The X-ray transient XTE J1859+226 in outburst and quiescence

We present optical photometry and spectroscopy of the X-ray transient XTE J1859+226, obtained during outburst and its subsequent decay to quiescence. Both the X-ray and optical properties are very similar to those of well-studied black hole soft X-ray transients. We have detected three mini-outbursts, when XTE J1859+226 was approaching quiescence, as has been previously detected in the soft X-ray transients GRO J0422+32 and GRS 1009-45. By 2000 August 24 the system had reached quiescence with R = 22.48 +/- 0.07. The estimated distance to the source is similar to11 kpc. Photometry taken during quiescence shows a sinusoidal modulation with a peak to peak amplitude of about 0.4 mag. A period analysis suggests that periods from 0.28 to 0.47 d are equally possible at the 68 per cent confidence level. The amplitude of the quiescent light curve and the relatively low ratio of X-ray to optical flux indicate that the binary inclination should be high. The measured colours during the outburst allow us to obtain the basic properties of the disc, which agrees well with irradiated disc model predictions.