Gradient expansion technique for inhomogeneous, magnetized quark matter

A quark-magnetic Ginzburg-Landau (qHGL) gradient expansion of the free energy of two-flavor inhomogeneous quark matter in a magnetic field $H$ is derived analytically. It can be applied away from the Lifshitz point, generalizing standard Ginzburg-Landau techniques. The thermodynamic potential is written as a sum of the thermal contribution, the non-thermal lowest Landau level contribution, and the non-thermal qHGL functional, which handles any arbitrary position-dependent periodic modulation of the chiral condensate as an input. The qHGL approximation has two main practical features: (1) it is fast to compute; (2) it applies to non-plane-wave modulations such as solitons even when the amplitude of the condensate and its gradients are large (unlike standard Ginzburg-Landau techniques). It agrees with the output of numerical techniques based on standard regularization schemes and reduces to known results at zero temperature ($T = 0$) in benchmark studies. It is found that the region of the $\mu$-$T$ plane (where $\mu$ is the chemical potential) occupied by the inhomogeneous phase expands, as $H$ increases and $T$ decreases.Comment: 16 pages, 5 figures. Accepted for publication in EPJ

Thermal luminosity degeneracy of magnetized neutron stars with and without hyperon cores

The dissipation of intense crustal electric currents produces high Joule heating rates in cooling neutron stars. Here it is shown that Joule heating can counterbalance fast cooling, making it difficult to infer the presence of hyperons (which accelerate cooling) from measurements of the observed thermal luminosity Lγ. Models with and without hyperon cores match Lγ of young magnetars (with poloidal-dipolar field Bdip ≳ 1014 G at the polar surface and Lγ ≳ 1034 erg s−1 at t ≲ 105 yr) as well as mature, moderately magnetized stars (with Bdip ≲ 1014 G and 1031 erg s−1 ≲ Lγ ≲ 1032 erg s−1 at t ≳ 105 yr). In magnetars, the crustal temperature is almost independent of hyperon direct Urca cooling in the core, regardless of whether the latter is suppressed or not by hyperon superfluidity. The thermal luminosities of light magnetars without hyperons and heavy magnetars with hyperons have Lγ in the same range and are almost indistinguishable. Likewise, Lγ data of neutron stars with Bdip ≲ 1014 G but with strong internal fields are not suitable to extract information about the equation of state as long as hyperons are superfluid, with maximum amplitude of the energy gaps of the order ≈1 MeV.FA is supported by The University of Melbourne through a Melbourne Research Scholarship. AM acknowledges funding from an Australian Research Council Discovery Project grant (DP170103625) and the Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav) (CE170100004). DV is supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (ERC Starting Grant "IMAGINE" No. 948582, PI DV). CD is supported by the ERC Consolidator Grant “MAGNESIA” (No. 817661, PI Nanda Rea) and this work has been carried out within the framework of the doctoral program in Physics of the Universitat Autònoma de Barcelona. JAP acknowledges support by the Generalitat Valenciana (PROMETEO/2019/071), AEI grant PGC2018-095984-B-I00 and the Alexander von Humboldt Stiftung through a Humboldt Research Award

Axion sourcing in dense stellar matter via CP-violating couplings

Compact objects such as neutron stars and white dwarfs can source axionlike particles and QCD axions due to CP-violating axion-fermion couplings. The magnitude of the axion field depends on the stellar density and on the strength of the axion-fermion couplings. We show that even CP-violating couplings one order of magnitude smaller than existing constraints source extended axion field configurations. For axionlike particles, the axion energy is comparable to the magnetic energy in neutron stars with inferred magnetic fields of the order of 1013 G and exceeds by more than one order of magnitude the magnetic energy content of white dwarfs with inferred fields of the order of 104 G. On the other hand, the energy stored in the QCD axion field is orders of magnitude lower due to the smallness of the predicted CP-violating couplings. It is shown that the sourced axion field can polarize the photons emitted from the stellar surface, and stimulate the production of photons with energies in the radio band.F. A., P. D. L., and A. M. are supported by the Australian Research Council (ARC) Centre of Excellence for Gravitational Wave Discovery (OzGrav), through Project No. CE170100004. P. D. L. is supported through ARC Discovery Project DP220101610. J. A. P. and A. G. acknowledge support from the Generalitat Valenciana Grants No. ASFAE/2022/026 (with funding from NextGenerationEU PRTR-C17.I1) and CIPROM/2022/ 13, and from the AEI Grant No. PID2021-127495NB I00 funded by MCIN/AEI/10.13039/501100011033 and by “ESF Investing in your future.

