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Published November 2025 | Version Published
Journal Article Open

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

Creators

  • 1. ROR icon University of Hertfordshire
  • 2. ROR icon Osservatorio Astronomico di Cagliari
  • 3. ROR icon Harvard-Smithsonian Center for Astrophysics
  • 4. ROR icon European Space Astronomy Centre
  • 5. INAF - IASF Palermo, Via U. La Malfa 153, I-90146 Palermo, Italy
  • 6. ROR icon Durham University
  • 7. INAF – Osservatorio Astrofisico di Torino, Strada Osservatorio 20, I-10025 Pino Torinese, Italy
  • 8. ROR icon University of Sydney
  • 9. Jet Propulsion Lab
  • 10. ROR icon California Institute of Technology
  • 11. ROR icon University of Cambridge
  • 12. ROR icon University of Southampton
  • 13. ROR icon Istituto Universitario di Studi Superiori di Pavia

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 (≳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.

Copyright and License

© The Author(s) 2025. Published by Oxford University Press on behalf of Royal Astronomical Society.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

Acknowledgement

The authors would like to thank the reviewer for their helpful feedback, which helped to improve the final version of the manuscript. DJW, TPR, and WNA acknowledge support from the Science and Technology Facilities Council (STFC; grant codes ST/Y001060/1, ST/X001075/1, andST/Y001982/1, respectively). PK acknowledges support from the National Aeronautics and Space Administration (NASA) through the NASA Hubble Fellowship grant HST-HF2-51534.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. CP is supported by PRIN MUR SEAWIND–European Union–NextGenerationEU. RS acknowledges support from the University of the Chinese Academy of Sciences during part of this work, as well as support from the Italian National Institute for Astrophysics (INAF) grant number 1.05.23.04.04. This research has made use of data obtained with NuSTAR, a project led by Caltech, funded by NASA and managed by NASA/JPL, and has utilized the nustardas software package, jointly developed by the ASDC (Italy) and Caltech (USA). This research has also made use of data obtained with XMM–Newton, a European Space Agency (ESA) science mission with instruments and contributions directly funded by ESA Member States.

Data Availability

The raw observational data underlying this article are all publicly available from ESA’s XMM–Newton Science Archive6 and NASA’s HEASARC archive.7

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2409.12241 (arXiv)

Funding

Science and Technology Facilities Council
ST/Y001060/1
Science and Technology Facilities Council
ST/X001075/1
Science and Technology Facilities Council
ST/Y001982/1
National Aeronautics and Space Administration
HST-HF2-51534.001-A
Space Telescope Science Institute
National Aeronautics and Space Administration
NAS5-26555
European Union
Chinese Academy of Sciences
National Institute for Astrophysics
1.05.23.04.04

Dates

Accepted
2025-09-15
Available
2025-09-30
Published online
Available
2025-10-13
Corrected and typeset

Caltech Custom Metadata

Caltech groups
NuSTAR, Space Radiation Laboratory, Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published