The soft X-ray transient EP241021a: A cosmic explosion with a complex off-axis jet and cocoon from a massive progenitor

TL;DR

EP241021a is a soft X-ray transient whose afterglow cannot be described by a single jet component. The authors develop a multi-component model comprising wide, low-Lorentz-factor jet wings, a narrowly collimated off-axis core, and two cocoon outflows to explain the optical, X-ray, and radio behavior across ~70 days. The early decay is dominated by the wings, the 7 d rebrightening by the off-axis core, and the radio emission by cocoons with γ≈2 and γ≈1, respectively, in a dense, anisotropic WR-like wind environment. This work demonstrates how XRFs can arise from complex jet–cocoon structures in collapsars and highlights the power of coordinated, multiwavelength campaigns for decoding such transients.

Abstract

X-ray flashes (XRFs) are fast X-ray transients thought to be softer analogs of gamma-ray bursts (GRBs). With its soft X-ray sensitivity, the Einstein Probe (EP) provides a unique opportunity to study these events. We report multiwavelength observations of EP241021a, a soft X-ray transient detected by EP, and interpret its afterglow in the context of leading XRF models. The prompt emission was observed by EP-WXT and Fermi-GBM, followed by a broad campaign across radio (uGMRT, ATCA, e-MERLIN, ALMA), optical (LBT, GTC, CAHA), and X-rays (EP-FXT). Light curves and spectra were analyzed with both empirical and physical models of GRBs and spherical expansions (both nonrelativistic and mildly relativistic cocoons). The afterglow shows multiple components, consistent with a structured jet interacting with a complex environment. The early optical and X-ray decline is explained by wide, low-Lorentz-factor ($γ\sim 40$) wings, while a rebrightening at approximately 7 days arises from the off-axis jet core. Radio data require an additional mildly relativistic cocoon ($γ\sim 2$), and a late (70 days) spectral component peaking at 50 GHz suggests a second, slower cocoon ($γ\sim 1$).

Figures (12)

• Figure 1: EP241021a broadband light curve. FXT observations are represented with green data points; optical observations are represented in shades of purple and pink; and radio data are represented in shades of blue. Inverted triangles indicate upper limits. The optical data represented with circles are taken from the literature Busmann2025Aryan2025Fu_GCNFu2_GCNLi_GCNJin_GCNPan_GCNRor_GCNKumar_GCNBusmann_GCNBochenek_GCNBochenek2_GCNBochenek3_GCNFreeburn_GCNFreeburn2_GCNQuirolaV_GCNMoskvitin_GCNMoskvitin2_GCNMoskvitin3_GCNSchneider_GCN. The solid black lines and shaded-colored regions represent the best fit and the 500 best likelihood fits of the data using power law models.
• Figure 2: Left panel: X-ray luminosities in the 0.3-10 keV band of Swift-detected GRBs Evans2007Evans2009. Right panel: Radio afterglow luminosities at 9 GHz of the GRB catalog presented in ChandraFrail2012. EP241021a is represented in magenta stars, while low-luminosity GRBs, such as GRB 060218 Campana2006Soderberg2006, GRB 100316D Starling2011Margutti2013, GRB 980425 Pian1999, and GRB 031203 Watson2004 are represented with light blue, yellow, light red, and green squares, respectively, in both panels.
• Figure 3: EP241021a broadband spectrum at $\sim$8 d (top panel) and $\sim$25 d (bottom panel). In the top panel, the solid lines represent a fit of two broken power law models. In the bottom panel, the solid line represents the sum of the radio and optical components, represented with dotted and dashed lines, respectively.
• Figure 4: EP241021a radio spectrum at 70 days. The 5 sigma upper limits from uGMRT are represented as triangles; ATCA observations are represented with dots; and ALMA observations are represented with stars. The spectra are fit with a broken power-law model, the colored regions represent the 500 highest-likelihood fits, while the black line represents the best fit.
• Figure 5: Top panel: Break frequency resulting from the fit as a function of time. The expected behavior is overplotted with a solid line in the case of a wind environment and with dashed and dot-dashed lines in the case of ISM. The vertical shaded line represents the assumed deceleration time. Bottom panel: Peak flux in the spectra as a function of time. The expected behavior as a function of time for the wind environment is constant, while that for the ISM is plotted in dashed lines.