Unveiling the nature of the Einstein Probe transient EP 241021a

TL;DR

EP 241021a represents a luminous, extragalactic FXT detected by the Einstein Probe, with a redshift of $z=0.7485$ and a peak X-ray luminosity of $L_{\rm X,peak}\approx2\times10^{48}$ erg s$^{-1}$. Via an intensive multi-wavelength campaign (X-ray to radio) and comprehensive SED/light-curve modeling, the authors show a three-epoch optical evolution: an initial decay, a pronounced re-brightening around $t\approx7.7$ d, and a late-time re-brightening near $t\approx19$ d. The data are best explained by a hybrid scenario combining non-thermal afterglow emission (consistently from radio to X-ray in Epochs I–II) with a thermal component emerging after day $\sim18$–$19$ d, interpreted as a broad-lined Type Ic SN within a collapsar framework, possibly powered by magnetar-like energy injection. This work demonstrates that some EP FXTs originate from massive stellar explosions, and emphasizes the importance of rapid, coordinated, long-term follow-up to disentangle multi-component engines and progenitor channels in FXTs.

Abstract

We present a multi-wavelength analysis of the fast X-ray transient EP 241021a, discovered by the Wide-field X-ray Telescope aboard the \emph{Einstein Probe} satellite on 2024 October 21. The event was not detected in gamma-rays. Follow-up observations from $\sim$1.5 to 100 days post-trigger were obtained across X-ray, UV, optical, near-infrared, and radio bands with ground- and space-based facilities. The redshift is constrained to $z = 0.7485$ from prominent optical spectral features. The optical light curve shows complex evolution: an initial $\sim t^{-0.7}$ decay, followed by a rapid re-brightening peaking at day 7.7 with $\sim t^{-1.7}$ decay, and a third phase peaking near day 19 with $\sim t^{-1.3}$ decay. The spectral energy distribution (SED) and its temporal evolution are consistent with a mix of non-thermal and thermal components. Early optical-to-X-ray spectral indices agree with optically thin synchrotron emission, while steepening of the optical SED after $\sim$20 days indicates either a shift in emission mechanism or the emergence of an additional component. Although broad-lined absorption features are absent, comparisons with type Ic-BL supernovae suggest a SN contribution at late times, suggesting a collapsar origin for EP 241021a. The likely SN in EP 241021a appears to require an additional energy source beyond $^{56}$Ni decay. These results support the view that some fast X-ray transients detected by the \emph{Einstein Probe} arise from massive stellar explosions.

Figures (18)

• Figure 1: $r$-band image of the X-ray transient EP 241021a; the position of the event is marked by magenta lines. The large panel is made from the GMOS observation of EP 241021a at day $\sim6.9$. The blue (10$^{\prime\prime}$ radius) and green (4.9$^{\prime\prime}$ radius) circles show the location of the transient reported by the EP Follow-up X-ray Telescope and Swift-XRT, respectively. The lateral panels display eight snapshots of the photometric evolution in $r$-band of the transient, from day $\sim2.76$ to $93.8$, taken by different telescopes.
• Figure 2: Apparent (left Y-axis) and absolute (right Y-axis) magnitude light curves of FXT EP 241021a in the observer- (bottom X-axis), and rest frame (top X-axis) in several optical and NIR bands. The times at which our spectroscopic observations were taken are marked by the vertical black dashed lines.
• Figure 3: Evolution of the optical spectra of the counterpart of FXT EP 241021a in the observer (bottom X-axis) and rest (top X-axis) frame. The spectrum obtained first is shown at the top, and the last one is at the bottom (the times in brackets next to the spectra indicate the days after the WXT trigger, in the observer frame). In addition, we also show the flux density measured using our four GTC/HiPERCAM observations ($ugriz$ bands). Vertical regions corresponding to telluric absorption are shaded.
• Figure 4: $r$-band light curve fit of the FXT EP 241021a using a smooth triple-power-law model (black line). The source flux decays initially following approximately a power-law, then it re-brightens around $t_1=5.6\pm0.2$ days reaching a peak flux at $t_2=7.7\pm0.1$ days, and it subsequently decays, which can be described approximately by another power-law. The Roman numbers at the top depict the three epochs of the light curve: (I) initial decay at $t\lesssim6$ days; (II) re-brightening and a peak emission followed by a decay at $6\lesssim t\lesssim18$ days; and (III) a re-brightening followed by power-law decay starting at $t\approx18$ days. We can expect a similar light curve behavior in other bands; however, the sampling cadence is not identical for all of them. Moreover, the inside figure depicts a comparison between the smooth triple-power-law model and $ugiz$ data. The units in the inside plot are the same as the large plot.
• Figure 5: Spectroscopic evolution of EP 241021a. The red dashed lines depict the power-law model that can describe the continuum of the transient (see §\ref{['sec:spectra']} for more details), as well as their best-fit power-law index. For visual purposes, offsets were applied to the spectra (see legend).