Time-Evolved X-ray Irradiation during the 1999$-$2000 Outburst of the Black-Hole Binary XTE J1859+226

TL;DR

This study tests theoretical models of X-ray irradiation in the black-hole binary XTE J1859+226 during outburst, revealing that models fit well at certain states but struggle with complex behaviors at low luminosity, indicating significant disc geometry changes.

Contribution

It provides observational validation and limitations of current irradiation models across different outburst states of XTE J1859+226.

Findings

Models reproduce reprocessing efficiency during flux decrease.

Spectrum softens from very high/intermediate to high/soft state.

Complex behaviors at low luminosity are not well modeled.

Abstract

X-ray irradiation in X-ray binaries is thought to control the behavior at the outer disc, which is observable mainly at optical wavelengths. It is generally parameterized phenomenologically, but it can also be predicted from theoretical models of irradiated discs and their coronae/winds. We test these models using five multi-wavelength Hubble Space Telescope (HST) and quasi simultaneous Rossi X-Ray Timing Explorer (RXTE) datasets from the black-hole binary XTE J1859$+$226. These show how the reprocessed optical emission changes during outburst as the source fades from the very high/intermediate state at $\sim$0.4$L_{\rm Edd}$ down through the high/soft state towards the transition to the hard state at $\sim$0.02$L_{\rm Edd}$. The models are able to reproduce the small change in reprocessing efficiency as the source flux decreases by a factor of two, and the spectrum softens from the very high/intermediate state to the bright high/soft state. However, the low luminosity high/soft state as well as the transition spectrum show more complex behaviour that is not well described by current models. We suggest the disc geometry has changed drastically during the outburst, probably due to tidal forces, and that the disc is no longer in steady state at the late stage of the outburst.