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

TL;DR

This study analyzes the spectral evolution of the X-ray nova XTE J1859+226 during its 1999 outburst using BeppoSAX and RXTE data, revealing correlations between spectral states, radio emission, and variability, and modeling the accretion processes.

Contribution

It introduces a comprehensive spectral analysis using the BMC model to describe accretion and Comptonization without physical assumptions, linking spectral states to variability and radio emission.

Findings

XTE J1859+226 exhibits all typical black-hole spectral states during outburst.

A correlation exists between the photon index and radio emission, with possible saturation at high radio levels.

Fast variability is linked to the innermost Compton cloud, possibly a jet.

Abstract

We report results of an extensive analysis of the X-ray nova XTE J1859+226 observed with BeppoSAX and RXTE during its 1999 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 disk, 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 disk contribution to the total luminosity is more than 70% and the root mean square variability > 5%, the high-energy photon index is closer to a hard state value (Gamma ~ 1.8). The BMC normalization and photon index Gamma well correlate with the radio emission, and we also observed a possible saturation effect of Gamma at the brightest radio emission levels. A strong positive correlation was found between the fraction of Comptonized seed photons and the observed integrated root mean square variability, which strengthens the idea that most of the fast variability in these systems is provided by the innermost Compton cloud, which may be also identified as a jet.