Axisymmetric magnetic activity on the Sun and Sun-like stars increases the frequencies of the modes of acoustic oscillation. However, it is unclear how a corotating patch of activity affects the oscillations, since such a perturbation is unsteady in the frame of the observer. In this paper we qualitatively describe the asteroseismic signature of a large active region in the power spectrum of the dipole and quadrupole p modes. In the corotating frame of the active region, the perturbations due to (differential) rotation and the active region completely lift the $(2ell + 1)$-fold azimuthal degeneracy of the frequency spectrum of modes with harmonic degree $ell$. In the frame of the observer, the unsteady nature of the perturbation leads to the appearance of $(2ell+1)^2$ peaks in the power spectrum of a multiplet. These peaks blend into each other to form asymmetric line profiles. In the limit of a small active region, we approximate the power spectrum of a multiplet in terms of $2times(2ell+1)$ peaks, whose amplitudes and frequencies depend on the latitude of the active region and the inclination angle of the stars rotation axis. In order to check the results and to explore the nonlinear regime, we also perform numerical simulations using the 3D time-domain pseudo-spectral linear pulsation code GLASS.
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