XMM-Newton multi-year campaign on NGC 55 ULX-1

Resolving the wind and its variability with RGS

¹ INAF – IASF Palermo Via U. La Malfa 153 I-90146 Palermo, Italy
² Università degli Studi di Palermo, Dipartimento di Fisica e Chimica Via Archirafi 36 I-90123 Palermo, Italy
³ Center for Astrophysics – Harvard & Smithsonian Cambridge MA, USA
⁴ Department of Astronomy & Physics, Saint Mary’s University 923 Robie Street Halifax NS B3H 3C3, Canada
⁵ Université de Toulouse, CNRS, IRAP, 9 Avenue du colonel Roche BP 44346 F-31028 Toulouse Cedex 4, France
⁶ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano Via A. Corti 12 20133 Milano, Italy
⁷ Institute of Space Sciences (ICE), CSIC, Campus UAB Carrer de Can Magrans s/n E-08193 Barcelona, Spain
⁸ INAF – Osservatorio Astronomico di Cagliari, Localita Poggio dei Pini Strada 54 09012 Capoterra, Italy
⁹ Department of Physics, Centre for Extragalactic Astronomy, Durham University South Road Durham DH1 3LE, UK
¹⁰ INAF – Osservatorio Astronomico di Brera Via E. Bianchi 46 23807 Merate (LC), Italy
¹¹ INAF – Osservatorio Astrofisico di Torino Strada Osservatorio 20 I-10025 Pino Torinese, Italy
¹² Centre for Astrophysics Research, University of Hertfordshire College Lane Hatfield AL10 9AB, UK

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

Received: 28 July 2025
Accepted: 16 December 2025

Abstract

Context. Winds are an important ingredient in the evolution of X-ray binary (XRB) systems, particularly those at high accretion rates such as ultra-luminous X-ray sources (ULXs), because they may regulate the accretion of matter onto the compact object.

Aims. Our aim is to understand the properties of ULX winds and their link with the source spectral and temporal behaviour.

Methods. We performed high-resolution X-ray spectroscopy of the variable source NGC 55 ULX-1 to resolve emission and absorption lines as observed with XMM-Newton at different epochs. Optically thin plasma models were used to characterise the wind.

Results. We confirmed and thoroughly strengthened previous evidence of outflows in NGC 55 ULX-1. The presence of radiative recombination signatures and the ratios of the fluxes of the emission lines favours photoionisation balance and low to moderate densities, which confirm that the lines originate from classical XRB disc winds. An in-depth parameter space exploration shows line emission from a slowly moving, cool, and variable plasma perhaps associated with a thermal wind. Mildly relativistic Doppler shifts (about −0.15c) associated with the absorption lines confirm, at higher confidence, the presence of powerful radiatively driven winds.

Conclusions. The comparison between the results obtained at different epochs revealed that the wind responds to the variability of the underlying continuum. These variations may be used to understand the actual accretion regime and the nature of the source.