Constraints on Relativistic Jets from the Fast X-Ray Transient 210423 Using Prompt Radio Follow-up Observations

• DOI: 10.3847/1538-4357/ad9397
• Publication Date: 2025-02-05T17:30:56Z
• Authors: Dina Ibrahimzade; R. Margutti; J. S. Bright; P. Blanchard; K. Paterson; D. Lin; H. Sears; A. Polzin; I. Andreoni; G. Schroeder; K. D. Alexander; E. Berger; D. L. Coppejans; A. Hajela; J. Irwin; T. Laskar; B. D. Metzger; J. C. Rastinejad; L. Rhodes
• Keywords: Astrophysical jet; Transient (computer programming); Relativistic beaming; X-ray transient

Abstract Fast X-ray transients (FXTs) are a new observational class of phenomena with no clear physical origin. This is at least partially consequence limited multiwavelength follow-up this in real time. Here we present deep optical ( g - and i -band) photometry Keck, prompt radio observations the Very Large Array FXT 210423 obtained δ t ≈ 14–36 days since trigger. We use these multiband observations, combined publicly available data sets, to constrain presence properties on-axis off-axis relativistic jets such as those that can be launched by neutron star mergers tidal disruption events, which among proposed theoretical scenarios FXTs. Considering wide range possible redshifts z ≤ 3.5, circumstellar medium density n = 10 −6 –10 −1 cm −3 , isotropic-equivalent jet kinetic energy E k ,iso 48 55 erg, find rule out opening angle θ j 15° viewed within 10° off-axis. For more collimated 3°) only obs 0°) orientations. study highlights constraining power FXTs discovered time current (e.g., Einstein Probe) future facilities.