  Anomalous X-ray Pulsars (AXPs) belong to a class of neutron stars believed to
harbor the strongest magnetic fields in the universe, as indicated by their
energetic bursts and their rapid spindowns. We have developed a theoretical
model that takes into account processes in the atmospheres and magnetospheres
of ultramagnetic neutron stars, as well as the effects of their strong
gravitational fields on the observable properties. Using this model, we have
analyzed the X-ray spectra of a number of AXPs. We find that in all cases, the
X-ray spectra are described very well with this emission model. The
spectroscopically measured magnetic field strengths of these sources are in
close agreement with the values inferred from their spindown properties and
provide independent evidence for their magnetar nature. The analysis of
spectral data using this physical model also sheds light on the long-term
evolution of AXPs.
