  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 z_g~0.2, and
magnetic field B~4x10^12 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 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 deg and the other angle =
20-45 deg, 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.
