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 \dot { \nu } = ( -4.3 \pm 2.3 ) \times 10 ^ { -16 } { Hz } { s } ^ { -1 } .
This constrains the dipole magnetic field to be < 3.7 \times 10 ^ { 13 } G at 2 \sigma , significantly less than the field of \sim 10 ^ { 14 } 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 \lesssim 3 \times 10 ^ { 14 } 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 .