Neutron stars challenge current models of highly dense matter. Despite be- ing the targets of numerous observational campaigns (e.g. gravitational-wave searches and X-ray observations), their equation of state is still unknown. One of the most exciting possibilities is that “unconventional” particles such as hy- perons may appear in neutron star cores. Hyperons have a major impact on the observed thermal luminosity, because they accelerate the cooling rate via direct Urca processes, which copiously increase the neutrino emission from the core. Such mechanism is often considered to be a key signature of hyperon concentrations at high densities. Hyperon superfluidity plays a major role as well, because it can suppress the neutrino emissivity exponentially. The hope is that a comparison of the theoretical cooling curves against the available data of thermally-emitting neutron star can hint towards the existence of hyperons and their superfluidity. There is one ingredient, however, that is often neglected in neutron star cooling models: internal heating. The magnetic field of neutron stars decays due to the dissipation of the electric currents circulating in the crust, generating substantial Joule heating in the shallower layers. The ther- mal power generated by this process can counterbalance hyperon fast cooling, making it difficult to infer the presence of hyperons from the available thermal luminosity data, and complicating the link between measured thermal emission and internal composition. We show that this is the case for magnetars, because their crustal temperature is almost independent of hyperon direct Urca cooling in the core, regardless of whether hyperons are superfluid or not. Likewise, ther- mal luminosity data of moderately magnetized neutron stars are not suitable to extract information about the internal composition, as long as hyperons are superfluid.

Axions 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 vacuo. 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 this talk we focus on 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, we calculate the axion-induced perturbations to the neutron star’ magnetic field, and show that they grow on relatively short time-scales. At the same time, intense electric currents form, leading to enhanced ohmic dissipation, which increases the stars’ observable thermal luminosity. The activation of such mechanisms depends on the axion decay constant and the axion mass, two long-sought parameters at the center of several experimental and theoretical investigations. Both parameters can be constrained by comparing our simulations to observations of thermally-emitting neutron stars. The latter do not exhibit uncontrolled growth of the magnetic field that causes enhanced ohmic dissipation, allowing us to place bounds on axion parameters. Our results open a new astrophysical avenue to constrain axions, extending significantly the parameter range that can be probed with direct axion searches.

We will have the 3rd workshop organized by KUIAS, iTHEMS and Heidelberg Univ. on "Medicine and Math" in Kyoto (hybrid style) on Sep.30-Oct.1, 2022.
Please join if you are interested in this interdisciplinary subject.
Most of the talks are in Japanese except for session 3.

For more information and registration, please visit the related links.

Co-organized by:
Center for Integrative Medicine and Physics, Kyoto University Institute for Advanced Study (KUIAS)
RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS)
Center for Science Adventure and Collaborative Research Advancement (SACRA), Kyoto University