Viscosity driven bar mode secular instabilities of rapidly rotating neutron stars are studied using LORENE/Nrotstar code. These instabilities set a more rigorous limit to the rotation frequency of neutron star than the Kepler frequency/mass shedding limit. The procedure employed in the code comprises of perturbing an axisymmetric and stationary configuration of a neutron star and studying its evolution by constructing a series of triaxial quasi-equilibrium configurations. Symmetry breaking point was found out for Polytropic as well as 10 realistic Equations of states (EOS) from the CompOSE database. The concept of piecewise polytropic EOSs has been used to comprehend the rotational instability of Realistic EOSs and validated with 19 different Realistic EOSs from CompOSE. The possibility of detecting quasi-periodic gravitational waves from viscosity driven instability with ground based LIGO/VIRGO interferometers is also discussed very briefly.
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