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Constraining the Spin-down of the Nearby Isolated Neutron Star RX J0806.4-4123, and Implications for the Population of Nearby Neutron Stars

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.