Broadband X‐Ray Observations of the Narrow‐Line X‐Ray Galaxy NGC 5506

Abstract

To address the nature of the Fe Kα line profile and the X-ray in the Seyfert 2 galaxy NGC 5506, we have performed a broadband X-ray analysis of data obtained with ASCA and ROSAT. Variations of up to 70% in the 2-10 keV band flux are detected during a 1 day ASCA observation performed in 1997 January, while no significant change in the 2-10 keV continuum shape is found. The ASCA spectrum consists of an absorbed power law, a soft excess below 2 keV, and an Fe Kα emission line at 6.4 keV. The soft excess can be well described either by thermal emission from very low-abundance material at a temperature kT0.8 keV or by scattered/leaking flux from the primary power law plus a small amount of thermal emission. The luminosity of the thermal emission in the former case is 1.2 × 1040 ergs s-1 over the 0.5-2 keV band, while the is ~1% of the intrinsic hard X-ray continuum in the latter case. Analysis of ROSAT HRI data confirms that the X-ray emission is extended on kpc scales in this object, and the extended component may account for most of the X-ray observed by the ASCA. The result suggests that in type 2 active galactic nuclei (AGNs), the soft excess comes at least partly from an extended region, causing a serious problem for the model in which the source is partially covered. We argue that the generally low abundances are a drawback for the single temperature thermal model and favor a scattering-dominated model. The scatterer is likely to be relatively cold (kT1 keV) in this object. The Fe Kα profile is complex and cannot be satisfactorily modeled by a single Gaussian. Models of either double Gaussians, or a narrow Gaussian plus a line from a relativistic accretion disk viewed at an inclination of about 40° ± 10° provide good fits to the data. However, the inclination of the disk can be substantially larger if there is a small amount of excessive Fe K edge absorption. The intermediate inclinations for narrow-line X-ray galaxies are consistent with a model in which the inner accretion disk is aligned with an outer obscuring torus.