2015MNRAS.450.2915W

XTE J1810-189 underwent an outburst in 2008, and was observed over ∼ 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 3.5-8.7kpc 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 ( ∼ 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 Γ increased from 1.84±0.01 to 2.25±0.04 when the unabsorbed X-ray luminosity at 3-25 keV dropped from 4x10³⁶ to 6x10³⁵erg/s. 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).