Letter to the Editor

Non-thermal radio emission in Sakurai’s Object

¹ Department of Geodesy, Institute of Geodesy and Civil Engineering, Faculty of Geoengineering, University of Warmia and Mazury, ul. Oczapowskiego 2, 10-719 Olsztyn, Poland
² Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
³ Jodrell Bank Centre for Astrophysics, Alan Turing Building, University of Manchester, Manchester M13 9PL, UK
⁴ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁵ Universidad Católica del Norte, Instituto de Astronomía, Av. Angamos 0610, Antofagasta, Chile
⁶ Universidad Central de Chile, Facultad de Ingeniería y Arquitectura, Av. Francisco de Aguirre 0405, La Serena, Coquimbo, Chile
⁷ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁸ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Morelia, 58089 Michoacán, Mexico

Received: 24 April 2024
Accepted: 25 July 2024

Abstract

Context. The very late thermal pulse (VLTP) affects the evolution of ∼20% of 1–8 M_⊙ stars, repeating the last red giant phases within a few years and leading to the formation of a new, but hydrogen-poor, nebula within the old planetary nebula. The strong dust formation in the latter obscures the optical and near-infrared radiation of the star.

Aims. We aimed to determine the reheating timescale of the central star in Sakurai’s Object, which is an important constraint for the poorly understood VLTP evolution.

Methods. We observed the radio continuum emission of Sakurai’s Object for almost 20 years, from 2004 to 2023. Continuous, multi-frequency observations proved to be essential for distinguishing between phases dominated by photoionization and shock ionization.

Results. The flux density fluctuates by more than a factor of 40 within months to years. The spectral index remained negative between 2006 and 2017 and has been close to zero since 2019. The emission region has been only barely resolved since 2021.

Conclusions. Non-thermal radio emission observed from 2004 to 2017 traces shocks induced by wind interactions due to discrete mass-loss events. Thermal emission dominates from 2019 to 2023 and may indicate photoionization of the nebula by the central star.