# Relativistic Neutron Stars: Rheological Type Extensions of the Equations of State

## TL;DR

This paper introduces a rheological extension to the equations of state for relativistic neutron stars, incorporating a new length-scale parameter that influences star compactness and pressure profiles.

## Contribution

It develops a novel rheological framework for neutron star equations of state, including a new parameter affecting star structure and applying it to the Lane-Emden model.

## Key findings

- Rheological extension makes neutron stars more compact.
- The new parameter decreases the star radius as it increases.
- Pressure profiles are significantly affected by rheological effects.

## Abstract

Based on the Rheological Paradigm, one has extended the equations of state for relativistic spherically symmetric static neutron stars, taking into consideration the derivative of the matter pressure along the so-called director four-vector. The modified equations of state are applied to the model of a zero-temperature neutron condensate. This model includes one new parameter with the dimensionality of length, which describes the rheological type screening inside the neutron star. As an illustration of the new approach, one has considered the rheological type generalization of the non-relativistic Lane-Emden theory and found the numerical profiles of the pressure for a number of values of the new guiding parameter. One has found that the rheological type self-interaction makes the neutron star more compact, since the radius of the star, related to the first null of the pressure profile, decreases when the modulus of the rheological type guiding parameter grows.