Unveiling the nature of the Einstein Probe transient EP241021a

We present a multiwavelength analysis of the fast X-ray transient EP241021a, discovered by the Wide-field X-ray Telescope aboard the Einstein Probe satellite on 2024 October 21. The event was not detected in gamma-rays. Follow-up observations from $\sim$1.5 to 100 d post-trigger were obtained across X-ray, ultraviolet, 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 d 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 an SN contribution at late times, suggesting a collapsar origin for EP241021a. The likely SN in EP241021a 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 Einstein Probe arise from massive stellar explosions.