Correlations Between Spectral Properties and Spin-Down Rate in Soft Gamma–Ray Repeaters and Anomalous X–ray Pulsars

Anomalous x–ray pulsars (AXPs) and soft gamma–ray repeaters (SGRs) are x–ray sources with unusual properties distinguishing them from both rotation–powered and most accretion–powered pulsars. Using archival ASCA data over the energy range 0.5 − 10.0 keV, we have studied the spectra of the persistent emission from these sources and their variation with spin-down rate. Using a single power law spectral model, we find that the overall hardness of the spectra increase with increasing spin-down rate, and therefore the spectral and spin-down mechanism are inextricably linked in these objects. In terms of the two-component blackbody plus power law spectral models, this correlation is seen as an increasing hardness of the high energy component with increasing spin-down rate, with the temperature of the low energy blackbody component remaining essentially constant. Also for the two component spectral model: the ratio of the 2 − 10 keV power law and bolometric blackbody luminosities gradually increases with the spin-down rate. We discuss these results in terms of the various theoretical models for SGRs and AXPs. Subject headings: Stars: neutron − pulsars: individual (SGRs, AXPs)