Magnetar Signature - The U Curve

TL;DR

This paper investigates magnetar signatures, focusing on period closures and X-ray decline, comparing dynamo and screened core models, and introduces a 'U' curve relationship between energy loss and X-ray luminosity.

Contribution

It proposes a screened core model explaining magnetar magnetic field evolution and the observed 'U' curve in the ratio of energy loss to X-ray luminosity.

Findings

The 'U' curve indicates exponential decline in X-ray luminosity leads to period closure.

Screened core model predicts increasing magnetic field to the final value over time.

Dynamo model predicts decreasing polar magnetic field with time.

Abstract

This work looks at some definitive signatures of magnetars, in particular of period closures accompanied by a decline of X-ray radiation in two models. We review some of the previous works which are based on the well known dynamo model in which the star is born with a period of a few milliseconds at high temperatures. In such a convection regime the dynamo mechanism can amplify the the magnetic fields to the magnetar value. This is in contrast to a screened core model which posits that a high density phase transition occurs in the inner core of magnetars that dynamically aligns all the neutron magnetic moments producing a large magnetic field in the core. The accompanying change of flux gives rise to shielding or screening currents in the surrounding high conductivity plasma that do not permit the field to exit to the surface. Ambipolar diffusion then transports the field to the crust dissipating energy in neutrinos and X-rays. The up-welling field cleaves the crust resulting in flares and X-ray radiation from ohmic dissipation in the crust till the screening currents are spent and the surface polar field attains its final value. In the dynamo model the polar magnetic field decreases with time whereas in our screened model it increases to its final value. One consequence of this is that in the latter model, as a function of time and period, the ratio of the dipole radiation loss, $\dot E$ to the X-ray luminosity, $ L_X$, is a 'U' curve, indicating that it is the exponential decline in $L_X$, that brings closure to the periods that are observed for magnetars.