2025ApJ...980...92I

Fast X-ray transients (FXTs) are a new observational class of phenomena with no clear physical origin. This is at least partially a consequence of limited multiwavelength follow-up of this class of transients in real time. Here we present deep optical (g- and i-band) photometry with Keck, and prompt radio observations with the Very Large Array of FXT 210423 obtained at δt ≈ 14-36 days since the X-ray trigger. We use these multiband observations, combined with publicly available data sets, to constrain the presence and physical properties of on-axis and off-axis relativistic jets such as those that can be launched by neutron star mergers and tidal disruption events, which are among the proposed theoretical scenarios of FXTs. Considering a wide range of possible redshifts z ≤ 3.5, circumstellar medium density n = 10^–6-10^–1 cm^–3, and isotropic-equivalent jet kinetic energy E_k,iso = 10⁴⁸-10⁵⁵ erg, we find that we can rule out wide jets with opening angle θ_j = 15° viewed within 10° off-axis. For more collimated jets (θ_j = 3°) we can only rule out on-axis (θ_obs = 0°) orientations. This study highlights the constraining power of prompt multiwavelength observations of FXTs discovered in real time by current (e.g., Einstein Probe) and future facilities.