2025PASJ...77..237Y

We compiled the X-ray and soft gamma-ray observations of the Galactic black hole binary XTE J1859$+$226 in the 1999-2000 outburst from RXTE, ASCA, BeppoSAX and CGRO. Throughout systematic spectral analysis using a two-component model consisting of a multi-temperature accretion disk plus a fraction of its flux convolved with an empirical Comptonized powerlaw component, we found that the innermost radius ($r_{\rm in}$) and temperature ($T_{\rm in}$) of the disk are very variable with time in the rising phase of soft X-ray flux where Type-A/-B/C low-frequency quasi-periodic oscillations (QPOs) were found. After this phase, $r_{\rm in}$ remains constant at around $60$ km assuming a distance of $8$ kpc and an inclination angle of $67^{\circ }$, and $T_{\rm in}$ smoothly decays with time. The constant $r_{\rm in}$ suggests a presence of the innermost stable circular orbit (ISCO), with $r_{\rm in}$ repeatedly moving closer and farther away from the ISCO in the rising phase. Both disk parameters are remarkably correlated with independently analyzed timing properties such as QPO frequency and rms variability. Type-A/-B QPOs are seen only when $r_{\rm in}$ is close to the ISCO, while Type-C are seen when $r_{\rm in}$ is truncated and the frequency changes with a relation of $r^{-1.0}_{\rm in}$, supporting that Type-C QPOs occur at the inner edge of the truncated disk. Accurate determinations of the frequency-$r_{\rm in}$ relation for various objects should be a powerful tool to discriminate plausible Type-C QPO models. Furthermore, we suggest that jet ejection events may occur when $r_{\rm in}$ rapidly approaches to the ISCO, along with rapid changes of the disk flux, the rms variability, and the hardness ratio. A rapid shrinkage of $r_{\rm in}$ down to the ISCO can be a useful index as a precursor of radio flares for triggering target-of-opportunity observations and would provide constraints on jet launching mechanisms.