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

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

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Publication Details

Journal
Astronomy and Astrophysics
Published
2025-09-25
DOI
https://doi.org/10.1051/0004-6361/202555450
Citations
3
Primary Topic
Particle Detector Development and Performance
Type
article
Field-Weighted Citation Impact
5.61

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article

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

Weimin Yuan, S.L. Yang, Sergiy Guziy, Han He et al.
3 citations
Astronomy and Astrophysics
Particle Detector Development and Performance
5.61
article

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

Weimin Yuan, S.L. Yang, Sergiy Guziy, Han He, Jinghua Yuan, Maria Gritsevich, G. Bruni, Giulia Gianfagna, Hendrik van Eerten, Aishwarya Linesh Thakur, R. Paladino, Y. Chen, E. Maiorano, Y. D. Hu, L. Piro, A. Rossi, Roberta Tripodi, A. J. Castro‐Tirado, R. Sánchez-Ramírez, M. D. Caballero‐García, Chen Zhang, Shashi B. Pandey, Shumei Jia, Y. Cheng, Zhixing Ling
article en
3 citations

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

Astronomy and AstrophysicsVol. 703
Ural Federal University (RU), University of Helsinki (FI), Guangxi University (CN), Wuhan University of Technology (CN), Chinese Academy of Sciences (CN), Astronomical Observatory of Rome (IT), Aryabhatta Research Institute of Observational Sciences (IN), Institute for Space Astrophysics and Planetology (IT), National Space Science Center (CN), Istituto di Radioastronomia di Bologna (IT), Astronomical Institute of the Slovak Academy of Sciences (SK), Astronomy and Space (AU), Instituto de Astrofísica de Andalucía (ES), National Astronomical Observatories (CN), University of Chinese Academy of Sciences (CN), University of Bath (GB), National Institute for Astrophysics (IT)
National Science Foundation, National Radio Astronomy Observatory, Australian Government, European Commission, Commonwealth Scientific and Industrial Research Organisation, Tata Institute of Fundamental Research, Chinese Academy of Sciences, Centre National d’Etudes Spatiales, Korea Astronomy and Space Science Institute, National Astronomical Observatory of Japan, National Science and Technology Council, Science and Technology Facilities Council, National Institutes of Natural Sciences
Openalex Percentile: Top 5%
Particle Detector Development and Performance
5.61
DOIOpen Access PDFOpenAlex
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