Magnetar-like x-ray bursts from anomalous x-ray pulsars

The leading model for understanding the energetics of anomalous X-ray pulsars (AXPs) is that they are "magnetars"---young, isolated neutron stars powered by the decay of their enormous magnetic fields. The identification of AXPs as magnetars is motivated by the similarity of AXPs to another enigmatic class of sources, the Soft Gamma Repeaters (SGRs). We report on long-term monitoring of AXPs using the Rossi X-ray Timing Explorer (RXTE). We monitor AXPs with RXTE to study their rotational stability, to search for variations in their pulsed flux and pulsed morphology. During our regular monitoring observations we discovered multiple bursts from two AXPs: this was the first time such behavior has been observed from these sources. Prior to our monitoring campaign, the detection of bursts solely from SGRs was the principle difference between AXPs and SGRs. The first detection of AXP bursts came in Fall 2001 when we discovered two SCR-like X-ray bursts from the direction of AXP 1E 1048.1--5937. Due to RXTE's large (1° x 1°) field-of-view we could not unambiguously identify the AXP as the burster. Recently, we detected a third burst from the direction of this source as well as a simultaneous increase in the pulsar's pulsed emission---this clearly identified the AXP as the burster. The most outstanding demonstration of AXP bursting behavior came on 2002 June 18 when AXP 1E 2259+586 underwent a major outburst involving 80 bursts and several changes in the persistent and pulsed emission, including a huge pulsed flux enhancement, a pulse morphology change and a rotational glitch. We also find variations in the persistent emission of AXPs in the absence of an obvious outburst. For example, we discovered two pulsed flux flares from AXP 1E 1048.1--5937. Both flares lasted several months and had well resolved few-week-long rises. The long rise tunes of the flares is a phenomenon not previously reported for this class of object, but has a clear explanation within the context of the magnetar model. All these results imply a close relationship between AXPs and SGRs, which we now believe are both magnetars, and have posed significant challenges to competing models