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

Year: 2025

Type: article

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

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202555450

Authors: 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

Institutions: Institute for Space Astrophysics and Planetology, National Institute for Astrophysics, University of Bath, Consejo Superior de Investigaciones Científicas, Instituto de Astrofísica de Andalucía, Chinese Academy of Sciences, Institute of High Energy Physics, Yunnan University, Ural Federal University, University of Helsinki, Wuhan University, Guangxi University, National Astronomical Observatories, University of Chinese Academy of Sciences, Osservatorio di Astrofisica e Scienza dello Spazio, Istituto di Radioastronomia di Bologna, Aryabhatta Research Institute of Observational Sciences, Astronomical Observatory of Rome

Language: English

Topic: Gamma-ray bursts and supernovae

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $1,865

OpenAlex ID: W4414532141