Possible rotation-power nature of SGRs and AXPs

Abstract

We investigate the possibility of some Soft Gamma-ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) could be described as rotation-powered neutron stars (NSs). The analysis was carried out by computing the structure properties of NSs, and then we focus on giving estimates for the surface magnetic field using both realistic structure parameters of NSs and a general relativistic model of a rotating magnetic dipole. We show that the use of realistic parameters of rotating neutron stars obtained from numerical integration of the self-consistent axisymmetric general relativistic equations of equilibrium leads to values of the magnetic field and radiation efficiency of SGRs/AXPs very different from estimates based on fiducial parameters. This analysis leads to a precise prediction of the range of NS masses, obtained here by making use of selected up-to-date nuclear equations of state (EOS). We show that 40% (nine) of the entire observed population of SGRs and AXPs can be described as canonical pulsars driven by the rotational energy of neutron stars, for which we give their possible range of masses. We also show that if the blackbody component in soft X-rays is due to the surface temperature of NSs, then 50% of the sources could be explained as ordinary rotation-powered pulsars. Besides, amongst these sources we find the four SGRs/AXPs with observed radio emission and six that are possibly associated with supernova remnants (including Swift J1834.9-0846 as the first magnetar to show a surrounding wind nebula), suggesting as well a natural explanation as ordinary pulsars.