Probing the inner region of Cygnus X-1 in the low/hard state through its X-ray broadband spectrum

We present the broadband X-ray spectrum of Cyg X-1 in the low/hard state as observed by the instruments on board BeppoSAX. The spectrum spans from 0.1 to 200 keV, allowing the total accretion luminosity to be observed rather than extrapolated, corresponding to similar to2% of the Eddington limit for a 10 M-. black hole. The broad bandpass allows us to determine the continu um shape with great accuracy. Simple models of Compton upscattering of seed photons from the accretion disk do not adequately match the spectrum. At l ow energies an additional continuum component is required, giving a complex soft excess which extends up to similar to4 keV, in line with previous res ults from ASCA. Moreover, we clearly detect a reflected component from the accretion disk that is smeared, probably because of relativistic and Dopple r effects. The reflecting material is not strongly ionized and does not sub tend a large solid angle as seen from the corona (Omega /2 pi similar to0.1 -0.3). The estimated inner radius of the disk, which depends on the inclina tion of the system, is most probably between 10 and 70 gravitational radii (R-g). An unsmeared reprocessed component, probably originating from the co mpanion star or the outer disk, could also be present. In this case, the in ner radius of the disk, as inferred from the smeared reflection, is smaller , between and 6R(g) and 20R(g).