  The so-called excess noise limits the energy resolution of transition-edge
sensor (TES) detectors, and its physical origin has been unclear, with many
competing models proposed. Here we present the noise and impedance data
analysis of a rectangular X-ray Ti/Au TES fabricated at SRON. To account for
all the major features in the impedance and noise data simultaneously, we have
used a thermal model consisting of three blocks of heat capacities, whereas a
two-block model is clearly insufficient. The implication is that, for these
detectors, the excess noise is simply thermal fluctuation noise of the internal
parts of the device. Equations for the impedance and noise for a three-block
model are also given.
