Implication for Super-Critical Accretion Flow in the Narrow-Line Seyfert 1 Galaxy PKS 0558-504

Abstract

We present results from XMM-Newton observations of the narrow-line Seyfert 1 galaxy PKS0558-504. The 2-12keV X-ray spectra can be well-reproduced by a single power law with a photon index of ∼2.2. The extrapolation of this power law continuum into the lower energy band reveals the existence of a strong soft excess component. When we applied a multicolor disk blackbody (MCD) model to the soft excess component, we found that the observed disk temperature, kT_in, is much higher than that predicted from a standard Shakura-Sunyaev accretion disk. Furthermore, when the luminosity of the soft excess component, L_disk, exceeds about 5×10⁴⁴ergs^-1, kT_in seems to decrease in spite of the increase of luminosity. This trend is inconsistent with the standard accretion disk. Therefore, these results strongly suggest that the standard accretion-disk picture is no longer appropriate in the nucleus of PKS0558-504. From the estimated bolometric luminosity (∼10⁴⁶ergs^-1) and the central mass (5×10⁷M_odot) of the black hole, PKS0558-504 may be shining at the super-Eddington luminosity. We discuss a possible origin of the soft excess component, and suggest that significant photon trapping in the disk could explain the observational results.