The Radio Counterpart to the Fast X-ray Transient EP240414a

TL;DR

This study presents the discovery and radio characterization of EP240414a, the second extragalactic fast X-ray transient (FXT) with a radio counterpart. Using MeerKAT, ATCA, and the Allen Telescope Array, complemented by archival radio surveys, the authors construct a multi-frequency radio light curve that peaks at about $30$ days after explosion with a peak flux around $F_{3\mathrm{GHz}} \sim 4.3\times10^{-4}$ Jy and a spectral index $α = 0.58 ± 0.15$, implying an electron distribution index $p ≈ 2.16$. Equipartition analyses yield a Newtonian minimum energy $E_{eq,N} \approx 1.5\times10^{49}$ erg and a corresponding size $R_{eq} \approx 1.8\times10^{17}$ cm, with a inferred expansion speed $β_{eq,N} ≈ 3.3 c$, indicating a modestly relativistic outflow. In the relativistic framework, the data point to an on-axis Lorentz factor $Γ_{on} ≈ 1.6$ (for small viewing angles) and allow off-axis jet scenarios, consistent with a long GRB-like afterglow originating from a collapsar, potentially with a cocoon or choked-jet geometry. The lack of a gamma-ray counterpart suggests a low isotropic gamma-ray energy or off-axis visibility, reinforcing the idea that many FXTs trace the deaths of massive stars and that rapid, comprehensive follow-up with facilities like Einstein Probe is crucial for mapping FXT progenitor diversity.

Abstract

Despite being operational for only a short time, the Einstein Probe mission, with its large field of view and rapid localisation capabilities, has already significantly advanced the study of rapid variability in the soft X-ray sky. We report the discovery of luminous and variable radio emission from the Einstein Probe fast X-ray transient EP240414a, the second such source with a radio counterpart. The radio emission at $3\,\rm{GHz}$ peaks at $\sim30$ days post explosion and with a spectral luminosity $\sim2\times10^{30}\,\rm{erg}\,\rm{s}^{-1}\,\rm{Hz}^{-1}$, similar to what is seen from long gamma-ray bursts, and distinct from other extra-galactic transients including supernovae and tidal disruption events, although we cannot completely rule out emission from engine driven stellar explosions e.g. the fast blue optical transients. An equipartition analysis of our radio data reveals that an outflow with at least a moderate bulk Lorentz factor ($Γ\gtrsim1.6$) with a minimum energy of $\sim10^{48}\,\rm{erg}$ is required to explain our observations. The apparent lack of reported gamma-ray counterpart to EP240414a could suggest that an off-axis or choked jet could be responsible for the radio emission, although a low luminosity gamma-ray burst may have gone undetected. Our observations are consistent with the hypothesis that a significant fraction of extragalactic fast X-ray transients are associated with the deaths of massive stars.

Figures (6)

• Figure 1: Optical and radio detections of EP240414a/AT2024gsa. (top) A Pan-STARRS i-band image of the field of EP240414a. The source position is marked by a pair of white lines. The diffuse source to the left is the putative host galaxy SDSS J124601.99$-$094309.3 aryan_optjonkerredshift at a redshift of $z = 0.4018 \pm 0.0010$ (srivastav2024, see also vandalen2024). (bottom) A subsection of our MeerKAT discovery image of EP240414a with the source position marked by a pair of black lines. The source has a flux density of $227\pm13\,\mu\rm{Jy}$. We detect clear emission from the possible host galaxy of EP240414a. The MeerKAT restoring beam is shown in the bottom left and is $3.2"\times3.2"$ at a position angle of 0 degrees. A scale bar in the bottom right shows $10\,\rm{kpc}$ at $z=0.4$.
• Figure 2: The radio lights curve of EP240414a at 3, 5.5, and $9\,\rm{GHz}$ from MeerKAT and ATCA. The upper limit is from the ATA and is shown with a downward facing arrow. Errors are $1\sigma$ and include both a statistical and absolute calibration uncertainty, but are smaller than the markers. We show other extragalactic transients to help put our data into context. We include (relativistic) supernova, LFBOTs coppejans2020, thermal TDEs alexander2020, relativistic TDEs eftekhari2018rhodes2022, short gamma-ray bursts fong2021, long gamma-ray bursts berger2002vanderhorst2008bright2019, and low-luminosity gamma-ray bursts kulkarni1998soderberg2006. This figure is based on the one presented in ho2020. The specific luminosity has been reduced corrected by a factor of (1+z) to account for the cosmological distances of some sources.
• Figure 3: The relativistic energy as a function of bulk Lorentz factor and jet angle for $\beta_{\rm{eq,N}}=3.3$, as is appropriate for EP240414a. The green line divides on- and off-axis solutions to the left and right, respectively. We only show solutions below $10^{10}$ times the minimum energy in the Newtonian limit. The solid white line marks the minimum energy for a given angle and Lorentz factor. The dashed white line shows the asymptotic limit of $\Gamma$ which satisfies the minimum energy condition for $\theta\rightarrow1$ according to \ref{['eq:3']}. The dotted white line shows the solution from matsumoto2023 which breaks down for moderate values of $\beta_{\rm eq,N}$.
• Figure 4: Solution for the minimum bulk Lorentz factor for \ref{['eq:3']} and the one given in matsumoto2023, which becomes significantly incorrect for $\beta_{\rm eq,N}\lesssim15$ at which point the ratio becomes larger than 1.05. At $\beta_{\rm eq,N}\lesssim9$ the ratio surpasses 1.1.
• Figure 5: The Doppler factor as a function of $\Gamma$ and $\theta$. The region of the plot coloured white violates the condition $\delta^{49/34}>3.3$, and therefore the boundary is defined as $\delta_{D}^{49/34}=3.3$. It can be seen that the minimum energy locus is approximated by the boundary, and they converge as $\beta\rightarrow1$ (see figure 29 in matsumoto2023, and fender2019). The grey lines are the same as the whites ones in \ref{['fig:min_en']}.