2022-10-24 Seminar Report

The main focus of Dr. Filippo Anzuini's talk was on axions, which are pseudo-Goldstone bosons that provide a solution to the strong CP problem, and are prominent candidates for dark matter. In neutron stars, it has been shown recently that the potential of the QCD axion acquires finite density corrections that shift the axion field expectation value, which can be large compared to the vanishing expectation value in vacuum. Such a shift leaves an imprint on typical neutron star observables such as the redshifted thermal luminosity, which can be used to constrain the axion parameter space. In his talk, Dr. Anzuini discussed the coupling of axions with photons, which modifies Maxwell’s equations and alters the neutron star magnetic field. By performing state-of-the-art magneto-thermal simulations, he performed his calculations of the axion-induced perturbations to the neutron star’ magnetic field, and showed that they grow on relatively short time-scales. Intense electric currents then lead to enhanced ohmic dissipation, increasing the stars’ observable thermal luminosity. The activation of such mechanisms depends on the axion decay constant and the axion mass. His results indeed opened a new astrophysical avenue to constrain axions, extending significantly the parameter range that can be probed with direct axion searches.

Reported by Ryo Namba

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