# Large polar caps for twisted magnetosphere of magnetars

## TL;DR

This paper proposes a model for twisted magnetospheres of magnetars, explaining large polar caps and magnetic energy release during outbursts, with implications for observed X-ray luminosity and hot spot evolution.

## Contribution

It introduces a new model linking twisted magnetic fields to large polar caps and energy release, explaining magnetar outburst characteristics.

## Key findings

- Magnetar polar caps can be larger due to twisted magnetic fields.
- Magnetic energy release correlates with hot spot shrinking.
- Model reproduces general trends of magnetar outbursts.

## Abstract

The magnetic field of magnetars may be twisted compared with that of normal pulsars. Previous works mainly discussed magnetic energy release in the closed field line regions of magnetars. For a twisted magnetic field, the field lines will inflate in the radial direction. Similar to normal pulsars, the idea of light cylinder radius is introduced. More field lines will cross the light cylinder and become open for a twisted magnetic field. Therefore, magnetars may have a large polar cap, which may correspond to the hot spot during outburst. Particle flow in the open field line regions will result in the untwisting of the magnetic field. Magnetic energy release in the open field line regions can be calculated. The model calculations can catch the general trend of magnetar outburst: decreasing X-ray luminosity, shrinking hot spot etc. For magnetic energy release in the open field line regions, the geometry will be the same for different outburst in one magnetar.