Measuring the density structure of an accretion hot spot

Magnetospheric accretion models predict that matter from protoplanetary disks accretes onto stars via funnel flows, which follow stellar magnetic field lines and shock on the stellar surfaces^1-3, leaving hot spots with density gradients^4-6. Previous work has provided observational evidence of varying density in hot spots⁷, but these observations were not sensitive to the radial density distribution. Attempts have been made to measure this distribution using X-ray observations^8-10; however, X-ray emission traces only a fraction of the hot spot^11,12 and also coronal emission^13,14. Here we report periodic ultraviolet and optical light curves of the accreting star GM Aurigae, which have a time lag of about one day between their peaks. The periodicity arises because 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 a magnetospheric accretion model, this finding indicates the presence of a radial density gradient in a hot spot on the stellar surface, because regions of the hot spot with different densities have different temperatures and therefore emit radiation at different wavelengths.