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).
| Original language | English |
|---|---|
| Article number | A92 |
| Number of pages | 18 |
| Journal | Astronomy and Astrophysics |
| Volume | 703 |
| Early online date | 7 Nov 2025 |
| DOIs | |
| Publication status | Published - 30 Nov 2025 |
Acknowledgements
The authors thank the anonymous referee for the useful comments. The authors also thank Bing Zhang for stimulating discussion.This work is based on data obtained with the Einstein Probe, a space mission
supported by the Strategic Priority Program on Space Science of the Chinese
Academy of Sciences, in collaboration with ESA, MPE and CNES (Grant No.
XDA15310000, No. XDA15052100). Based on observations collected at the
Centro Astronómico Hispano en Andalucía (CAHA) at Calar Alto, proposal
24B-2.2-012, operated jointly by Junta de Andalucía and Consejo Superior de
Investigaciones Científicas (IAA-CSIC). Also based on observations made with
the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del
Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the
island of La Palma. The Australia Telescope Compact Array is part of the Australia Telescope National Facility (grid.421683.a), which is funded by the Australian Government for operation as a National Facility managed by CSIRO. We acknowledge the Gomeroi people as the traditional owners of the Observatory site. We thank the staff of the GMRT that made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. This paper makes use of the following ALMA
data: ADS/JAO.ALMA#2024.A.00019.T. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC
(Canada), NSTC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by
ESO, AUI/NRAO and NAOJ.
Funding
We acknowledge support by the European Union Horizon 2020 programme under the AHEAD2020 project (grant agreement number 871158). This work has been also supported by ASI (Italian Space Agency) through the Contract no. 2019-27-HH.0 AJCT acknowledges support from the Spanish Ministry project PID2023-151905OB-I00 and Junta de Andalucía grant P20_010168 and from the Severo Ochoa grant CEX2021-001131-S funded by MCIN/AEI/10.13039/501100011033. MCG acknowledges support from the Spanish Ministry project PID2023-149817OB-C31. eMERLIN is a National Facility operated by the University of Manchester at Jodrell Bank Observatory on behalf of STFC. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101004719.
Keywords
- gamma-ray burst: general
- radiation mechanisms: non-thermal
- relativistic processes
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science