We present the analysis of six XMM-Newton observations of the Anomalous X–ray Pulsar CXOU J010043.1–721134 , the magnetar candidate characterized by the lowest interstellar absorption .
In contrast with all the other magnetar candidates , its X–ray spectrum can not be fit by an absorbed power-law plus blackbody model .
The sum of two ( absorbed ) blackbody components with kT _ { 1 } = 0.30 \pm 0.02 keV and kT _ { 2 } = 0.7 \pm 0.1 keV gives an acceptable fit , and the radii of the corresponding blackbody emission regions are R ^ { \infty } _ { BB 1 } = 12.1 ^ { +2.1 } _ { -1.4 } km and R ^ { \infty } _ { BB 2 } = 1.7 ^ { +0.6 } _ { -0.5 } km .
The former value is consistent with emission from a large fraction of a neutron star surface and , given the well known distance of CXOU J010043.1–721134 , that is located in the Small Magellanic Cloud , it provides the most constraining lower limit to a magnetar radius ever obtained .
A more physical model , where resonant cyclotron scattering in the magnetar magnetosphere is taken into account , has also been successfully applied to this source .