On the nature of the ultraluminous X-ray source Holmberg II X-1

TL;DR

This study analyzes the broadband X-ray spectra of Holmberg II X-1 over 19 years, revealing accretion properties consistent with a stellar-mass black hole and evidence of nitrogen-rich donor star material.

Contribution

It provides a comprehensive spectral analysis of Holmberg II X-1, constraining accretion disc properties and suggesting a super-critical accretion regime with a stellar-mass black hole.

Findings

Disc temperature-luminosity trends align with thin disc expectations.

Deviations from power-law trends at high luminosity suggest super-critical accretion.

Detection of nitrogen-rich gas indicates a nitrogen-rich donor star.

Abstract

We present a comprehensive spectral analysis of the ultraluminous X-ray source Holmberg II X-1 using broadband and high-resolution X-ray spectra taken with the XMM-Newton satellite over a period of 19 years benefiting from a recent campaign. We tested several models for the broadband spectra among which a double thermal component provided a reasonable description for the continuum between 0.3-10 keV and enabled us to constrain the properties of the accretion disc. The Luminosity-Temperature trends of the inner and outer disc components broadly agree with the expectations for a thin disc, although the exact values of the slopes are slightly sensitive to the adopted model. However, all tested models show L-T trends which deviate from a power law above a bolometric luminosity of about 5 $\times \ 10^{39} $erg/s, particularly for the hot thermal component associated to the inner accretion flow. Assuming that such deviations are due to the accretion rate exceeding its Eddington limit or, most likely, the super-critical rate, a compact object with a mass 16-36 Msun, i.e. a stellar-mass black hole, is inferred. The time-averaged (2021) high resolution spectra present narrow emission lines at 1 keV primarily from Ne IX-X and a very strong at 0.5 keV from N VII, which indicate Ne-N-rich gas with non-Solar abundances. This favours a nitrogen-rich donor star, such as a blue/red supergiant, which has escaped from its native stellar cluster characterised by a low-metallicity environment.