EP251129a: A Fast X-ray Transient with Two X-Ray Plateaus and Its Off-axis Afterglow

@misc{wu2026ep251129a,
  title = {{EP251129a: A Fast X-ray Transient with Two X-Ray Plateaus and Its Off-axis Afterglow}},
  author = {Yue Wu and Ailing Wang and Senlin Pang and Yu-Han Yang and Biao Zhang and Zi-Gao Dai and Tao An and Jin-Jun Geng and Xue-Feng Wu and Hui Sun and Di Xiao and Jia Ren and Chen Deng and Yehao Cheng and Fa-Yin Wang and Yuan-Pei Yang and Eleonora Troja and Brendan O'Connor and Wen-Jie Zhang and Yuan Liu and Weimin Yuan and Yahui Jiang and Yun Wang and Cuiyuan Dai and Hucheng Ding and Tao Wu and Kangrui Ni},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.34810},
  eprint = {2610.34810},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.34810},
}

TY  - UNPB
TI  - EP251129a: A Fast X-ray Transient with Two X-Ray Plateaus and Its Off-axis Afterglow
AU  - Yue Wu
AU  - Ailing Wang
AU  - Senlin Pang
AU  - Yu-Han Yang
AU  - Biao Zhang
AU  - Zi-Gao Dai
AU  - Tao An
AU  - Jin-Jun Geng
AU  - Xue-Feng Wu
AU  - Hui Sun
AU  - Di Xiao
AU  - Jia Ren
AU  - Chen Deng
AU  - Yehao Cheng
AU  - Fa-Yin Wang
AU  - Yuan-Pei Yang
AU  - Eleonora Troja
AU  - Brendan O'Connor
AU  - Wen-Jie Zhang
AU  - Yuan Liu
AU  - Weimin Yuan
AU  - Yahui Jiang
AU  - Yun Wang
AU  - Cuiyuan Dai
AU  - Hucheng Ding
AU  - Tao Wu
AU  - Kangrui Ni
PY  - 2026
DA  - 2026/10/26
PB  - arXiv
DO  - 10.48550/arxiv.2610.34810
UR  - https://arxiv.org/abs/2610.34810
AB  - Extragalactic fast X-ray transients (EFXTs) detected by the Einstein Probe are extending the parameter space of relativistic stellar explosions toward softer and longer-lived transients. Here we report EP251129a/GRB 251129A at z=0.505, which exhibits two X-ray plateaus: an early plateau lasting ∼ 10^4 s followed by a rapid decay with spectral softening, and a second plateau extending to 60 days, placing it at the extreme long-duration end of Gamma Ray Burst (GRB) plateaus. The early optical emission evolves independently of the X-ray, indicating a different origin; whereas the unusual radio afterglow shows a rapid rise of F_ν∝ t^1.9−2.5 and a delayed peak at ∼36−40 days, providing strong evidence for an off-axis geometry. We find that the early X-ray behavior resembles those of ultra-long GRBs (ULGRBs) and can be naturally explained by the internal dissipation from a newborn millisecond magnetar undergoing gravitational-wave-dominated spin-down before collapsing into a black hole. The early optical, radio and late X-ray emission are well described by an off-axis two-component jet afterglow with additional energy injection, possibly powered by fallback accretion onto black hole. EP251129a therefore connects the emerging EFXT population with ULGRB-like central-engine activity and off-axis afterglow, offering a rare opportunity to probe both the evolution of central engine and jet structure in a single event.
ER  -