Indian Physicists Explore Axionic Dark Matter’s Impact on Neutron Stars

Researchers from the Birla Institute of Technology and Science in India have conducted a study on the impact of axionic dark matter on non-rotating magnetized neutron stars. Using a modified Tolman-Oppenheimer-Volkoff system of equations, they determined the luminosities of direct photons, neutrinos, and axions for a specific axion mass in the presence of a magnetic field. The study also explored the possibility of axion-to-photon conversion in the magnetosphere of neutron stars and the impact of the magnetic field on the energy spectrum of axions and axion-converted photon flux. The research could provide valuable insights into the nature of dark matter.

What is the Impact of Axionic Dark Matter on Neutron Stars?

The study conducted by Shubham Yadav, M Mishra, and Tapomoy Guha Sarkar from the Department of Physics at Birla Institute of Technology and Science in Pilani, Rajasthan, India, explores the impact of axionic dark matter on non-rotating magnetized neutron stars. The researchers used a modified Tolman-Oppenheimer-Volkoff (TOV) system of equations to determine the luminosities of direct photons, neutrinos, and axions for a particular axion mass in the presence of a magnetic field. They employed two different equations of states (EoSs), namely APR and FPS, to generate the profiles of mass and pressure for spherically symmetric and non-rotating Neutron stars (NSs).

The researchers computed the axions and neutrino emission rates by employing the Cooper-pair-breaking and formation process (PBF) in the core using the NSCool code. They also examined the possibility of axion-to-photon conversion in the magnetosphere of NSs. Furthermore, they investigated the impact of the magnetic field on the actual observables such as the energy spectrum of axions and axion-converted photon flux for three different NSs. Their comparative study indicates that axions’ energy spectrum and axion-converted photon flux change significantly due to an intense magnetic field.

What is the Role of Axions in Dark Matter?

Various astrophysical observations indicate that a significant fraction (30%) of the universe’s matter-energy budget is in the form of dark matter. There has been a tremendous effort to understand this mysterious matter in terms of particles and in the framework of particle physics. The QCD axion is a promising candidate for such dark matter. These hypothetical particles are postulated to explain the CP conservation in QCD, commonly called a strong CP problem. The axion field is introduced through a derivative coupling to a fermion field ψf with an interaction Lagrangian LintCf2faψfγµγ5ψfµa where fa is the decay constant of axion. This term allows for the conversion of a fermion to an axion. Two possible axion models exist in the literature, the Kim-Shifman-Weinstein-Zakharov (KSVZ) hadronic and the Dean-Fischler-Srednitsky-Zhitnitsky (DFSZ) model, depending on whether the axions couple only with hadrons or leptons.

How are Axions Detected?

Several theoretical and experimental attempts have been carried out so far to find the properties of the axions and to explore their detection possibilities. The Axionic Dark Matter experiment (ADMX) located at the University of Washington, US, is one such experiment and will cover much of the axions parameter space. According to the ADMX experiment, if these very light particles exist, then it could be possible that they could decay into a pair of light particles, thus making them difficult to detect. Also, the various collider experiments continue to search for these weakly interacting particles, so-called axions.

What is the Role of Neutron Stars in Axion Detection?

At the end stage of the stars’ life, massive stars undergo a violent transition to produce compact objects such as Neutron Stars (NSs), White Dwarfs, and Black holes. NSs provide an excellent laboratory for constraining the properties of light and weakly interacting QCD axions. Axions may be produced in the core/crust of a NSs through Cooper pair formation/breaking mechanism and bremsstrahlung in nucleon-electron scattering processes. The axions emitted from the core/crust may resonantly convert into X-ray photons due to a strong magnetic field inside the magnetospheres of the NSs. Axions in extended NSs magnetospheres can couple to virtual photons and produce real photons due to the Primakoff effect.

How Does the Magnetic Field Affect Axion Emission?

Recent observations suggest a specific group of NSs called Magnificent Seven magnetars. Magnetars are strongly magnetized NSs that exhibit a wide array of X-ray activity with extraordinarily strong magnetic field intensities of up to 10^18 Gauss. Many authors have proposed that the internal structure and cooling properties are affected by the distribution of a strong magnetic field in the interior of NSs. The emission properties of highly magnetized stars get modified due to the change in the stellar structure equations (TOV equations) and the significant change in the conductive and convective processes in the heat blanketing layer of the NSs. It is found that the NS cooling shall be affected by the strong magnetic field, which will further affect the emission of axions and their subsequent conversion to photons. Furthermore, the equation of state (EoS) of the NSs shall also have an imprint on the emission properties of various observables as a result of NS cooling.