Soft gamma repeaters and anomalous X-ray pulsars: magnetar candidates

Introduction Baade and Zwicky (1934) were the first to envision the formation of neutron stars as the end product of a supernova explosion. Their forward thinking was not vindicated for another three decades, with the discovery of the first radio pulsars by Bell and Hewish (Hewish et al. 1968). What Baade and Zwiscky could not have anticipated, however, was the menagerie of astrophysical objects that are now associated with neutron stars. Today, we observe them as magnetically braking pulsars, accreting pulsars in binary systems, isolated cooling blackbodies, sources of astrophysical jets, and emitters of high-luminosity bursts of X-rays. Here, we focus on two of the most extraordinary evolutionary paths of a neutron star, namely soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs). Soft gamma repeaters were discovered as high-energy transient burst sources; some were later found also to be persistent X-ray pulsars, with periods of several seconds, that are spinning down rapidly. Anomalous X-ray pulsars are identified through their persistent pulsations and rapid spin down; some have also been found to emit SGR-like bursts. In spite of the differing methods of discovery, this convergence in the observed properties of the SGRs and AXPs has made it clear that they are, fundamentally, the same type of object. What distinguishes them from other neutron stars is the likely source of energy for their radiative emissions, magnetism.