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We report results of an extensive analysis of the X-ray nova XTE J1859+226observed with BeppoSAX and RXTE during its 1999 outburst. We modelled thesource spectrum with a multicolour blackbody-like feature plus the genericComptonization model BMC which has the advantage of providing spectraldescription of the emitted-radiation properties without assumptions on theunderlying physical process. The multicolour component is attributed to thegeometrically thin accretion disk, while the Comptonization spectrum is claimedto originate in the innermost sub-Keplerian region of the system (transitionlayer). We find that XTE J1859+226 covers all the spectral states typical ofblack-hole sources during its evolution across the outburst, however during thevery high state, when the disk contribution to the total luminosity is morethan 70% and the root mean square variability > 5%, the high-energy photonindex is closer to a hard state value (Gamma ~ 1.8). The BMC normalization andphoton index Gamma well correlate with the radio emission, and we also observeda possible saturation effect of Gamma at the brightest radio emission levels. Astrong positive correlation was found between the fraction of Comptonized seedphotons and the observed integrated root mean square variability, whichstrengthens the idea that most of the fast variability in these systems isprovided by the innermost Compton cloud, which may be also identified as a jet.

Spectral evolution of the X-ray nova XTE J1859+226 during its outburst observed by BeppoSAX and RXTE