Sayantan Ghosh

We investigate the impact of adiabatic sound speed on the radial oscillations and stability of neutron stars (NSs), using five representative equations of state (EOSs): three hadronic (FSU2R, HPUC, SINPA) and two hybrid (Gibbs 40, KW 55). The adiabatic case, which assumes fixed particle composition during compression, corresponds to slow reaction scenarios and leads to an extension of the stable branch toward smaller radii and higher compactness, even beyond the maximum mass configuration. This extended regime, known as the “slow stable” branch, remains dynamically stable only when reactions are sufficiently slow. To assess stability, we subject the NSs to radial perturbations and examine their fundamental radial (f-mode) oscillations, identifying the transition point where the mode frequency vanishes. The adiabatic sound speed, compared to the equilibrium sound speed, delays the onset of instability, allowing NSs to sustain stability at higher compactness and thus exhibit smaller radii — a more physically realistic outcome for slow reaction rates. Notably, the delays in the onset of instability are more pronounced for hybrid EOSs than for hadronic ones.

Neutron stars (NSs) are incredibly versatile for studying various important aspects of high-energy and compact-object physics. These celestial objects contain extreme matter at incredibly high densities in their interiors, leading to the risk of instabilities that may cause them to collapse into a black hole (BH). This paper focuses on exploring the stability and gravitational collapse of NSs. For a more realistic approach we have considered the pressure to be locally anisotropic. We utilize the BL model to describe the anisotropy inside the NS. The presence of quarks in the core of an NS can heavily affect its stability. Hence, along with pure hadronic EOSs, we have also considered hadron-quark phase transition (HQPT) EOSs for this paper’s analysis. We subject the anisotropic NSs to radial perturbations to study their stability against radial oscillations. NSs exhibiting imaginary eigenfrequencies are identified as unstable, and their inevitable destiny is gravitational collapse, resulting in the formation of a BH. We consider the interior of these unstable anisotropic NSs to be a nonideal fluid in a nonadiabatic background in order to study its dynamical evolution during the collapse. We examine the temporal evolution of key properties of NSs, such as mass, density, heat flux, and anisotropy during the process of gravitational collapse. We present an innovative and viable approach to detect such high-energy gravitational collapse events, providing valuable insights into the properties of the static NS before its collapse.

Non-radial oscillations of Neutron Stars (NSs) provide a means to learn important details regarding their interior composition and equation of state. We consider the effects of Δ-baryons on non-radial f-mode oscillations and other NS properties within the Density-Dependent Relativistic Mean Field formalism. Calculations are performed for Δ-admixed NS matter with and without hyperons. Our study of the non-radial f-mode oscillations revealed a distinct increase in frequency due to the addition of the Δ-baryons with upto 20% increase in frequency being seen for canonical NSs. Other bulk properties of NSs, including mass, radii, and dimensionless tidal deformability (Λ) were also affected by these additional baryons. Comparing our results with available observational data from pulsars (NICER) and gravitational waves (LIGO-VIRGO collaboration), we found strong agreement, particularly concerning Λ.