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The EP250302a flare showed two jets of matter catching up with each other long after the explosion.
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In a distant galaxy, at a distance that light takes 11 billion years to cross, something flashed. The Einstein Probe X-ray telescope caught a brief, needle-prick-like burst: it lasted only a few dozen seconds. But the most interesting part came later: a few days afterward, in ordinary visible light, the object smoothly brightened again.
This is like two race cars that started from the same line but at different speeds. The fast car pulled ahead, while the slow one lagged behind. After some time, the fast one unexpectedly eased off the gas, the slow one caught up, and they collided. In space, the role of the cars is played by ejections of matter — jets flying at tremendous speeds. The fast jet caught up with the slow one, and the collision gave birth to a new burst of light.
Such a jet collision had previously been seen only in the first minutes after a star explosion. Here, the impact occurred days later — meaning the 'engine' inside the source had restarted.
Scientists had long suspected that sources of short X-ray bursts could wake up again. But catching the moment of awakening had eluded them. Now we have the first recorded case of two ejecta colliding at a late stage, and this opens a new window into studying what lies hidden deep inside these cosmic 'engines'.
