EP250226a: A fast X-ray transient associated with GRB 250226A and its multiwavelength afterglow properties

Recent advances in fast X-ray transient studies have been driven by the wide-field, sensitive soft X-ray monitoring capabilities of the Einstein Probe (EP), enabling the prompt detection and localization of previously underexplored events. EP observations offer new opportunities to investigate the origins, emission mechanisms, and population properties of newly discovered fast X-ray transients. We present multiwavelength observations of EP250226a, an extragalactic fast X-ray transient associated with GRB 250226A. We jointly analyze the prompt emission from soft X-rays to gamma rays and the X-ray afterglow spectra. We collect optical photometric data, perform empirical and afterglow model-based analyses, and present optical-to-infrared spectroscopy, measuring the equivalent widths of absorption lines. The spectroscopic redshift is $z = 3.314$, and the isotropic-equivalent gamma-ray energy is Eᵢso7.32×10⁵³ erg. Its location in the Amati diagram is consistent with a massive-star core-collapse origin (Type II GRB). The fluence ratio S(25-50\,keV)/S(50-100\,keV)=0.57±0.06 supports its classification as a classical GRB. The X-ray and optical afterglows show achromatic behavior. Among the models considered, a top-hat jet propagating into a uniform interstellar medium best describes the multiwavelength afterglow. This model suggests a highly collimated, nearly on-axis jet, with an isotropic-equivalent kinetic energy of Eₖ,iso4.07×10⁵⁵ erg, a gamma-ray radiative efficiency of η_γ 1.77%, and a low magnetic-field energy fraction of log εB,FS -6.52. EP250226a exhibits typical long-duration GRB properties, with the first soft X-ray peak coinciding with the gamma-ray prompt emission, while the soft X-ray emission lasts significantly longer than the gamma-ray emission.