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

¹ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, Via Archirafi 36, I-90123 Palermo, Italy
² INAF – IASF Palermo, Via U. La Malfa 153, I-90146 Palermo, Italy
³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁵ School of Physics & Astronomy, University of Southampton, Southampton, Southampton SO17 1BJ, UK
⁶ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁷ INAF – Osservatorio Astronomico di Brera, Via Brera 28, I-20121 Milano, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 19 December 2025
Accepted: 1 March 2026

Abstract

Context. Ultraluminous X-ray sources (ULXs) are non-nuclear X-ray binary systems that exceed the Eddington luminosity for a 10 M_⊙ 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 ∼ 10¹⁰ cm⁻³) and temperature (T_e ≥ 1.5 × 10⁵ K). A hybrid plasma whose ionisation balance is affected by both collisions and radiation is favoured.