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

• 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.

Ämnesord

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Nyckelord

accretion

accretion discs

black hole physics

stars: individual: Cyg X-1

X- rays: binaries

X- rays: individual: Cyg X-1

Publikations- och innehållstyp

ref (ämneskategori)

art (ämneskategori)