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# The broadband spectral variability of Holmberg IX X-1

Walton, D.J., Fürst, F., Harrison, F.A. and Middleton, Matthew (2017) The broadband spectral variability of Holmberg IX X-1. The Astrophysical Journal.

Record type: Article

## Abstract

We present results from four new broadband X-ray observations of the extreme ultraluminous X-ray source Holmberg IX X-1 (${L}_{{\rm{X}}}\gt {10}^{40}$ erg s−1), performed by Suzaku and NuSTAR in coordination. Combined with the archival data, we now have broadband observations of this remarkable source from six separate epochs. Two of these new observations probe lower fluxes than seen previously, allowing us to extend our knowledge of the broadband spectral variability exhibited. The spectra are well fit by two thermal blackbody components that dominate the emission below 10 keV, as well as a steep (${\rm{\Gamma }}\sim 3.5$) power-law tail that dominates above ~15 keV. Remarkably, while the 0.3–10.0 keV flux varies by a factor of ~3 between all these epochs, the 15–40 keV flux varies by only ~20%. Although the spectral variability is strongest in the ~1–10 keV band, both of the thermal components are required to vary when all epochs are considered. We also revisit the search for iron absorption features by leveraging the high-energy NuSTAR data to improve our sensitivity to extreme velocity outflows in light of the ultra-fast outflow recently detected in NGC 1313 X-1. Iron absorption from a similar outflow along our line of sight can be ruled out in this case. We discuss these results in the context of super-Eddington accretion models that invoke a funnel-like geometry for the inner flow, and propose a scenario in which we have an almost face-on view of a funnel that expands to larger radii with increasing flux, resulting in an increasing degree of geometrical collimation for the emission from intermediate-temperature regions.

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The Broadband Spectral Variability of Holmberg IX X-1 - Accepted Manuscript

Accepted/In Press date: 17 March 2017
e-pub ahead of print date: 21 April 2017
Additional Information: Arxiv record 1610.006611 author Middleton confirms AM
Organisations: Astronomy Group

## Identifiers

Local EPrints ID: 412027
URI: http://eprints.soton.ac.uk/id/eprint/412027
ISSN: 0004-637X
PURE UUID: dd5d12e7-6c78-4812-83bd-c96e5f4c4e47

## Catalogue record

Date deposited: 05 Jul 2017 16:31

## Contributors

Author: D.J. Walton
Author: F. Fürst
Author: F.A. Harrison