Modelling Dissipative Dynamics of r-mode Instability in Hybrid Stars

Abstract

Compact star cores reach extreme densities and may contain exotic dense-matter phases. Information about the exotic interiors of rapidly rotating pulsars can be inferred from r-mode oscillations, whose stability is governed by viscous dissipation. In this work, we model a compact star containing a possible mixed phase of hadronic and quark matter and employ a hybrid statistical framework based on Bayesian inference to infer the dissipation time scales associated with the hybrid phase. Using low-mass X-ray binaries (LMXB) timing observations together with mass-radius constraints from the Neutron Star Interior Composition Explorer (NICER) mission, we estimate the shear and bulk viscosity contributions to r-mode damping for a hybrid star of two layers. Our inference yields shear and bulk viscous dissipation time scales of $τ_s=(4.99^{+0.49}_{-0.52}) \times 10^8 T^{\frac{5}{3}}$s and $τ_B=(2.150^{+1.23}_{-0.60}) \times 10^{19} (T^4 10^{-12}+T^2 10^{-6})^{-1}Ω^{-2}$s respectively. The timescales thus obtained can be implemented to obtain the minima of the star's rotation frequency at $Ω=451.87$ Hz at temperature $T=0.259$ MeV for a hybrid star of mass $1.5$ $M_{\odot}$ and $Ω=517.47$ Hz at $T=0.234$ MeV for $M=1.75 M_{\odot}$. We find that the instability window obtained through the inference framework effectively explains the observed stability of millisecond pulsars in both the radio and LMXB populations, particularly for XTE J0929-314 and XTE J1807-294, J0437-4715, J2124-3358, respectively. These results demonstrate that Bayesian inference combined with r-mode phenomenology provides a powerful and observationally consistent framework for constraining the transport properties of dense hybrid matter.