An ASCA X-ray spectrum of a prototype bare seyfert 1 nucleus in MCG -2-58-22

We present new X-ray spectral data for the Seyfert 1 galaxy MCG-2-58-22 obtained with the Advanced Satellite for Cosmology and Astrophysics (ASCA). The simplicity of the spectrum between 0.5 and 6 keV due to a lack of complex absorption or an appreciable contribution from Compton reflection allows a measure of the true continuum slope. Accounting for excess flux at ∼ 1.0 keV, the intrinsic photon index is Γ= 1.75 ± 0.05. This is flatter than Γ ∼ 2.0 predicted by current pair/reflection models and thermal Comptonization models although no conclusions can be drawn until the distribution of intrinsic indices for Seyfert galaxies is known. The 6-10 keV data do not require or rule out Compton reflection. Assuming no reflection component is present in the ASCA bandpass, we confirm a strong Fe Kα fluorescence line at 6.2 ± 0.17 keV that is resolved with a velocity width of 31, 000_-18,000^+29,500 km s^-1 FWHM and an equivalent width (EW) of 340_-140⁺¹⁵⁰ eV. Although the line width is consistent with arising in some portion of the broad line region, the EW is larger than ∼50-150 eV expected for typical broad line region column densities of 10^22-23 cm^-2. Alternatively, we derive a self-consistent model of the X-ray spectrum if we assume that the predicted amount of Compton reflection from a face-on accretion disk is present. In this case, the Fe K line appears to be unresolved with an upper limit of 47,400 km s^-1 FWHM, while the EW of 230_-120⁺¹⁴⁰ eV is less and consistent with production by the disk. The Fe K line parameters constrain the inclination of the disk to our line of sight to be about 40°. We use the MCG-2-58-22 data to illustrate unique problems and limitations encountered in fitting ASCA spectra of Seyfert galaxies. In particular, how the continuum is modeled has profound effects on the parameters inferred for the Fe Kα line.