We present an XMM-Newton EPIC observation of HE 0450-2958, which may be a ``naked'' quasar as suggested by Magain and coworkers. The XMM-Newton EPIC spectra show a substantial soft X-ray excess and a steep photon index, as well as marginal evidence for a weak Fe Kα line. The X-ray absorption is consistent with the Galactic level. The 0.3-10 keV EPIC spectra can be fitted by a power-law plus blackbody model; however, the fit by a relativistically blurred photoionized disk reflection model is better. We estimate a black hole mass of 2⁺⁷_-1.3×10⁷ M_solar from the X-ray variability. This broadly agrees with the value derived from the optical Hβ line width. These results support a high-state Seyfert galaxy as the source. HE 0450-2958 shares properties with transitionary objects from ultraluminous infrared galaxies to quasars. We suggest that HE 0450-2958 is just in the beginning of an optical quasar window.