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Analysis of NuSTAR and Suzaku observations of Cyg X-1 in the hard state: evidence for a truncated disc geometry
KTH, School of Engineering Sciences (SCI), Physics. Polish Academy of Sciences, Poland.
2017 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 472, no 4, 4220-4232 p.Article in journal (Refereed) Published
Abstract [en]

The geometry of the accretion flow in black hole X-ray binaries in the hard state, in particular the position of the disc inner edge, has been a subject of intense debate in recent years. We address this issue by performing a spectral study of simultaneous observations of Cyg X-1 in the hard state by NuSTAR and Suzaku. The same data were analysed before, and modelled by a lamppost containing hybrid electrons and located very close to the horizon, whose emission was incident on a surrounding disc extending almost to the innermost stable circular orbit. We re-analyse the incident continuum model and show that it suffers from the lack of physical self-consistency. Still, the good fit to the data provided by this model indicates that the real continuum has a similar shape. We find it features a strong soft X-ray excess below a few keV, which we model as a soft thermal-Comptonization component, in addition to the main hard thermal-Compton component. This continuum model with reflection of both components yields the overall lowest chi(2) and has a geometry with a hot inner accretion flow and a disc truncated at similar or equal to 13-20 gravitational radii. On the other hand, we have also found spectral solution with a lamppost at a large height and a disc that can extend to the innermost stable circular orbit, though somewhat statistically worse. Overall, we find that the fitted truncation radius depends on the assumed continuum and geometry.

Place, publisher, year, edition, pages
Oxford University Press, 2017. Vol. 472, no 4, 4220-4232 p.
Keyword [en]
accretion, accretion discs, black hole physics, stars: individual: Cyg X-1, X- rays: binaries, X- rays: individual: Cyg X-1
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:kth:diva-219492DOI: 10.1093/mnras/stx2283ISI: 000415652500035OAI: oai:DiVA.org:kth-219492DiVA: diva2:1163559
Note

QC 20171207

Available from: 2017-12-07 Created: 2017-12-07 Last updated: 2017-12-07Bibliographically approved

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