Dark matter (DM) and its non interacting nature with Standard Model particles has been a mystery since decades. Its presence can be felt only because of gravity on astrophysical scale and the key to solve the dark matter mystery lies in measuring the non-gravitational interactions of dark matter with ordinary particles. Ordinary matter contributes approximately 5% of the total energy content of the universe while the dark matter contributes approximately 1/4 of the total content of the universe and remaining the 70% is dark energy. The structure of galaxies and their evolution depend strongly on the nature of dark matter. The understanding of dark matter helps understand the role of particle physics in the evolution of the universe.
Neutron stars (NS) may be viewed as astrophysical laboratories to test and enhance our knowledge of behaviour of nuclear matter at the extremes of pressure, energy density, temperature. They have also been used to test proposals concerning the nature of DM. Now, Husain and Thomas presented a study of neutron star models that contain dark matter (DM) in the core. The DM is assumed to have a particle nature and to be self-interacting. Using constraints on the mass and radius of neutron stars, they investigated the allowed properties of either bosonic or fermionic DM particles.
For this purpose, they considered three different models of neutron stars, the first involving nucleons only, the second including hyperons, and the last involving strange matter in the core. Different equation of states (EoS’s) are constructed for the various cases of fermionic and bosonic DM. For ex: for pure nuclear matter and hyperons at the core they have considered N-QMC700 (pure nuclear matter EoS), F-QMC700 (EoS includes hyperons), strange matter EoS. These EoSs are solved for selected properties of the DM particles and the results are tested against mass, radius and tidal deformability constraints for neutron stars.
They manifested that the models available to explain DM accretion on a neutron star suggested that it takes thousands, millions or maybe more years for a neutron star to accumulate DM matter mass that contributes 5% of the total mass. Having 5% of the mass contributed by DM is a large amount of mass, in the sense that it leads to significant changes in the properties of the neutron stars. Having 5% or less DM mass in the core seems a good and realistic approximation to see the changes of the neutron star properties. From mass vs radius plots it is evident that in general having DM in the core will reduce the radius and maximum mass of the neutron star. Bosonic DM neutron stars may be heavier and bigger than fermionic DM neutron stars.
“As more dark matter is accreted by the neutron star, the total mass and radius of neutron star decreases. As the DM mass increases further the maximum mass of neutron star moves below 2 solar masses, with the corresponding decrease in radii.”— told Husain, first author of the study
They have also shown a comparative study of three models of neutron stars with DM of dark matter particle mass, mχ = 1 GeV (for bosonic DM self-scattering length lχ= 1 fm and for fermionic DM self-interacting particle mass mI = 100 MeV). They found that the strange matter EoS produces neutron stars with a maximum mass very close to 2M when 5% of the mass is contributed by DM. F-QMC700 is also very close to producing a maximum mass of 2M for the neutron star but strange matter EoS is closer. While, the N-QMC700 EoS can produce neutron stars with a maximum mass of more than 2 solar masses, even when 15% of the mass is contributed by DM. For selected values of the DM particle properties and type of interaction, they found that the constraint of maximum mass of at least 2 solar masses and radius in the range 9 to 13 kms stars is respected by all three types of EoSs.
Furthermore, the distribution of the energy density of DM inside the neutron stars suggested that fermionic DM covers the whole neutron star right from the centre to the surface and outside, where it constitutes a halo, while bosonic DM sits inside the surface of the neutron star.
“Bosonic DM condenses inside the core and remains inside the surface of the neutron star within a radius of a few kilometers. While, fermionic DM does not condense, and covers the whole neutron star from core to outside the surface, because the fermionic DM pressure does not vanish before the nuclear matter pressure.”— told Husain, first author of the study
In light of constraint on tidal deformability, in general the neutron stars can possess greater amount of bosonic DM than fermionic DM. As the fermionic DM contribution can be at most 10% of the total neutron star mass for N-QMC700 and F-QMC700 while it can 18% for bosonic DM with N-QMC700 for have a tidal deformability in the range Λ = 400-800. For a strange matter neutron star the tidal deformability constraint suggests that whether it is fermionic DM or bosonic DM, the DM mass inside the neutron star can be no more than 5% of the total neutron star mass.
“As the DM increases inside the neutron star, the tidal deformability of the neutron star decreases, because neutron star becomes more complicated. We have shown that, for neutron stars with F-QMC700 and bosonic DM the tidal deformability decreases with Increasing DM content inside the neutron star. “told Thomas, second author of the study.
“As shown by us in our paper, the distribution of energy density of fermionic and bosonic DM is very different. In future it will be interesting to see the gravitational lensing around the neutron star containing DM inside it, that may suggest the nature of DM because gravitational lensing will be different for different distributions of energy densities.”— concluded authors of the study.
Reference: Wasif Husaina, Anthony W. Thomas, “Possible nature of Dark Matter”, ArXiv, pp. 1-17, 2021. https://arxiv.org/abs/2104.01540
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