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-ra y sources with unusual properties distinguishing them from both rotation-po wered 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 e mission 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 model, this correla tion is seen as an increasing hardness of the high-energy component with in creasing spin-down rate, with the temperature of the low-energy blackbody c omponent remaining essentially constant. Also, for the two component spectr al model, the ratio of the 2-10 keV power-law and bolometric blackbody lumi nosities gradually increases with the spin-down rate. We discuss these resu lts in terms of the various theoretical models for SGRs and AXPs.