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arXiv · 2505.05444

The soft X-ray transient EP241021a: A cosmic explosion with a complex off-axis jet and cocoon from a massive progenitor

Giulia Gianfagna·Luigi Piro·Gabriele Bruni·Aishwarya Linesh Thakur·Hendrik Van Eerten·Maria D. Caballero-García·Alberto Castro-Tirado·Yong Chen·Ye-hao Cheng·Maria Gritsevich·Sergiy Guziy·Han He·You-Dong Hu·Shumei Jia·Zhixing Ling·Elisabetta Maiorano·Rosita Paladino·Shashi B. Pandey·Roberta Tripodi·Andrea Rossi·Rubén Sánchez-Ramírez·Shuaikang Yang·Jianghui Yuan·Weimin Yuan·Chen Zhang
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Abstract

X-ray flashes (XRFs) are fast X-ray transients thought to be softer analogs of gamma-ray bursts (GRBs). With its soft X-ray sensitivity, the Einstein Probe (EP) provides a unique opportunity to study these events. We report multiwavelength observations of EP241021a, a soft X-ray transient detected by EP, and interpret its afterglow in the context of leading XRF models. The prompt emission was observed by EP-WXT and Fermi-GBM, followed by a broad campaign across radio (uGMRT, ATCA, e-MERLIN, ALMA), optical (LBT, GTC, CAHA), and X-rays (EP-FXT). Light curves and spectra were analyzed with both empirical and physical models of GRBs and spherical expansions (both nonrelativistic and mildly relativistic cocoons). The afterglow shows multiple components, consistent with a structured jet interacting with a complex environment. The early optical and X-ray decline is explained by wide, low-Lorentz-factor ($γ\sim 40$) wings, while a rebrightening at approximately 7 days arises from the off-axis jet core. Radio data require an additional mildly relativistic cocoon ($γ\sim 2$), and a late (70 days) spectral component peaking at 50 GHz suggests a second, slower cocoon ($γ\sim 1$).

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Giulia Gianfagna, Luigi Piro, Gabriele Bruni, Aishwarya Linesh Thakur, Hendrik Van Eerten, Maria D. Caballero-García, Alberto Castro-Tirado, Yong Chen, Ye-hao Cheng, Maria Gritsevich, Sergiy Guziy, Han He, You-Dong Hu, Shumei Jia, Zhixing Ling, Elisabetta Maiorano, Rosita Paladino, Shashi B. Pandey, Roberta Tripodi, Andrea Rossi, Rubén Sánchez-Ramírez, Shuaikang Yang, Jianghui Yuan, Weimin Yuan, Chen Zhang. 2025-09-24. The soft X-ray transient EP241021a: A cosmic explosion with a complex off-axis jet and cocoon from a massive progenitor. https://doi.org/10.1051/0004-6361%2F202555450

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