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Bulk viscosity of neutron star cores is responsible for a number of interesting phenomena, e.g., r-mode stabilization [1] and magnetic inclination-angle evolution [2]. It originated because of non-equilibrium particle mutual transformations in dense stellar plasma, and strongly depends on the actual core composition. It is well-known that account for hyperons in the core dramatically increases the bulk viscosity comparing to purely nucleonic one [3]. Modern equations of state, calibrated to the up-to-date hypernuclear data (e.g., [4,5]), predict that hyperons are mainly presented in the form of $\Lambda$'s and $\Xi^-$'s, while all the existing calculations of the non-equilibrium reaction rates ([3,6,7] and others) have been performed for $\Sigma^-\Lambda$ hyperonic composition.

In the present work we fill this gap by calculating the bulk viscosity for $npe\mu\Lambda\Xi^-$ matter. A number of viscosity-generating nonequilibrium reactions is considered, some of them have never been studied in the neutron-star literature before. The calculated reaction rates and bulk viscosity are approximated, for a number of realistic equations of state, by simple analytic formulas, in order to facilitate their use in applications. Possible consequences of our results for the r-mode physics are briefly discussed.

References

[1] Haskell, B. 2015, IJMPE, 24, 1541007.

[2] Lander, S. K. & Jones, D. I. 2018, arXiv:1807.01289.

[3] Lindblom, L. & Owen, B. J. 2002, PRD, 65, 63006.

[4] Fortin, M., Avancini, S. S., Provid$\hat{\rm e}$ncia, C., et al. 2017, PRC, 95, 65803.

[5] Jie Li, J., Long, W. H. & Sedrakian, A. 2018, arXiv:1801.07084.

[6] Haensel, P., Levenfish, K. P. & Yakovlev, D. G. 2002, A&A, 381, 1080.

[7] van Dalen, E. N. & Dieperink, A. E. 2004, PRC, 69, 25802.