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arXiv · 2606.17170

Discovery of a Supernova Following the Einstein Probe Transient EP250302a at z = 1.131

Brendan O'Connor·Malte Busmann·James Freeburn·Lei Hu·Noel Klingler·Daniel Gruen·Igor Andreoni·Julius Gassert·Xander J. Hall·Antonella Palmese
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Abstract

We present a multi-wavelength analysis of the Einstein Probe (EP) fast X-ray transient (FXT) EP250302a located at redshift $z=1.131$. Despite its luminous prompt X-ray emission, the event was not detected in gamma-rays. Multi-wavelength follow-up identified a bright optical and X-ray source that displayed rapid chromatic flaring before returning to the standard decay of a gamma-ray burst afterglow. We interpret the chromatic flare as either due to a refreshed shock caused by a discrete shell collision or as reverse shock emission. Using the early optical data, we place constraints on the Lorentz factor of the outflow, requiring an ultrarelativistic jet with $\Gamma_0>25$. We additionally obtained deep late-time imaging with the Gemini North Telescope that reveals the presence of an optical excess at $20-30$ d post-explosion. We interpret this as supernova (SN) emission and find good agreement with the canonical broad-lined Ic SN 1998bw with a flux-scaling factor of $k_\textrm{98bw}>0.3$. This adds to the growing evidence that the majority of EP FXTs are associated with the deaths of massive stars.

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Brendan O'Connor, Malte Busmann, James Freeburn, Lei Hu, Noel Klingler, Daniel Gruen, Igor Andreoni, Julius Gassert, Xander J. Hall, Antonella Palmese. 2026-06-15. Discovery of a Supernova Following the Einstein Probe Transient EP250302a at z = 1.131. https://arxiv.org/abs/2606.17170

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