paper

Weak Radio and Bright Early Optical Emission in EP260321a: A Comparative Study with XRO 080109 Reveals the Role of Circumstellar Material Mass

arXiv:2610.30973 · doi:10.3847/2041-8213/ae9eba

Abstract

Shock breakout (SBO) occurs when a radiation-mediated shock wave, formed during a core-collapse supernova (SN) explosion, breaks out from the stellar surface of the progenitor or from the circumstellar material (CSM), if the CSM is optically thick. The X-ray outburst XRO 080109, associated with a Type Ib/c SN 2008D, has been suggested to be an SBO event from a regular supernova. The recent detection of EP260321a, associated with a broad-lined Type Ic SN 2026gzf, by the Einstein Probe adds a new SBO event. However, the two SBO events differ markedly in their spectra, energetics, late-time radio emission and early optical emission within the first day. We show that the non-detections in radio follow-up observations of EP260321a imply a shock velocity , whereas the radio data of XRO 080109 suggest a shock velocity of . These velocities are consistent with those inferred from the properties of the X-ray outbursts and explain the difference in their X-ray spectra. The X-ray energy and duration of EP260321a indicate that the shock breaks out from the CSM at a radius of . Its bright optical emission at can be explained by cooling emission from a dense, extended CSM shell heated by the radiation-mediated shock, requiring a total CSM mass of , substantially larger than that inferred for XRO 080109. Finally, the lower shock velocity and larger CSM mass can be unified in a picture in which a more massive CSM decelerates the radiation-mediated shock more significantly before it breaks out.

arXiv comment21 pages, 5 figures. Published in ApJL (submitted on 29 July 2026)

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