Central flashes during stellar occultations

Effects of diffraction, interferences, and stellar diameter

¹ Laboratoire Temps Espace (LTE), Observatoire de Paris, Université PSL, CNRS UMR 8255, Sorbonne Université, LNE, 61 Av. de l’Observatoire, 75014 Paris, France
² Université Jean Monnet Saint-Etienne, CNRS, Institut d Optique Graduate School, Laboratoire Hubert Curien UMR 5516, 42023 Saint-Etienne, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 May 2025
Accepted: 7 January 2026

Abstract

Context. Central flashes may occur during stellar occultations by objects in the Solar System.

Aims. Catalog diffraction effects on the flash with point-like stars, monochromatic waves, and different cases of spherical transparent atmosphere; describe the corrections due to stellar diameters.

Methods. To describe diffraction, we used the Huygens principle, the Sommerfeld lemma, and the stationary phase method, and we treated the effects of finite stellar diameter using Clausius’ theorem.

Results. For point-like stars, the central flash shape is that of the classical Poisson spot, but with a greater height. For tenuous atmospheres that cannot focus the stellar rays at the shadow center, the flash is amplified by the factor (R₀/r₀)² compared to the Poisson spot, where R₀ and r₀ are the object and the shadow radii, respectively. For denser atmospheres that can focus the rays at the shadow center, the flash peaks at 2π²(R_CF/λ_F)²ϕ_⊥(0), where R_CF is the central flash layer radius, λ_F is the Fresnel scale, and φ_⊥(0) is the flux that would be observed at the shadow center without focusing. For isothermal atmospheres with scale height H, the height is 2π²(R_CFH)/λ_F². Fringes surrounding the central flash are separated by λ_P = λ_F²/R_CF, which is related to the separation between the primary and secondary stellar images. For a projected stellar diameter D_* ≫ λ_P, the flash is described by complete elliptic integrals, and has a full width at half maximum of 1.14D_* and a peak value of 8H/D_*.

Conclusions. For Earth-based occultations by Pluto and Triton observed in the visible with point-like stars, diffraction causes flashes with very large heights of ∼10⁴−10⁵, spread over a very small meter-sized region in the shadow plane. In practice, the flash is usually smoothed by the stellar diameter, but still reaches high values of ∼50 and ∼200 during Pluto and Triton occultations, respectively. Diffraction dominates when using millimeter wavelengths or longer. We discuss the effects of departure from sphericity, atmospheric waves, and stellar limb darkening.