Cosmic projectiles collided before our eyes express
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.
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'.
Original abstract
as written by the authors arXiv:2604.14341
Extragalactic Fast X-ray Transients (EFXTs) represent an emerging class of high-energy phenomena characterized by X-ray outbursts lasting from tens to hundreds of seconds. However, for more than half of the EFXTs, their physical origins remain elusive. In this Letter, we report the discovery of EP250302a, a luminous EFXT detected by the Einstein Probe (EP) at a redshift of $$z = 1.131$$. The multi-wavelength light curves of EP250302a reveal remarkable temporal features that distinguish it from the previously known EP-detected EFXT population, most notably a needle-like X-ray flare accompanied by smooth optical rebrightening during the afterglow phase. We suggest that the distinct X-ray and optical behaviors constitute the first observed instance of late-time violent collision of two relativistic shells in an EFXT. Drawing on insights from GRB studies, such a collision process strongly indicates the reactivation of a central engine, making EP250302a-like transients a unique laboratory for probing the late-time activity and jet physics of EFXT central engines.
The Einstein Probe X-ray telescope sees flashes that last less than a minute yet shine billions of times brighter than the Sun.