THE KINEMATICS AND PHYSICAL CONDITIONS OF THE IONIZED GAS IN NGC 4593 FROM CHANDRA HIGH-ENERGY GRATING SPECTROSCOPY

摘要

We observed the Seyfert 1 galaxy NGC 4593 with the Chandra high-energy transmission gratings and present a detailed analysis of the soft X-ray spectrum. We measure strong absorption lines from He-like O, Ne, Mg, Si, H-like N, O, Ne, Mg, Si, and highly ionized Fe XIX-Fe XXV. The weighted mean of the offset velocity of the strongest absorption profiles is -140 +- 35 km s~(-1). However, the individual profiles are consistent with the systemic velocity of NGC 4593, and many profiles hint at the presence of either multiple kinematic components or blending. Only the N Ⅶ Lyα (λ24.781), O Ⅷ Lyα (λ18.969), and Mg Ⅻ Lyα (λ8.421) lines appear to be marginally resolved. We identify a spectral feature at ～0.707 keV with a neutral Fe L edge, which might suggest that there is dust along the line of sight to NGC 4593, although this is not the only interpretation of this feature. A search for neutral O absorption, which would reasonably be expected from dust absorption, is complicated by contamination of the Chandra ACIS CCDs. Neutral Si absorption, which might also be expected from absorption due to dust, is present (though not significantly) in the form of a weak neutral Si edge. The neutral Si column (<4 x 10~(17) cm~(-2)) corresponding to the Si edge is consistent with the neutral Fe column (～1.5 x 10~(17) cm~(-2)) from the Fe L edge. We also detect, at marginal significance, N Ⅶ Lyα (λ24.781) and O Ⅶ (r) 1s~2-1s2p (λ21.602) absorption at z ～ 0, due to a hot medium in our Local Group. The soft X-ray spectrum of NGC 4593 is adequately described by a simple single-zone photoionized absorber with an equivalent hydrogen column density of 5.37_(-0.79)~(+1.45) x 10~(21) cm~(-2) and an ionization parameter of logξ= 2.52_(-0.04)~(+0.06) ergs cm s~(-1), although there remain some features that are not identified. Although the photoionized gas almost certainly is composed of matter in more than one ionization state and may consist of several kinematic components, data with better signal-to-noise ratio and better spectral resolution are required to justify a more complex model. Finally, in emission we detect only weak forbidden lines of [Ne Ⅸ] and [O Ⅶ].