  In the last few years considerable observational resources have been devoted
to study the thermal emission from isolated neutron stars. Detailed XMM and
Chandra observations revealed a number of features in the X-ray pulse profile,
like asymmetry, energy dependence, and possible evolution of the pulse profile
over a time scale of months or years. Here we show that these characteristics
may be explained by a patchy surface temperature distribution, which is
expected if the magnetic field has a complex structure in which higher order
multipoles contribute together with the dipole. We reconsider these effects
from a theoretical point of view, and discuss their implications to the
observational properties of thermally emitting neutron stars.
