Constraining the geometry of the neutron star RX J1856.5-3754

RXJ1856.5- 3754 is one of the brightest, nearby isolated neutron stars ( NSs), and considerable observational resources have been devoted to its study. In previous work, we found that our latest models of a magnetic, hydrogen atmosphere match well the entire spectrum, from X- rays to optical ( with best- fitting NS radius R approximate to 14 km, gravitational redshift z(g) similar to 0.2, and magnetic field B approximate to 4 x 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 approximate to 1 per cent 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 degrees. and the other angle approximate to 20 degrees - 45 degrees, though the solutions are not definitive, given the observational and model uncertainties. These angles indicate a close alignment between the rotation and the 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.