Unveiling the biconical geometry of the outflow in the ultraluminous X-ray source NGC 5204 X-1

Context. Ultraluminous X-ray sources (ULXs) are non-nuclear X-ray binary systems that exceed the Eddington luminosity for a 10 solar masses black hole. The majority of these sources are thought to be stellar-mass compact objects accreting at super-Eddington rates, exhibiting powerful relativistic winds. These winds have been identified through the detection of absorption lines with a blueshift as high as 0.3c and emission lines typically found at their laboratory wavelengths. Aims. In this work, we analysed the XMM-Newton data of the ULX NGC 5204 X-1, which has been observed to exhibit emission lines with a blueshift of about 0.3c. The aim of this study is to examine the geometry and physical properties of the accretion disc and the relativistic outflows. In addition, we aim to explore the factors that influence the ULX spectral transitions. Methods. We undertook an observing campaign with XMM-Newton to explore the source behaviour at different luminosities. In this first paper of the series, we performed high-resolution X-ray spectroscopy, including archival data, with the Reflection Grating Spectrometer (RGS) instrument, which allowed us to resolve both emission and absorption lines. The outflows features were characterised using physical models of plasma in collisional ionisation and photoionisation equilibrium. Results. We identify collisionally ionised blueshifted and redshifted components at about 0.3c. These findings have a high statistical significance and suggest a biconical structure for the outflow. Additionally, the analysis of the O VII line triplet observed in the spectrum enables us to infer physical properties of the low-velocity line-emitting plasma; for example, electron density (n_e similar to 10^(10) cm^(-3)) and temperature (T_e >= 1.5 x 10^5 K). A hybrid plasma whose ionisation balance is affected by both collisions and radiation is favoured.