Constraints on Relativistic Jets from the Fast X-ray Transient 210423 using Prompt Radio Follow-Up Observations

Abstract

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 multi-wavelength 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 VLA of FXT 210423 obtained at $\hat{I}´t\approx 14-36$ days since the X-ray trigger. We use these multi-band 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 $z\le 3.5$, circumstellar medium (CSM) density $n=10^{-6}-10^{-1}\mathrm{c}{\mathrm{m}}^{-3}$, isotropic-equivalent jet kinetic energy $E_{k,iso}=10^{48}-10^{55}\mathrm{erg}$, we find that we can rule out wide jets with opening angle $\hat{I}{¸}_j=15^{\circ }$ viewed within $10^{\circ }$ off-axis. For more collimated jets ($\hat{I}{¸}_j=3^{\circ }$) we can only rule out on-axis ($\hat{I}{¸}_{obs}=0^{\circ }$) orientations. This study highlights the constraining power of prompt multi-wavelength observations of FXTs discovered in real time by current (e.g., Einstein Probe) and future facilities.

14 pages, 6 figures