Pulsars have been divided into three distinct categories according to the source of energy that powers the emitted radiation. Radio pulsars are powered by the loss of rotational energy. X-ray pulsars are binary systems in which the X-ray emission is powered by the release of gravitational potential energy as material is accreted from a massive companion. But until recently, the energy source for anomalous X-ray pulsars (AXP) was a mystery – hence the term anomalous in the name.
The pulse periods of AXPs are long and clustered between 6 and 12 seconds, a rotation rate far too slow to power X-ray emission. An alternative model where remnant material from the supernova explosion falls back onto the neutron star and is heated enough to produce X-rays has also been dismissed. The currently favoured model for AXPs is that they are a type of magnetar (a neutron star with an ultra-strong magnetic field), and the X-rays are produced as the magnetic field decays over time.
Support for this theory resides in the observation that AXPs have very similar properties to soft gamma repeaters (SGRs) which are also thought to be magnetars. Their pulse periods, spin-down rates and quiescent X-ray properties are all similar, with the main difference being that AXPs are less active than SGRs (though some AXPs do show outbursts). Additional evidence is that the rotation rates of AXPs slow down rapidly, an indicator of magnetic braking and a very strong magnetic field.
It is now thought that SGRs evolve into AXPs once their rotation slows and magnetic field weakens. The lifespan of a SGR is estimated to be about 10,000 years, during which time its activity lessens as it slowly turns into an AXP. Due to the rapid slow-down of their rotation rate, AXPs are only expected to survive for ~10,000 years, meaning that although we should only observe a handful of active AXPs, many dead ones should also exist. There are currently 6 known and 2 candidate AXPs.