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Analysis of NuSTAR and Suzaku observations of Cyg X-1 in the hard state: evidence for a truncated disc geometry

Basak, Rupal and Zdziarski, Andrzej A. and Parker, Michael and Islam, Nazma (2017) Analysis of NuSTAR and Suzaku observations of Cyg X-1 in the hard state: evidence for a truncated disc geometry. Monthly Notices of the Royal Astronomical Society, 472 (4). pp. 4220-4232. ISSN 0035-8711. https://resolver.caltech.edu/CaltechAUTHORS:20170520-161631457

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Abstract

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 χ^2 and has a geometry with a hot inner accretion flow and a disc truncated at ≃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.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1093/mnras/stx2283DOIArticle
https://academic.oup.com/mnras/article/472/4/4220/4104653PublisherArticle
http://arxiv.org/abs/1705.06638arXivDiscussion Paper
Additional Information:© 2017 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2017 September 1. Received 2017 September 1; in original form 2017 May 18. Published: 05 September 2017. We thank John Tomsick for providing us with the wind-absorption model used in this work, Thomas Dauser, Chris Done, Javier García, Andrzej Niedźwiecki, Jörn Wilms for valuable discussions, and the referee, Javier García, for important suggestions. This research has used data obtained through the HEASARC Online Service, provided by the NASA/GSFC, in support of NASA High Energy Astrophysics Programs. RB has been supported by the 2016 START programme of the Polish Science Foundation. This research has been supported in part by the Polish National Science Centre grants 2013/10/M/ST9/00729, 2015/18/A/ST9/00746 and 2016/21/P/ST9/04025.
Group:NuSTAR
Subject Keywords:accretion, accretion discs – black hole physics – stars: individual: Cyg X-1 – X-rays: binaries – X-rays: individual: Cyg X-1
Issue or Number:4
Record Number:CaltechAUTHORS:20170520-161631457
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20170520-161631457
Official Citation:Rupal Basak, Andrzej A. Zdziarski, Michael Parker, Nazma Islam; Analysis of NuSTAR and Suzaku observations of Cyg X-1 in the hard state: evidence for a truncated disc geometry, Monthly Notices of the Royal Astronomical Society, Volume 472, Issue 4, 21 December 2017, Pages 4220–4232, https://doi.org/10.1093/mnras/stx2283
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:77612
Collection:CaltechAUTHORS
Deposited By: Joy Painter
Deposited On:21 May 2017 19:02
Last Modified:03 Oct 2019 17:59

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