Topology of magnetars external field. I. Axially symmetric fields

TL;DR

This paper explores complex, multipolar magnetic field configurations in magnetars, analyzing their spectral and observational signatures, and comparing models with actual X-ray observations to understand the magnetic topology.

Contribution

It introduces a method to generate multipolar force-free magnetosphere solutions and compares their spectral features with observations, extending beyond simple dipolar models.

Findings

Multipolar fields produce distinct spectral signatures.

Observations suggest these magnetars are not globally twisted.

Models match observed spectra better with localized twists.

Abstract

There is an increasing theoretical and observational evidence that the external magnetic field of magnetars may contain a toroidal component, likely of the same order of the poloidal one. Such "twisted magnetospheres" are threaded by currents flowing along the closed field lines which can efficiently interact with soft thermal photons via resonant cyclotron scatterings (RCS). Actually, RCS spectral models proved quite successful in explaining the persistent ~1-10 keV emission from the magnetar candidates, the soft gamma-ray repeaters (SGRs) and the anomalous X-ray pulsars (AXPs). Moreover, it has been proposed that, in presence of highly relativistic electrons, the same process can give rise to the observed hard X-ray spectral tails extending up to ~200 keV. Spectral calculations have been restricted up to now to the case of a globally twisted dipolar magnetosphere, although there are indications that the twist may be confined only to a portion of the magnetosphere, and/or that the large scale field is more complex than a simple dipole. In this paper we investigate multipolar, force-free magnetospheres of ultra-magnetized neutron stars. We first discuss a general method to generate multipolar solutions of the Grad- Schluter-Shafranov equation, and analyze in detail dipolar, quadrupolar and octupolar fields. The spectra and lightcurves for these multipolar, globally twisted fields are then computed using a Monte Carlo code and compared with those of a purely dipolar configuration. Finally the phase-resolved spectra and energy-dependent lightcurves obtained with a simple model of a locally sheared field are confronted with the INTEGRAL observations of the AXPs 1RXS J1708-4009 and 4U 0142+61. Results support a picture in which the field in these two sources is not globally twisted.