2018MNRAS.481..639J

Previous X-ray spectral analysis has revealed an increasing number of active galactic nuclei (AGNs) with high accretion rates where an outflow with a mildly relativistic velocity originates from the inner accretion disc. Here we report the detection of a new ultrafast outflow (UFO) with a velocity of v_ out_=0.319+0.005^_–0.008c in addition to a relativistic disc 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 XXVI absorption feature and the Suzaku data also show evidence for an Ar XVIII with the same blueshift. A supersolar argon abundance (Z^′^_ Ar_> 6 Z_☉) and a slight iron overabundance (Z^′^_ Fe_=2.6+1.9^_–2.0 Z_☉) are found in our spectral modelling. Based on Monte Carlo simulations, the detection of the UFO is estimated to be at around 3σ significance. The fast wind most likely arises from a radius of >=20r_g away from the central black hole. The disc is accreting at a high Eddington ratio (L_bol = 0.4-0.7L_Edd). The mass outflow rate of the UFO is comparable with the disc mass inflow rate ({dot}M_ out_> 30%{dot}M_ in_), assuming a maximum covering factor. The kinetic power of the wind might not be high enough to have an influence on AGN feedback ({dot}E_ wind_/L bol_~3-5 per cent) due to a relatively small column density (12⁺⁹_–4×10²² 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⁺³_–2 ^○^).