Weak Radio and Bright Early Optical Emission in EP260321a: A Comparative Study with XRO 080109 Reveals the Role of Circumstellar Material Mass
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 ≲0.1c, whereas the radio data of XRO 080109 suggest a shock velocity of ≃0.2c. 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 ∼2×1013 cm. Its bright optical emission at t≲1 d can be explained by cooling emission from a dense, extended CSM shell heated by the radiation-mediated shock, requiring a total CSM mass of ∼0.06M⊙, 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.
Versions
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BibTeX
@misc{dai2026weak,
title = {{Weak Radio and Bright Early Optical Emission in EP260321a: A Comparative Study with XRO 080109 Reveals the Role of Circumstellar Material Mass}},
author = {Cui-Yuan Dai and Xiang-Yu Wang and Jian-He Zheng},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.30973},
eprint = {2610.30973},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.30973},
}
RIS
TY - UNPB
TI - Weak Radio and Bright Early Optical Emission in EP260321a: A Comparative Study with XRO 080109 Reveals the Role of Circumstellar Material Mass
AU - Cui-Yuan Dai
AU - Xiang-Yu Wang
AU - Jian-He Zheng
PY - 2026
DA - 2026/10/22
PB - arXiv
DO - 10.48550/arxiv.2610.30973
UR - https://arxiv.org/abs/2610.30973
AB - 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 ≲0.1c, whereas the radio data of XRO 080109 suggest a shock velocity of ≃0.2c. 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 ∼2×10^13 cm. Its bright optical emission at t≲1 d can be explained by cooling emission from a dense, extended CSM shell heated by the radiation-mediated shock, requiring a total CSM mass of ∼0.06M_⊙, 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.
ER -