X-ray spectral softening of Swift J1753.5-0127

We report on a pointed Swift observation of the black hole X-ray transient Swift J1753.5-0127. The source was first detected by Swift/BAT on 2005 May 30 (Atel #546; Palmer et al. 2005) and it is has been active since then. This is a rather unusual behaviour for a black-hole transient, since these systems usually undergo weeks-to-months long outbursts. Although a dynamical measure of the mass of the accretor is still missing, the source is suggested to be a black hole candidate based on its timing and spectral properties (Cadolle Bel et al.