The physics governing magnetic field evolution in magnetars.

Abstract

Neutron stars are ultra-dense remnants of massive stellar cores, observable across the electromagnetic spectrum. Their emission reflects a delicate interplay of magnetic, thermal, and structural processes under extreme conditions of density, temperature, and magnetic fields—regimes unattainable in terrestrial laboratories. Among neutron stars, magnetars stand out, hosting the strongest magnetic fields in the Universe. Their intense X-ray activity, recurrent outbursts, and unusually slow rotation point to powerful interior dynamics, yet the origin of the large-scale dipolar fields that drive their long spin periods remains a longstanding mystery. In this talk, I will introduce MATINS, a new open-access 3D code for modeling the coupled magneto-thermal evolution of isolated neutron stars. Focusing on the evolution of magnetic fields in magnetar interiors, I will discuss the key mechanisms operating under these extreme conditions, in particular the chiral magnetic effect—a phenomenon arising from the chiral anomaly that enables the conversion between magnetic helicity and electron chiral asymmetry. I will present simulations for both non-zero and vanishing initial magnetic helicity, the latter reflecting realistic conditions at the birth of a proto–neutron star. Our results show that the chiral magnetic effect can naturally transfer energy from small-scale magnetic fields at birth into the large-scale dipolar fields observed in magnetars without requiring external energy input. This provides a compelling mechanism for magnetar formation.