dc.contributor.author Walton, Dominic en dc.contributor.author Furst, F en dc.contributor.author Harrison, FA en dc.contributor.author Middleton, MJ en dc.contributor.author Fabian, Andrew en dc.contributor.author Bachetti, M en dc.contributor.author Barret, D en dc.contributor.author Miller, JM en dc.contributor.author Ptak, A en dc.contributor.author Rana, V en dc.contributor.author Stern, D en dc.contributor.author Tao, L en dc.date.accessioned 2017-06-05T08:16:01Z dc.date.available 2017-06-05T08:16:01Z dc.date.issued 2017-04-21 en dc.identifier.issn 0004-637X dc.identifier.uri https://www.repository.cam.ac.uk/handle/1810/264622 dc.description.abstract We present results from four new broadband X-ray observations of the extreme ultraluminous X-ray source Holmberg IX X-1 (L$_{x}$ > 10$^{40}$ erg s$^{-1}$), performed by $\textit{Suzaku}$ and $\textit{Nu}$STAR 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 ($\Gamma$ $\sim$ 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. dc.description.sponsorship D.J.W. and M.J.M. acknowledge support from STFC through Ernest Rutherford fellowships, A.C.F. acknowledges support from ERC Advanced Grant 340442, and D.B. acknowledges financial support from the French Space Agency (CNES). 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, an ESA science mission with instruments and contributions directly funded by ESA Member States, and with Suzaku, a collaborative mission between the space agencies of Japan (JAXA) and the USA (NASA). dc.language eng en dc.language.iso en en dc.publisher Institute of Physics dc.subject black hole physics en dc.subject X-rays: binaries en dc.subject X-rays: individual (Holmberg IX X-1) en dc.title The Broadband Spectral Variability of Holmberg IX X-1 en dc.type Article prism.issueIdentifier 2 en prism.number 105 en prism.publicationDate 2017 en prism.publicationName The Astrophysical Journal en prism.volume 839 en dc.identifier.doi 10.17863/CAM.10207 dcterms.dateAccepted 2017-03-17 en rioxxterms.versionofrecord 10.3847/1538-4357/aa683e en rioxxterms.version VoR en rioxxterms.licenseref.uri http://www.rioxx.net/licenses/all-rights-reserved en rioxxterms.licenseref.startdate 2017-04-21 en dc.contributor.orcid Walton, Dominic [0000-0001-5819-3552] dc.contributor.orcid Fabian, Andrew [0000-0002-9378-4072] dc.identifier.eissn 1538-4357 rioxxterms.type Journal Article/Review en pubs.funder-project-id European Research Council (340442) pubs.funder-project-id SCIENCE & TECHNOLOGY FACILITIES COUNCIL (ST/N000927/1)
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