Discovery of ultra-fast outflows with v_ out >0.3 c in local bright active galactic nuclei

Ultra-fast outflows (UFOs) are mildly relativistic (outflow velocity ̌out>0.1̊m c) nuclear winds detected as blueshifted absorption lines from highly ionized, dense gas in the X-ray spectra of active galactic nuclei (AGNs). The AGN feedback mechanism is believed to be powered by these outflows, which can inject a large amount of energy and momentum into the surrounding interstellar medium, shaping the coevolution of the AGNs and their host galaxies. We performed a systematic search and rigorous statistical assessment of the presence of UFOs in the 7-12 keV band, in a sample of bright local AGNs. This study also aims to understand whether the presence and characteristics of UFOs depend on the state of the sources, by studying the relations between the incidence of UFOs and the accretion properties of AGNs. We collected X-ray spectroscopic flux-limited data of 33 observations of local (z<0.2) type 1 AGNs. We modeled their spectra in the 2-12 keV band using a combination of direct-continuum and reflection components and searched for absorption features. This represents the first systematic search for UFOs up to 12 keV. We performed Monte Carlo (MC) simulations to assess the statistical significance of the detected lines. We report strong detections of UFOs in six sources of the sample at the ≥ 95% confidence level via MC simulations, corresponding to a fraction of sim18% in our sample. From the observed energies of each absorption line, we evaluated the respective wind velocities, which in some cases exceed 40% of the speed of light. The velocity distribution found in this work is therefore shifted to higher energies than those found in previous searches for UFOs in local sources, which were limited to 10 keV. Moreover, our analysis shows no correlation between the accretion properties of the super massive black holes (SMBHs) and the presence of winds. Finally, the comparison with previous studies of nuclear winds in local AGNs highlights the temporal variability of UFOs.