Radio observations point to a moderately relativistic outflow in the fast X-ray transient EP241021a

TL;DR

This study of EP241021a demonstrates that fast X-ray transients lacking gamma-ray counterparts can still harbor energetic, relativistic outflows. Through an extended radio monitoring campaign and ISS-based size constraints, the authors infer a mildly relativistic outflow (Γ ≈ 4 at ~1.5 d) and, via broadband afterglow modeling with Afterglowpy, show that a top-hat jet seen near-axis or a highly relativistic structured jet seen off-axis can reproduce the data with a kinetic energy of order 10^{51} erg and a low radiative efficiency (η_gamma ≲ 1%). The findings support the interpretation that gamma-ray-dark FXRTs constitute a population of relativistic explosions (GRB-like or relativistic TDEs) whose gamma-ray output is suppressed by spectral softness or viewing geometry, underscoring the diagnostic power of radio scintillation for jet physics in high-energy transients.

Abstract

Fast X-ray transients (FXRTs) are short-lived X-ray outbursts with diverse progenitor scenarios, including compact object mergers, stellar core-collapses and tidal disruption events. The Einstein Probe (EP) has enabled the rapid discovery and follow-up of dozens of FXRTs, revealing that while some of them overlap with traditional gamma-ray bursts (GRBs), a larger fraction of FXRTs have no associated gamma-ray counterpart down to deep limits. The origin of these gamma-ray dark FXRTs and their connection to the diverse landscape of stellar explosions remains an open question, which can be tackled through the study of their multi-wavelength counterparts and environment. In this paper, we present long-term radio observations of the gamma-ray dark EP241021a, which exhibits sustained radio emission for over 100 days, placing it among the longest-lived radio afterglows. We detect signature of interstellar scintillation in early epochs, allowing us to constrain the angular size and Lorentz factor of the emitting region. Our observations point to an outflow that is at least mildly relativistic with Lorentz factor > 4. Afterglow modeling favors a moderately relativistic and collimated outflow interacting with a low-density interstellar medium. The derived beaming-corrected kinetic energy and low radiative efficiency are consistent with a standard relativistic explosion which did not produce bright gamma-rays. Alternatively, a highly-relativistic structured jet remains consistent with our observations if seen substantially off-axis. In the latter case, the initial X-ray flare detected by EP would be caused by the slower ejecta from the lateral wings intercepting our line of sight rather than by traditional prompt-emission mechanisms within the jet core.

Figures (10)

• Figure 1: e-MERLIN image of EP241021a. The transient is co-located with the 90% confidence-level from the Fraunhofer Telescope Wendelstein (orange circle; Busmann2025) at coordinates RA (J2000): 1$^{\mathrm{h}}$55$^{\mathrm{m}}$23.430$^{\mathrm{s}}$, Dec (J2000): +5$^\circ$56$^\prime$17.86$^{\prime\prime}$ with an uncertainty of 0.15$^{\prime\prime}$. It is also consistent with the 90% C.L. Chandra X-ray localization (yellow circle; see Sec. \ref{['sec:chandra']}). The e-MERLIN radio data (see Sec. \ref{['sec:e_MERLIN']}) provide an even tighter positional constraint at 1$\sigma$ confidence, with milliarcsecond-level precision.
• Figure 2: Observed gamma-ray flux (10--1000 keV) versus X-ray flux (0.5--4 keV) for EP-discovered FXRTs. EP sources with joint gamma-ray detections (EP-GRBs) are shown by orange circles, and gamma-ray dark FXRTs by upward gray triangles (which mark $3\sigma$ upper limits to the gamma-ray emission, based on 8.192 s time-averaged fluxes from Fermi-GBM; (see Section \ref{['sec:sec2']}). The X-ray fluxes represent the time-averaged, unabsorbed 0.5--4 keV fluxes derived from WXT observations; uncertainties are shown at the 90% confidence level. EP241021a is indicated by the downward red triangle. Dashed lines represent power-law spectra with photon indices $\Gamma = 0.5$, $1.0$, and $1.5$.
• Figure 3: ATCA and e-MERLIN radio observations of EP241021a. Flux densities are listed in Table \ref{['tab:flux_density_ATCA']} and \ref{['tab:flux_density_grouped']}. The inset panel displays the intra-observation light curve from the first epoch at 5.5 GHz, segmented into 40-min intervals to highlight short-term variability.
• Figure 4: Variability timescale on October 29, 2024 with data sampled at 40-minute intervals.
• Figure 5: Spectral index evolution between 5.5–9 GHz (red circles) and 9–17 GHz (blue squares) over time since the EP trigger. The dashed line indicates $\beta = 0.75$.