Work by Strong Members is a Feature by the Staff Picks on Wolfram Community. The work, Forward ray tracing and hot spots in Kerr spacetime, was developed by Lihang Zhou a visiting student at Strong, Zhen Zhong, Yifan Chen and Vitor Cardoso and published in Physical Review D.

Near the event horizon of a black hole, swirling gas and plasma can flare up in sudden bursts, forming what astronomers call “hotspots” that outshine their surroundings for brief moments. In Sagittarius A*—the supermassive black hole at the center of our galaxy—such hotspots are frequently observed and offer tantalizing clues about the black hole’s properties.

However, interpreting these signals is far from straightforward. Light from these hotspots doesn’t travel in straight lines—it loops and bends through the curved spacetime, sometimes circling the black hole multiple times before reaching Earth. This produces multiple, increasingly faint images on the observer’s sky—each distorted and delayed by gravity.

To better reconstruct these images, we developed a forward ray tracing method to solve the geodesic equations and track all light paths from a hotspot to a distant observer. This method captures both direct and higher-order images caused by strong lensing, along with their shapes and brightness. This provides valuable insights for interpreting data from the Event Horizon Telescope and its next-generation upgrades, deepening our understanding of the spacetime near black holes.