2013MNRAS.428.3295F

We report results of an extensive analysis of the X-ray nova XTE J1859+226 observed with BeppoSAX and the Rossi X-ray Timing Explorer during the 1999 source outburst. We modelled the source spectrum with a multicolour blackbody-like feature plus the generic Comptonization model bmc which has the advantage of providing spectral description of the emitted-radiation properties without assumptions on the underlying physical process. The multicolour component is attributed to the geometrically thin accretion disc, while the Comptonization spectrum is claimed to originate in the innermost sub-Keplerian region of the system (transition layer). We find that XTE J1859+226 covers all the spectral states typical of black hole sources during its evolution across the outburst; however, during the very high state, when the disc contribution to the total luminosity is more than 70 per cent and the root mean square variability ≲ 5 per cent, the high-energy photon index is closer to a hard state value (Γ ∼ 1.8). The bmc normalization and photon index Γ well correlate with the radio emission, and we also observed a possible saturation effect of Γ at the brightest radio emission levels. A strong positive correlation was found between the fractions of Comptonized seed photons and the integrated root mean square variability, which strengthens the idea that most of the fast variability in these systems is attributable to the innermost Compton cloud, which may be also identified as a jet.