RX J1856.5 - 3754 is one of the brightest , nearby isolated neutron stars , and considerable observational resources have been devoted to its study .
In previous work , we found that our latest models of a magnetic , hydrogen atmosphere matches well the entire spectrum , from X-rays to optical ( with best-fitting neutron star radius R \approx 14 km , gravitational redshift z _ { g } \sim 0.2 , and magnetic field B \approx 4 \times 10 ^ { 12 } G ) .
A remaining puzzle is the non-detection of rotational modulation of the X-ray emission , despite extensive searches .
The situation changed recently with XMM-Newton observations that uncovered 7 s pulsations at the \approx 1 \% level .
By comparing the predictions of our model ( which includes simple dipolar-like surface distributions of magnetic field and temperature ) with the observed brightness variations , we are able to constrain the geometry of RX J1856.5 - 3754 , with one angle < 6 ^ { \circ } and the other angle \approx 20 - 45 ^ { \circ } , though the solutions are not definitive given the observational and model uncertainties .
These angles indicate a close alignment between the rotation and magnetic axes or between the rotation axis and the observer .
We discuss our results in the context of RX J1856.5 - 3754 being a normal radio pulsar and a candidate for observation by future X-ray polarization missions such as Constellation-X or XEUS .