A soft X-ray study of type I active galactic nuclei observed with Chandra high-energy transmission grating spectrometer

Abstract

We present the results of a uniform analysis of the soft X-ray spectra of 15 type I active galactic nuclei (AGN) observed with the high-resolution X-ray gratings onboard Chandra. We found that 10 out of the 15 AGN exhibit signatures of an intrinsic ionized absorber. The absorbers are photoionized and outflowing, with velocities in the range ∼101− 103 km s−1. The column density of the warm absorbing gas is ∼1020−23 cm−2. Nine out of the 10 AGN exhibiting warm absorption are best fitted by multiple ionization components and three out of the 10 AGN require multiple kinematic components. The warm absorbing gas in our AGN sample has a wide range of ionization parameter, spanning roughly four orders of magnitude (ξ∼ 100−4 erg cm s−1) in total, and often spanning three orders of magnitude in the same gas. Warm absorber components with ionization parameter ξ < 10 generate an unresolved transition array due to Fe in seven out of the 10 AGN exhibiting warm absorption. These low ionization state absorbers may also carry away the largest mass outflows from the AGN. The mass outflow rate depends critically on the volume filling factor of the gas, which cannot yet be directly measured. However, upper limits on the mass outflow rates for filling factors of unity can be much greater than the expected accretion rate on to the central supermassive black hole and filling factors as small as 1 per cent can give outflow rates comparable to the accretion rate. There appears to be a gap in the outflow velocities in our sample between ∼300 and 500 km s−1, the origin of which is not clear. The outflow components with velocities below this gap tend to be associated with lower column densities than those with velocities above the gap.