  The nearby isolated neutron stars are a group of seven relatively slowly
rotating neutron stars that show thermal X-ray spectra, most with broad
absorption features. They are interesting both because they may allow one to
determine fundamental neutron-star properties by modeling their spectra, and
because they appear to be a large fraction of the overall neutron-star
population. Here, we describe a series of XMM-Newton observations of the nearby
isolated neutron star RX J0806.4-4123, taken as part of larger program of
timing studies. From these, we limit the spin-down rate to
dnu/dt=(-4.3+/-2.3)*10^{-16} Hz/s. This constrains the dipole magnetic field to
be <3.7e13 G at 2sigma, significantly less than the field of 1e14 G implied by
simple models for the X-ray absorption found at 0.45 keV. We confirm that the
spectrum is thermal and stable (to within a few percent), but find that the
0.45 keV absorption feature is broader and more complex than previously
thought. Considering the population of isolated neutron stars, we find that
magnetic field decay from an initial field of 3e14 G accounts most naturally
for their timing and spectral properties, both qualitatively and in the context
of the models for field decay of Pons and collaborators.
