The evolving accretion disc in the black hole X-ray transient XTE J1859+226

We present HST, RXTE, and UKIRT observations of the broad-band spectra of the black hole X-ray transient XTE J1859+226 during the decline from its 1999–2000 outburst. Our UV spectra define the 2175-Å interstellar absorption feature very well and based on its strength we estimate E(B-V)=0.58±0.12. Hence we deredden our spectra and follow the evolution of the spectral energy distribution on the decline from outburst. We find that the UV and optical data, and the X-ray thermal component when detectable, can be fitted with a simple blackbody model of an accretion disc heated by internal viscosity and X-ray irradiation, and extending to close to the last stable orbit around the black hole, although the actual inner radius cannot be well constrained. During the decline we see the disc apparently evolving from a model with the edge dominated by irradiative heating towards one where viscous heating is dominant everywhere. The outer disc radius also appears to decrease during the decline; we interpret this as evidence of a cooling wave moving inwards and discuss its implications for the disc instability model. Based on the normalization of our spectral fits we estimate a likely distance range of 4.6–8.0 kpc, although a value outside this range cannot securely be ruled out.