Evolution of the spectral curvature in the ULX Holmberg II X-1

TL;DR

This study analyzes the spectral curvature evolution in the ULX Holmberg II X-1, revealing a luminosity-dependent spectral break at around 4 keV, which challenges previous trends observed in similar sources.

Contribution

The paper presents new XMM-Newton observations showing a luminosity-dependent spectral break in Holmberg II X-1, providing insights into the spectral evolution of ULXs.

Findings

Spectral break at ~4 keV observed in Holmberg II X-1

Break energy increases with luminosity in this source

Contrasts with opposite trend in Holmberg IX ULX

Abstract

Ultraluminous X-ray sources (ULXs) are interesting systems as they can host intermediate mass black holes. Alternatively, ULXs can represent stellar-mass black holes accreting at super-Eddington rates. Recently spectral curvature or breaks at energies above a few keV have been detected in high quality ULX spectra. These spectral features have been taken as evidence against the intermediate-mass black hole case. In this paper, we report on a new XMM-Newton observation of the ULX Holmberg II X-1 that also shows a clear spectral break at approximately 4 keV. This observation was performed during a low luminosity state of the system and by comparing this new data to a high luminosity state XMM-Newton observation, we can conclude that the spectral break energy increases with luminosity. This behaviour is different to a ULX in the Holmberg IX galaxy,where an opposite trend between the luminosity and the spectral break energy has been claimed. We discuss mechanisms that could explain this complex behaviour.