Accreting Schwarzschild-like compact object: Plasma-photon interaction and stability

Accretion is a common phenomenon associated with any astrophysical compact object, which is best described by plasma, a state of matter composed of electrons and heavy ions. In this paper, we analyze the linear dynamics of electromagnetic (EM) fields propagating through the accreting plasma around static and spherically symmetric horizonless, exotic compact objects (ECOs). The general equations governing the propagation of EM waves in such a background exhibits quasibound states, whose characteristic frequencies differ from the black hole (BH) values, for both the axial and the polar modes, as well as for homogeneous and inhomogeneous plasma distributions. Moreover, the real and imaginary parts of these quasibound frequencies depict an oscillatory behavior with the plasma frequency, characteristic of the ECOs considered. The amplitude of these oscillations depend on the nonzero reflectivity of the surface of the compact object, while the oscillation length depends on its compactness. This results in slower decay of the quasibound states with time for certain parameter space of the plasma frequency, compared to BHs, making these ECOs more prone to instabilities.