The Ultra-Fast Outflow of WKK 4438: Suzaku and NuSTAR X-ray Spectral Analysis

Abstract

Previous X-ray spectral analysis has revealed an increasing number of AGNs with high accretion rates where an outflow with a mildly relativistic velocity originates from the inner accretion disk. Here we report the detection of a new ultra-fast outflow (UFO) with a velocity of $v_{\mathrm{out}}=0.319_{-0.008}^{+0.005}c$ in addition to a relativistic disk reflection component in a poorly studied NLS1 WKK~4438, based on archival \nustar and \suzaku observations. The spectra of both \suzaku and \nustar observations show an Fe~\textsc{xxvi} absorption feature and the \suzaku data also show evidence for an Ar~\textsc{xviii} with the same blueshift. A super-solar argon abundance ($Z_{\mathrm{Ar}}^{\prime }>6Z_{\odot }$) and a slight iron over-abundance ($Z_{\mathrm{Fe}}^{\prime }=2.6_{-2.0}^{+1.9}{Z}_{\odot }$) are found in our spectral modelling. Based on Monte-Carlo simulations, the detection of the UFO is estimated to be around at 3$\sigma$ significance. The fast wind most likely arises from a radius of $\ge 20r_g$ away from the central black hole. The disk is accreting at a high Eddington ratio ($L_{\mathrm{bol}}=0.4-0.7L_{\mathrm{Edd}}$). The mass outflow rate of the UFO is comparable with the disk mass inflow rate (${\dot{M}}_{\mathrm{out}}>30\%{\dot{M}}_{\mathrm{in}}$), assuming a maximum covering factor. The kinetic power of the wind might not be high enough to have influence in AGN feedback (${\dot{E}}_{\mathrm{wind}}/L_{\mathrm{bol}}\approx 3-5\%$) due to a relatively small column density ($12_{-4}^{+9}\times 10^{22}$~cm${}^{-2}$). However note that both the inferred velocity and the column density could be lower limits owing to the low viewing angle ($i=23_{-2}^{+3}$${}^{\circ }$).

7 pages, 3 figures, accepted by MNRAS