XMM-Newton observations of CXOU J010043.1-721134:: The first deep look at the soft X-ray emission of a magnetar

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 cannot be fitted by an absorbed power-law plus blackbody model. The sum of two (absorbed) blackbody components with kT(1) = 0.30 +/- 0.02 LeV and kT(2) = 0.7 +/- 0.1 keV gives an acceptable fit, and the radii of the corresponding blackbody emission regions are R-BB1(infinity) = 12.1(-1.4)(+2.1) km R-BB2(infinity) = 1.7(-0.5)(+0.6) 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, which 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.