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On the X-ray Spectra of Anomalous X-ray Pulsars and Soft Gamma Repeaters

We revisit the apparent correlation between soft X-ray band photon index and spin-down rate nudot previously reported for Anomalous X-ray Pulsars (AXPs) and Soft Gamma Repeaters (SGRs) by Marsden & White (2001). Our analysis, improved thanks to new source discoveries, better spectral parameter measurements in previously known sources, and the requirement of source quiescence for parameter inclusion, shows evidence for the previously noted trend, although with greater scatter. This trend supports the twisted magnetosphere model of magnetars although the scatter suggests that factors other than nudot are also important. We also note possible correlations involving the spectra of AXPs and SGRs in the hard X-ray band. Specifically, the hard-band photon index shows a possible correlation with inferred nudot and B, as does the degree of spectral turnover. If the former trend is correct, then the hard-band photon index for AXP 1E 1048.1-5937 should be ~0--1. This may be testable with long integrations by INTEGRAL, or by the upcoming focussing hard X-ray mission NuSTAR.