Constraining the geometry of the neutron star RX J 1856 . 5 − 3754

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 ≈ 14 km, gravitational redshift zg ∼ 0.2, and magnetic field B ≈ 4 × 1012 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 ≈ 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 and the other angle ≈ 20−45, 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.