Fast X-ray Bursts and Their Optical Surprises: The Case of EP241021a

* EP241021a was discovered as a soft X-ray trigger but was not detected at gamma-ray frequencies.

* The prompt soft X-ray emission spectrum is consistent with **non-thermal radiation**, suggesting a **mildly relativistic outflow with a bulk Lorentz factor Γ≳ 4**.

* The optical and near-infrared light curve shows a **two-component behavior**: an initial fading component (∼ t⁻¹) followed by a **rapid rise (steeper than ∼ t⁴)**, peaking at an absolute magnitude of **Mr ≈−22 mag**, before quickly returning to the initial decay. This peak magnitude is **the most luminous optical emission associated with an FXT**, surpassing EP240414a.

* Standard supernova models cannot explain either the **absolute magnitude or the rapid timescale (< 2 days rest frame)** of the rebrightening.

* The X-ray, optical, and near-infrared spectral energy distributions indicate a **red color (r− J ≈ 1 mag)** and suggest a **non-thermal origin (∼ ν⁻¹)** for the broadband emission.

* Considering a gamma-ray burst (GRB) as a possible scenario, the authors favor a **refreshed shock as the cause of the rebrightening**. This is consistent with the inferred mildly relativistic outflow.

* The results suggest a **likely link between EP-discovered FXTs and low-luminosity gamma-ray bursts**.

The source also compares EP241021a to another peculiar EP transient, **EP240414a**, which showed a roughly similar multi-wavelength behavior. Both events share features like the lack of gamma-ray emission, multiple optical emission components, a relatively flat X-ray light curve, and luminous, late-peaking radio emission. However, EP241021a has a **more luminous peak in its second optical component** and **longer timescales** for its light curve variations. Unlike EP240414a, which showed spectroscopic evidence of a supernova, **no clear supernova features were identified in the HET spectra of EP241021a**.

The authors explore various interpretations for the rebrightening, including off-axis structured jets and refreshed shocks. They disfavor a simple forward shock from an off-axis structured jet due to the steep rise observed but suggest that a **reverse shock from off-axis material in a shallow structured jet** or a **refreshed shock** are more plausible explanations. The consistency of the temporal and spectral indices with standard afterglow closure relations in a wind environment (expected for a massive star progenitor) supports the refreshed shock scenario.

The paper concludes that both EP241021a and EP240414a are likely produced by the **death of a massive star**. The non-thermal prompt emission necessitates at least a mildly relativistic outflow. The rapid optical rebrightening is challenging for supernova models and may be due to refreshed shocks or a reverse shock from off-axis material, both favoring a mildly relativistic outflow and non-thermal synchrotron radiation. The authors emphasize the need for future observations of similar events to better understand their nature.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Chinese Academy of Sciences (CAS).