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^1,2,3, leaving hot spots with density gradients^4,5,6. 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^8,9,10; 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.

磁层吸积模型预测，来自原行星盘的物质通过漏斗流吸积到恒星上，漏斗流跟随恒星磁场线并冲击恒星表面^{1,2,3 ，留下密度梯度4,5,6}的热点。以前的工作提供了热点⁷中不同密度的观测证据，但这些观测对径向密度分布不​​敏感。已经尝试使用 X 射线观测^8,9,10来测量这种分布；然而，X 射线发射仅追踪热点^11,12的一小部分，还有日冕发射^13,14. 在这里，我们报告了吸积星 GM Aurigae 的周期性紫外线和光学光曲线，它们的峰值之间有大约一天的时间滞后。周期性的出现是因为紫外线和光发射源随着恒星旋转而进出视野。时间滞后表明恒星表面上紫外线和光学亮度的空间分布存在差异。在磁层吸积模型的框架内，这一发现表明恒星表面的热点存在径向密度梯度，因为不同密度的热点区域具有不同的温度，因此会发射不同波长的辐射。