The soft X-ray transient EP241021A: A cosmic explosion with a complex off-axis jet and cocoon from a massive progenitor

¹ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, I-00133 Rome, Italy
² Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK
³ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁴ Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
⁵ South-Western Institute for Astronomy Research, Yunnan University, Kunming, Yunnan 650500, China
⁶ Faculty of Science, University of Helsinki, Gustaf Hällströmin katu 2a, P.O. Box 64 FI-00014 Helsinki, Finland
⁷ Institute of Physics and Technology, Ural Federal University, Mira str. 19, 620002 Ekaterinburg, Russia
⁸ Department of Astronomy, School of Physics and Technology, Wuhan University, Wuhan 430072, China
⁹ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China
¹⁰ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
¹¹ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China
¹² INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, I-40129 Bologna, Italy
¹³ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
¹⁴ ARIES – Aryabhatta Research Institute of Observational Sciences, Nainital 263001, India
¹⁵ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, Monte Porzio Catone 00078, Italy

^⋆ Corresponding author: giulia.gianfagna@inaf.it

Received: 8 May 2025
Accepted: 7 September 2025

Abstract

Context. X-ray flashes (XRFs) are fast X-ray transients discovered by the BeppoSAX satellite. Diverse evidence indicates that XRFs are connected to gamma ray bursts (GRBs) and likely represent their softer analogs. With its soft X-ray bandpass and exquisite sensitivity, the Einstein Probe (EP) offers a novel opportunity to disclose the nature of such puzzling events.

Aims. Several models have been proposed to explain the observed properties of XRFs, mostly in the context of the collapsar scenario, where such soft events could have different geometrical or physical conditions of the progenitor with respect to GRBs. These include off-axis GRBs and baryon-loaded explosions, which either produce a low-Lorentz-factor jet or a spherical, mildly (or non-) relativistic ejecta, known as cocoons. In this paper, we present multiwavelength observations of the afterglow of EP241021a, a soft X-ray transient detected by EP. We attempt to connect the complex, multicomponent afterglow emission with leading XRF models.

Methods. We first characterize the prompt emission of EP2410121a by EP-WXT and Fermi-GBM. Then, we present the results of our multiwavelength campaign from radio (uGMRT, ATCA, e-MERLIN, and ALMA), optical (LBT, GTC, and CAHA) and X-rays (EP-FXT). We perform an analysis of light curves and broad-band spectra using both empirical and physical models of GRBs and spherical expansions (both nonrelativistic and mildly relativistic cocoons).

Results. The EP241021a afterglow is characterized by multiple components, which represent the imprints of the interaction of a jet with the complex environment of the preexisting progenitor that is likely shaping its structure. In particular, the optical and X-ray afterglows are well described by a structured jet with wide and low-Lorentz-factor (γ ∼ 40) wings, which produce the decreasing light curve before 6 days. A re-brightening at 7 days in the optical and X-ray data is due to the jet core, which is off-axis and coming into view. The radio emission can be modeled with a mildly relativistic cocoon (γ ∼ 2). Finally, in the radio spectrum at 70 days, we find an additional component peaking at ∼50 GHz, which is well described by a second cocoon with γ ∼ 1