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

Abstract

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 horizon-less, exotic compact objects (ECOs). The general equations governing the propagation of EM waves in such a background exhibit quasi-bound states whose characteristic frequencies differ from the 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 quasi-bound frequencies depict an oscillatory behaviour with the plasma frequency, characteristic of the ECOs considered. The amplitude of these oscillations depends on the non-zero reflectivity of the surface of the compact object, while the oscillation length depends on its compactness. This results in slower decay of the quasi-bound states with time for a certain parameter space of the plasma frequency, compared to BHs, making these ECOs more prone to instabilities.