X-ray softening during the 2008 outburst of XTE J1810-189

XTE J1810-189 underwent an outburst in 2008, and was observed over similar to 100 d by RXTE. Performing a time-resolved spectral analysis on the photospheric radius expansion burst detected on 2008 May 4, we obtain the source distance in the range 3.5-8.7 kpc for the first time. During its outburst, XTE J1810-189 did not enter into the high/soft state, and both the soft and hard colours decreased with decreasing flux. The fractional rms remained at high values (similar to 30 per cent). The RXTE/PCA spectra for 3-25 keV can be described by an absorbed power-law component with an additional Gaussian component, and the derived photon index I" increased from 1.84 +/- A 0.01 to 2.25 +/- A 0.04 when the unabsorbed X-ray luminosity at 3-25 keV dropped from 4 x 10(36) to 6 x 10(35) erg s(-1). The relatively high flux, dense observations and broad-band spectra are strong evidence that the softening behaviour detected in the outburst of XTE J1810-189 originates from the evolution of a non-thermal component rather than the thermal component (i.e. neutron star surface emission).