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

TL;DR

This study uses prompt radio and optical follow-up observations of the Fast X-ray Transient 210423 to constrain the properties and presence of relativistic jets, providing insights into their physical characteristics and orientations.

Contribution

It presents the first constraints on relativistic jet properties of FXT 210423 using multi-wavelength follow-up, highlighting the importance of real-time observations for understanding FXTs.

Findings

Wide jets with 15° opening angle viewed within 10° off-axis are ruled out.

On-axis collimated jets with 3° opening angle are only constrained at on-axis orientations.

Prompt multi-wavelength follow-up effectively constrains jet properties of FXTs.

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 ${\delta t \approx 14-36}$ 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 $z\le3.5$, circumstellar medium (CSM) density $n={10^{-6}-10^{-1}\,\rm{cm^{-3}}}$, isotropic-equivalent jet kinetic energy $E_{k,iso}={10^{48}-10^{55}\,\rm{erg}}$, we find that we can rule out wide jets with opening angle ${\theta_{j}=15^{\circ}}$ viewed within ${10^{\circ}}$ off-axis. For more collimated jets (${\theta_{j}=3^{\circ}}$) we can only rule out on-axis (${\theta_{obs}=0^{\circ}}$) 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.