The curious case of EP241021a: Unraveling the mystery of its exceptional rebrightening

¹ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
² McWilliams Center for Cosmology and Astrophysics, Department of Physics, Carnegie Mellon University, Pittsburgh, PA, 15213 USA
³ Excellence Cluster ORIGINS, Boltzmannstr. 2, 85748 Garching, Germany
⁴ Department of Natural Sciences, The Open University of Israel, PO Box 808 Ra’anana, 43537 Israel
⁵ Astrophysics Research Center of the Open university (ARCO), The Open University of Israel, PO Box 808 Ra’anana, 43537 Israel
⁶ Department of Physics, The George Washington University, Washington, DC, 20052 USA
⁷ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Antigua Carretera a Pátzcuaro # 8701, Ex-Hda. San José de la Huerta, Morelia, Michoacán, C.P. 58089 Mexico
⁸ Astrophysics Science Division, NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MD, 20771 USA
⁹ NASA Postdoctoral Program Fellow, NASA Goddard Space Flight Center, Greenbelt, MD, 20771 USA
¹⁰ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
¹¹ Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, I-00133 Rome, Italy
¹² Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA, 16802 USA
¹³ INAF-Istituto di Radioastronomia, Via Gobetti 101, I-40129 Bologna, Italy
¹⁴ Center for Space Sciences and Technology, University of Maryland, Baltimore County, Baltimore, MD, 21250 USA
¹⁵ Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, Maryland, 20771 USA
¹⁶ Perimeter Institute for Theoretical Physics, Waterloo, Ontario, N2L 2Y5 Canada
¹⁷ University of Texas, Hobby–Eberly Telescope, McDonald Observatory, TX, 79734 USA

^⋆ Corresponding authors: m.busmann@physik.lmu.de, boconno2@andrew.cmu.edu

Received: 18 March 2025
Accepted: 3 July 2025

Abstract

Context. Fast X-ray transients (FXTs) are a rare and poorly understood phenomenon with a variety of possible progenitors. The launch of the Einstein Probe (EP) mission has facilitated a rapid increase in the real-time discovery and follow-up of FXTs.

Aims. We focus on the recent EP discovered transient EP241021a, which shows a peculiar panchromatic behavior, with the aim of understanding its origin.

Methods. We obtained optical and near-infrared multiband imaging and spectroscopy with the Fraunhofer Telescope at Wendelstein Observatory, the Hobby-Eberly Telescope, and the Very Large Telescope of the newly discovered EP transient EP241021a over the first 100 days of its evolution.

Results. EP241021a was discovered by EP as a soft X-ray trigger, but was not detected at gamma-ray frequencies. The observed soft X-ray prompt emission spectrum is consistent with nonthermal radiation, which requires at least a mildly relativistic outflow with a bulk Lorentz factor Γ ≳ 4. The optical and near-infrared light curve displays a two-component behavior, where an initially fading component, ∼ t⁻¹, transitions to a rise steeper than ∼ t³ after a few days, before peaking at an absolute magnitude of M_r ≈ −21.8 mag and quickly returning to the initial decay. Standard supernova models are unable to reproduce either the absolute magnitude or the rapid timescale (< 2 d) of the rebrightening. The X-ray, optical and near-infrared spectral energy distributions display a red color, r − J ≈ 0.8 mag, and point to a nonthermal origin (∼ ν⁻¹) for the broadband emission. Considering a gamma-ray burst as a plausible scenario, we favor a refreshed shock as the cause of the rebrightening. This is consistent with the inference of an at least mildly relativistic outflow based on the prompt trigger.

Conclusions. Our results suggest a link between EP-discovered FXTs and gamma-ray bursts, despite the lack of gamma-ray detections for the majority of EP transients.