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[4.18] Evidence for an Ionized Accretion Disk in the Seyfert~2 Galaxy NGC~1068

E. J. M. Colbert (JHU), K. A. Weaver (NASA/GSFC), J. S. Mulchaey (OCIW), R. F. Mushotzky (NASA/GSFC)

We present results from analyses of RXTE, ASCA and BeppoSAX X-ray spectral data from the archetypal Seyfert~2 galaxy NGC~1068. Simultaneous RXTE and ASCA data (spanning 4 - 100 keV) are best fit with a power-law continuum with photon index \Gamma ~ 1.7 (in agreement with the canonical value for type~1 Seyferts), plus reflection from ionized matter with \xi ~ 1000. Reflection from ionized matter is significantly preferred over reflection from cold matter (\Delta\chi² \approx 50 for 320 dof). When the Fe line complex is modelled with three narrow Gaussians at 6.4, 6.7 and 6.97 keV, we find that the 6.7 keV line flux increases by a factor of \approx 2 in four months, between the RXTE/ASCA and BeppoSAX observations. Thus we argue that the 6.7 keV line emission comes to us directly from the accretion disk, and not from the electron scattering region further out from the nucleus. We find no evidence for variability in the line fluxes at 6.4 and 6.97 keV. Although ionized accretion disks are thought to be present in NLS1 nuclei, we are only now finding evidence for them in ``broad-line'' Seyfert nuclei (type~1: 1E~1615+061 and type~2: NGC~1068, this work). We shall discuss the implications of these results on the particular geometry required in NGC~1068.