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Ultraluminous X-ray sources are powered by super-Eddington accretion onto stellar-mass compact objects, at least a fraction of them being magnetized neutron stars. The supercritical flows of ULXs are therefore expected to launch powerful, radiation-driven outflows of ionised material from their accretion discs. Considering their extragalactic distances, ULX fluxes are low and therefore any observational detections of such outflows are challenging and rare. The first detection of a fast wind, with a velocity of 0.2c was achieved in NGC 1313 X-1 using the RGS gratings onboard XMM-Newton. Following that study, we found the first evidence for a fast outflow in a neutron star ULX, NGC 300 ULX-1. Most recently, a very large XMM campaign on the soft ULX in NGC 247 revealed a spectacular outflow with a velocity of 0.17c. In parallel to these in-depth studies on individual ULXs, we searched the soft X-ray spectra of the ULX population for any spectral lines. Over 90% of studied sources show at least 1 spectral line, and roughly a third of the sources at least 5 line detections. We found that most detected emission lines are consistent with rest-frame elemental transitions, indicating low velocity ionised plasma. Conversely, most absorption lines are shifted from their rest-frame and are therefore likely imprinted by ionised absorbers with sub-relativistic velocities (0.1-0.2c), as unambiguously shown in the few ULXs with best quality high-resolution datasets. In my talk I will present these recent results on fast outflows in ULXs.

In this thesis I present the results of my PhD research into the physics of accretion (infall) and ejection (outflow) of matter in compact objects. Accretion disc winds are formed of hot and ionised material launched by magnetic forces, radiation pressure or thermally from the discs of accretors. They have been discovered in most types of accreting systems including supermassive black holes, classical X-ray binaries and Ultraluminous X-ray sources. Outflows form a ubiquitous and important part of the accretion flow, carrying away a considerable fraction of the originally infalling mass. With velocities as high as 30 per cent of the speed of light, their kinetic energy budget can also be significant and have a strong impact on the accretor surroundings. Ultra-fast outflows from supermassive black holes could contribute to or even drive active galactic nucleus feedback in galaxies. Accretion disc winds can be observed through Doppler-shifted spectral lines in the X-ray part of the electromagnetic spectrum, the wind physical properties can thus be studied with X-ray spectroscopy. In this work, I present the detection of disc winds and I study their physics in a number of accreting systems. I particularly make use of the high-spectral resolution Reflection Grating Spectrometer onboard the XMM-Newton observatory. For efficient data analysis I develop and use systematic automated routines for search of wind signatures in X-ray spectra. The first two chapters of this thesis introduce the theory of accretion and describe different accreting systems, as well as the X-ray observatories and data analysis methods used in this study. The third chapter contains a systematic search for disc winds in a sample of Ultraluminous X-ray sources, powered by super-Eddington accretion onto stellar-mass black holes and neutron stars. In the fourth chapter I achieve the first discovery of an ultra-fast wind in a neutron star Ultraluminous X-ray source. In the following two chapters I present the detection of ultra-fast outflows from two accreting supermassive black holes PG 1448+273 and 1H 0707-495. The outflow energetics show that these winds are more than capable to drive feedback in the accretor host galaxies. Furthermore, both show evidence for a multi-phase wind structure, PG 1448+273 also exhibiting variability in time. In the seventh chapter, I present the discovery of a disc wind in the unique X-ray binary Hercules X-1 known for a precessing, warped accretion disc. I leverage the warped disc precession to sample the vertical distribution of the disc wind, constraining its launching angle as well as the total mass outflow rate, two crucial quantities which are difficult to measure in other accreting systems. The final chapter contains the conclusions of the thesis as well as potential future research avenues in this field and the promising upcoming X-ray observatories.

Ultra-fast outflows (UFOs) have been detected in the high-quality X-ray spectra of a number of active galactic nuclei (AGN) with fairly high accretion rates and are thought to significantly contribute to the AGN feedback. After a decade of dedicated study, their launching mechanisms and structure are still not well understood, but variability techniques may provide useful constraints. In this work, therefore, we perform a flux-resolved X-ray spectroscopy on a highly accreting and variable NLS1 AGN, 1H 0707-495, using all archival XMM-Newton observations to study the structure of the UFO. We find that the wind spectral lines weaken at higher luminosities, most likely due to an increasing ionization parameter as previously found in a few similar sources. Instead, the velocity is anticorrelated with the luminosity, which is opposite to the trend observed in the NLS1 IRAS 13224-3809. Furthermore, the detection of the emission lines, which are not observed in IRAS 13224-3809, indicates a wind with a larger opening angle in 1H 0707-495, presumably due to a higher accretion rate. The emitting gas is found to remain broadly constant with the luminosity. We describe the variability of the wind with a scenario where the strong radiation extends the launch radius outwards and shields the outer emitting gas, similarly to super-Eddington compact objects, although other possible explanations are discussed. Our work provides several hints for a multi-phase outflow in 1H 0707-495., Comment: 20 pages, 9 figures, accepted for publication in MNRAS

1ES 1927+654 is a paradigm-defying AGN and one of the most peculiar X-ray nuclear transients. In early 2018, this well-known AGN underwent a changing-look event, in which broad optical emission lines appeared and the optical flux increased. Yet, by July 2018, the X-ray flux had dropped by over two orders of magnitude, indicating a dramatic change to the inner accretion flow. With three years of observations with NICER, XMM-Newton, and NuSTAR, we present the X-ray evolution of 1ES 1927+654, which can be broken into three phases-(1) an early super-Eddington phase with rapid variability in X-ray luminosity and spectral parameters, (2) a stable super-Eddington phase at the peak X-ray luminosity, and (3) a steady decline back to the pre-outburst luminosity and spectral parameters. For the first time, we witnessed the formation of the X-ray corona, as the X-ray spectrum transitioned from thermally-dominated to primarily Comptonized. We also track the evolution of the prominent, broad 1 keV feature in the early X-ray spectra and show that this feature can be modeled with blueshifted reflection (z = -0.33) from a single-temperature blackbody irradiating spectrum using xillverTDE, a new flavor of the xillver models. Thus, we propose that the 1 keV feature could arise from reflected emission off the base of an optically thick outflow from a geometrically thick, super-Eddington inner accretion flow, connecting the inner accretion flow with outflows launched during extreme accretion events (e.g. tidal disruption events). Lastly, we compare 1ES 1927+654 to other nuclear transients and discuss applications of xillverTDE to super-Eddington accretors., Comment: 25 pages, 10 figures, accepted for publication in ApJ