What can Fermi tell us about magnetars?

TL;DR

This paper examines Fermi observations of magnetars, highlighting the lack of significant detections, conflicts with existing models, and proposing wind braking as an alternative explanation for their spin-down behavior.

Contribution

It introduces the wind braking scenario for magnetars, challenging the traditional magnetic dipole braking model and suggesting new observational tests.

Findings

No significant Fermi detections of magnetars.

Conflicts between the outer gap model and observations.

Wind braking predicts a pulsar wind nebula and smaller braking index.

Abstract

We have analyzed the physical implications of Fermi observations of magnetars. Observationally, no significant detection is reported in Fermi observations of all magnetars. Then there are conflicts between outer gap model in the case of magnetars and Fermi observations. One possible explanation is that magnetars are wind braking instead of magnetic dipole braking. In the wind braking scenario, magnetars are neutron stars with strong multipole field. A strong dipole field is no longer required. A magnetism-powered pulsar wind nebula and a braking index smaller than three are the two predictions of wind braking of magnetars. Future deeper Fermi observations will help us make clear whether they are wind braking or magnetic dipole braking. It will also help us to distinguish between the magnetar model and the accretion model for AXPs and SGRs.