Magnetic dynamo caused by axions in neutron stars

The coupling between axions and photons modifies Maxwell's equations, introducing a dynamo term in the magnetic induction equation. In neutron stars, for critical values of the axion decay constant and axion mass, the magnetic dynamo mechanism increases the total magnetic energy of the star. We show that this generates substantial internal heating due to enhanced dissipation of crustal electric currents. These mechanisms would lead magnetized neutron stars to increase their magnetic energy and thermal luminosity by several orders of magnitude, in contrast to observations of thermally-emitting neutron stars. To prevent the activation of the dynamo, bounds on the allowed axion parameter space can be derived.Comment: 5 pages, 2 figures, 1 table + Supplemental Materia

Magnetic Dynamo Caused by Axions in Neutron Stars

The coupling between axions and photons modifies Maxwell’s equations, introducing a dynamo term in the magnetic induction equation. In neutron stars, for critical values of the axion decay constant and axion mass, the magnetic dynamo mechanism increases the total magnetic energy of the star. We show that this generates substantial internal heating due to enhanced dissipation of crustal electric currents. These mechanisms would lead magnetized neutron stars to increase their magnetic energy and thermal luminosity by several orders of magnitude, in contrast to observations of thermally emitting neutron stars. To prevent the activation of the dynamo, bounds on the allowed axion parameter space can be derived.F. A., P. D. L., and A. M. are supported by the Australian Research Council (ARC) Centre of Excellence for Gravitational Wave Discovery (OzGrav), through Project No. CE170100004. P. D. L. is supported through ARC Discovery Project No. DP220101610. J. A. P. acknowledges support by the Generalitat Valenciana (PROMETEO/2019/071) and AEI Grant No. PID2021-127495NB-I00

Fast cooling and internal heating in hyperon stars

Neutron star models with maximum mass close to 2 M⊙ reach high central densities, which may activate nucleonic and hyperon direct Urca neutrino emission. To alleviate the tension between fast theoretical cooling rates and thermal luminosity observations of moderately magnetized, isolated thermally emitting stars (with Lγ ≳ 1031 erg s−1 at t ≳ 105.3 yr), some internal heating source is required. The power supplied by the internal heater is estimated for both a phenomenological source in the inner crust and Joule heating due to magnetic field decay, assuming different superfluidity models and compositions of the outer stellar envelope. It is found that a thermal power of W(t) ≈ 1034 erg s−1 allows neutron star models to match observations of moderately magnetized, isolated stars with ages t ≳ 105.3 yr. The requisite W(t) can be supplied by Joule heating due to crust-confined initial magnetic configurations with (i) mixed poloidal–toroidal fields, with surface strength Bdip = 1013 G at the pole of the dipolar poloidal component and ∼90 per cent of the magnetic energy stored in the toroidal component; and (ii) poloidal-only configurations with Bdip = 1014 G.FA is supported by the University of Melbourne through a Melbourne Research Scholarship. AM acknowledges funding from an Australian Research Council Discovery Project grant (DP170103625). DV is supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (ERC Starting Grant ‘IMAGINE’ No. 948582, PI DV). CD is supported by the ERC Consolidator Grant ‘MAGNESIA’ (No. 817661, PI Nanda Rea) and this work has been carried out within the framework of the doctoral program in Physics of the Universitat Autònoma de Barcelona. JAP acknowledges support by the Generalitat Valenciana (PROMETEO/2019/071), AEI grant PGC2018-095984-B-I00, and the Alexander von Humboldt Stiftung through a Humboldt Research Award