Radio Observations Point to a Moderately Relativistic Outflow in the Fast X-Ray Transient EP241021a

作者：Muskan Yadav, E. Troja, R. Ricci, Yu-Han Yang, M. H. Wieringa, Brendan O’Connor, Yacheng Kang, R. L. Becerra, Geoffrey Ryan, Malte Busmann · 发表于：The Astrophysical Journal · 年份：2025 · DOI：10.3847/1538-4357/ae1746 · 被引用次数：7 · 研究领域：Gamma-ray bursts and supernovae、Astrophysics and Cosmic Phenomena、Earth Systems and Cosmic Evolution

Abstract Fast X-ray transients (FXRTs) are short-lived X-ray outbursts with diverse progenitor scenarios, including compact object mergers, stellar core collapses, and tidal disruption events. The Einstein Probe (EP) has enabled the rapid discovery and follow-up of dozens of FXRTs, revealing that while some of them overlap with traditional gamma-ray bursts (GRBs), a larger fraction of FXRTs have no associated gamma-ray counterpart down to deep limits. The origin of these gamma-ray dark FXRTs and their connection to the diverse landscape of stellar explosions remains an open question, which can be tackled through the study of their multiwavelength counterparts and environment. In this paper, we present long-term radio observations of the gamma-ray dark EP241021a, which exhibits sustained radio emission for over 100 days, placing it among the longest-lived radio afterglows. We detect signature of interstellar scintillation in early epochs, allowing us to constrain the angular size and Lorentz factor of the emitting region. Our observations point to an outflow that is at least mildly relativistic with Γ ≳ 4. Afterglow modeling favors a moderately relativistic (Γ ≈ 40) top-hat jet ( θ c ≈ 0.16 rad) interacting with a low-density interstellar medium. The derived beaming-corrected kinetic energy, E k ∼ 1.6 × 10 51 erg, and low radiative efficiency, η γ ≲ 1%, are consistent with a standard relativistic explosion which did not produce bright gamma rays. Alternatively, a highly relati...