A New Broadband Spectral State in the Ultraluminous X-ray Source Holmberg IX X-1

TL;DR

This study reports a new broadband spectral state in Holmberg IX X-1, showing variable high-energy emission and potential luminosity-temperature behavior similar to other ultraluminous X-ray sources, with implications for super-Eddington accretion models.

Contribution

It identifies a novel spectral state in Holmberg IX X-1 and analyzes its luminosity-temperature behavior, exploring the nature of high-energy emission and accretion flow structures.

Findings

High soft X-ray flux with collapsed high-energy emission in some epochs.

Recovery of hard X-ray emission in later observations.

Potential similarity to NGC 1313 X-1's luminosity-temperature tracks.

Abstract

We present a series of five new broadband X-ray observations of the ultraluminous X-ray source Holmberg IX X-1, performed by $XMM$-$Newton$ and $NuSTAR$ in coordination. The first three of these show high soft X-ray fluxes but a near total collapse of the high-energy ($\gtrsim$15 keV) emission, previously seen to be surprisingly stable across all prior broadband observations of the source. The latter two show a recovery in hard X-rays, remarkably once again respecting the same stable high-energy flux exhibited by all of the archival observations. We also present a joint analysis of all broadband observations of Holmberg IX X-1 to date (encompassing 11 epochs in total) in order to investigate whether it shows the same luminosity-temperature behaviour as NGC 1313 X-1 (which also shows a stable high-energy flux), whereby the hotter disc component in the spectrum exhibits two distinct, positively-correlated tracks in the luminosity-temperature plane. Holmberg IX X-1 may show similar behaviour, but the results depend on whether the highest energy emission is assumed to be an up-scattering corona or an accretion column. The strongest evidence for this behaviour is found in the former case, while in the latter the new 'soft' epochs appear distinct from the other high-flux epochs. We discuss possible explanations for these new 'soft' spectra in the context of the expected structure of super-Eddington accretion flows around black holes and neutron stars, and highlight a potentially interesting analogy with the recent destruction and re-creation of the corona seen in the AGN 1ES 1927+654.