2011ApJ...727L..51P

Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are young neutron stars (NSs) characterized by high X-ray quiescent luminosities, outbursts, and, in the case of SGRs, sporadic giant flares. They are believed to be powered by ultra-strong magnetic fields (hence dubbed magnetars). The diversity of their observed behaviors is however not understood and made even more puzzling by the discovery of magnetar-like bursts from "low-field" pulsars. Here, we perform long-term two-dimensional simulations that follow the evolution of magnetic stresses in the crust; these, together with recent calculations of the breaking stress of the NS crust, allow us to establish when starquakes occur. For the first time, we provide a quantitative estimate of the burst energetics, event rate, and location on the NS surface, which bear a direct relevance for the interpretation of the overall magnetar phenomenology. Typically, an "SGR-like" object tends to be more active than an "AXP-like" object or a "high-B radio pulsar," but there is no fundamental separation among what constitutes the apparent different classes. Among the key elements that create the variety of observed phenomena, age is more important than a small variation in magnetic field strength. We find that outbursts can also be produced in old, lower-field pulsars (B∼ a few x10¹² G), but those events are much less frequent than in young, high-field magnetars.