A new broadband spectral state in the ultraluminous x-ray source Holmberg IX X-1

Abstract

We present a series of five new broad-band 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 (≥15 keV) emission, previously seen to be surprisingly stable across all prior broad-band 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 broad-band observations of Holmberg IX X-1to date (encompassing 11 epochs in total) in order to investigate whether it shows the same luminosity–temperature behaviouras NGC 1313 X-1 (which also shows a stable high-energy flux), whereby the hotter disc component in the spectrum exhibits twodistinct, positively-correlated tracks in the luminosity–temperature plane. Holmberg IX X-1 may show similar behaviour, butthe results depend on whether the highest energy emission is assumed to be an up-scattering corona or an accretion column. Thestrongest evidence for this behaviour is found in the former case, while in the latter the new ‘soft’ epochs appear distinct fromthe other high-flux epochs. We discuss possible explanations for these new ‘soft’ spectra in the context of the expected structureof super-Eddington accretion flows around black holes and neutron stars, and highlight a potentially interesting analogy withthe recent destruction and re-creation of the corona seen in the AGN 1ES 1927 + 654

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