Anomalous x-ray pulsars

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1E 2259+586 was the first anomalous x-ray pulsar to be discovered (Fahlman & Gregory 1981). In the early nineties these ob jects began to form a unique class (Mereghetti & Stella 1995; Corbet et al. 1995; Van Paradijs et al. 1995; Vasisht & Gotthelf 1997). They were found to have much in common with the soft-gamma repeaters. These ob jects typically have pulsed X-ray emission with steadily increasing periods of several seconds, X-ray luminosities of ∼ 1035 − 1036 ergs s−1, soft spectra, and no detected companions or accretion disks. Furthermore, they are typically observed through hydrogen column densities of ∼ 1022 cm−2, indicating that they are not common.

Two of the five confirmed AXPs are located near the centres of supernova remnants 1E 2259+586 and 1E 1841-045(Fahlman & Gregory 1981; Vasisht & Gotthelf 1997) as well as the AXP candidate AX J1845-0258 (Vasisht et al. 2000). The remaining objects are 4U 0142+61 (Israel et al. 1994), 1E 1048.1-5937 (Seward et al. 1986), 1RXS J170849.0-400910 (Sugizaki et al. 1997) and possibly XTE J1810-197 (Ibrahim et al. 2004). The emission from the AXPs fits neatly within the magnetar model (Thompson & Duncan 1996). Heyl & Hernquist (1997) argued that the thermal flux passing through an ultramagnetized, hydrogen or helium envelope is sufficient to account for the x-ray emission from these objects. Heyl & Kulkarni (1998) examined how magnetic field decay can augment the thermal energy budget. For fields less than about 1015 G magnetic field decay in a realistic model does not strongly affect the emission from young (less than 10,000 years) AXPs but can greatly increase their lifetime as observable x-ray sources. Alternative models such as accretion have fallen by the wayside because even very low mass companions have not been discovered orbiting these neutron stars nor has the tell-tale optical emission from even a truncated accretion been detected.