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