We use gravitational wave ( GW ) and electromagnetic ( EM ) observations of GW170817 to constrain the extent of pressure anisotropy in it .
While it is quite likely that the pressure inside a neutron star is mostly isotropic , certain physical processes or characteristics , such as phase transitions in nuclear matter or the presence of strong magnetic fields , can introduce pressure anisotropy .
In this work , we show that anisotropic pressure in neutron stars can reduce their tidal deformability substantially .
For the anisotropy-pressure model of Bowers and Liang and a couple of relativistic EOSs – DDH \delta and GM1 – we demonstrate that this reduction in spherical neutron stars with masses in the range of 1 to 2 M _ { \odot } can be 23 % to 46 % .
This suggests that certain EOSs that are ruled out by GW170817 observations , under assumptions of pressure isotropy , can become viable if the stars had a significant enough anisotropic pressure component , but do not violate causality .
We also show how the inference of the star radius can be used to rule out certain EOSs ( such as GM1 ) , even for high anisotropic pressure , because their radii are larger than what the observations find .