Asteroseismic detection of an internal magnetic field in the B0.5V pulsator HD 192575

Context. Internal magnetic fields are an elusive component of stellar structure. However, they can play an important role in stellar structure and evolution models through efficient angular momentum transport and through their impact on internal mixing. Aims. We strive to explain the nine components of one frequency multiplet identified as a low-order quadrupole gravity mode detected in the light curve of the β Cep pulsator HD 192575 assembled by the Transiting Exoplanet Survey Satellite (TESS). Methods. We updated the frequencies of the quadrupole mode under investigation using a standard pre-whitening method applied to the 1951.46 d TESS light curve. This showed that an internal magnetic field is required to simultaneously explain all nine components. We implemented theoretical pulsation computations applicable to the low-order modes of a β Cep pulsator including the Coriolis force as well as a magnetic field that is misaligned with respect to the rotation axis. We applied the theoretical description to perform asteroseismic modelling of the amplitudes and frequencies in the multiplet of the quadrupole g-mode of this evolved β Cep star. Results. Pulsation predictions based on the measured internal rotation frequency of the star cannot explain the observed nine-component frequency splittings of the quadrupole low-order gravity mode. By contrast, we show that the combined effect of the Coriolis force caused by the near-core rotation with a period of ∼5.3 d and the Lorentz force due to an internal inclined magnetic field with a maximum strength of ∼24 kG does provide a proper explanation of the nine multiplet frequencies and their relative amplitudes. Conclusions. Given HD 192575’s stellar mass of about 12 M⊙, this work presents the detection and magneto-gravito-asteroseismic modelling of a stable internal magnetic field buried inside an evolved rotating supernova progenitor.