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

Journal article

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 δ t ≈ 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.

Authors: Ibrahimzade, D; Margutti, R; Bright, JS; Blanchard, P; Paterson, K

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal
• Volume: 980
• Issue: 1
• Article number: 92
• Publication date: 2025-02-05
• Acceptance date: 2024-11-07
• DOI: 10.3847/1538-4357/ad9397
• EISSN: 1538-4357
• ISSN: 0004-637X
• Language: English
• Keywords: X-ray transient sources