Radio emission from radio-bright fast blue optical transients powered by trans-relativistic shocks in confined circumstellar material
Year: 2026
Type: article
Abstract: Context. Fast blue optical transients (FBOTs) are luminous, rapidly evolving explosions whose radio emission provides a vital probe of the shock dynamics and the density structure of the circumstellar material (CSM). Most existing radio studies rely on epoch-by-epoch spectral fits; however, this approach lacks dynamical consistency and fails to reproduce the steep post-peak fading observed in several well-sampled events. Aims. We aim to develop a self-consistent physical framework for interpreting the radio emission of FBOTs and for constraining the immediate mass-loss history of their progenitors. Methods. We present a forward-shock synchrotron model for ejecta interacting with a dense but radially confined CSM described by a broken power-law density profile. By incorporating synchrotron self-absorption and external free-free absorption, we modeled the multi-frequency radio light curves and broadband spectra of a representative sample of FBOTs. Results. We show that the diversity of FBOT radio morphologies can be naturally explained by shock interaction within a finite CSM shell. Absorption regulates the early-time evolution, while the rapid post-peak decline marks the forward shock’s exit from the dense inner region into a tenuous environment. The inferred shock velocities are trans-relativistic ( v ∼ 0.1–0.5 c ) and are associated with high mass-loading rates ( Ṁ ∼ 10 −4 –10 −3 M ⊙ yr −1 ), yet modest total CSM masses ( M csm ∼ 10 −4 –10 −2 M ⊙ ). Conclusions. These properties point to brief episodes of enhanced mass loss occurring years to decades prior to explosion, rather than long-lived steady winds. Our framework provides a physically motivated interpretation of FBOT radio emission and a robust diagnostic for reconstructing the immediate mass-loss history of their progenitors.
Source: Astronomy and Astrophysics
Institution: Central China Normal University
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
OpenAlex ID: W7220832668