  Magnetospheric accretion models predict that matter from protoplanetary disks
accretes onto the star via funnel flows which follow the stellar field lines
and shock on the stellar surface leaving a hot spot with a density gradient.
Previous work has inferred different densities in the hot spot, but has not
been sensitive to the radial density distribution. Attempts have been made to
measure this with X-ray observations, but X-ray emission only traces a fraction
of the hot spot and also coronal emission. Here we report periodic ultraviolet
and optical light curves of the accreting star GM Aur that display a time lag
of about 1 day between their peaks. The periodicity arises as the source of the
ultraviolet and optical emission moves into and out of view as it rotates along
with the star. The time-lag indicates a difference in the spatial distribution
of ultraviolet and optical brightness over the stellar surface. Within the
framework of the magnetospheric accretion model, this indicates a radial
density gradient in a hot spot on the stellar surface since different density
parts of the hot spot are expected to emit radiation at different wavelengths.
These results are the first observational confirmation of the magnetospheric
accretion model's prediction of a density gradient in the hot spot and
demonstrate the insights gained from focusing on the wavelengths where the bulk
of the accretion energy can be observed.
