XMM-Newton multi-year campaign on NGC 55 ULX-1: Resolving the wind and its variability with RGS

TL;DR

This study uses high-resolution X-ray spectroscopy over multiple years to analyze the winds in NGC 55 ULX-1, revealing their properties, variability, and connection to the source's accretion behavior.

Contribution

It provides detailed characterization of ULX winds, confirming outflows, and demonstrates their variability and response to the source's spectral changes using multi-epoch high-resolution data.

Findings

Confirmed presence of outflows in NGC 55 ULX-1

Detected radiative recombination signatures indicating photoionisation

Observed mildly-relativistic Doppler shifts (~ -0.15c) confirming powerful winds

Abstract

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. We aim at understanding the properties of ULX winds and their link with the source spectral and temporal behavior. 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 are used to characterise the wind. We confirmed and thoroughly strengthened previous evidence of outflows in NGC 55 ULX-1. The presence of radiative recombination signatures and the ratios between 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. The comparison between results obtained at different epochs revealed that the wind responds to the variability of the underlying continuum and these variations may be used to understand the actual accretion regime and the nature of the source.