  We report the results of a 2-month campaign conducted with the Chandra X-ray
observatory to monitor the ultraluminous X-ray source (ULX) NGC 5204 X-1. This
was composed of a 50-ks observation, followed by ten 5-ks follow-ups spaced
initially at ~ 3, then at ~ 10 day intervals. The ULX flux is seen to vary by
factors ~ 5 on timescales of a few days, but no strong variability is seen on
timescales shorter than an hour. There is no evidence for a periodic signal in
the X-ray data. An examination of the X-ray colour variations over the period
of the campaign shows the ULX emission consistently becomes spectrally harder
as its flux increases. The X-ray spectrum from the 50-ks observation can be
fitted by a number of disparate spectral models, all of which describe a smooth
continuum with, unusually for a ULX, a broad emission feature evident at 0.96
keV. The spectral variations, both within the 50-ks observation and over the
course of the whole campaign, can then be explained solely by variations in the
continuum component. In the context of an optically-thick corona model (as
found in other recent results for ULXs) the spectral variations can be
explained by the heating of the corona as the luminosity of the ULX increases,
consistent with the behaviour of at least one Galactic black hole system in the
strongly-Comptonised very high state. We find no new evidence supporting the
presence of an intermediate-mass black hole in this ULX.
