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

RX J1856.5-3754 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 ~ 14 km, gravitational redshift z_g ~ 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 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 , 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.