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A Coherent Timing Solution for the Nearby, Thermally Emitting Isolated Neutron Star RX J0420.0-5022

We present a phase-coherent timing solution for RX J0420.0-5022, the coolest (kT=45 eV) and fastest-spinning (P=3.45 s) of the seven so-called isolated neutron stars (INSs). Using 14 observations with the XMM-Newton spacecraft in 2010-2011, we were able to measure a spin-down rate nudot=(-2.3+/-0.2)e-15 Hz/s (Pdot=(2.8+/-0.3)e-14 s/s), from which we infer a dipolar magnetic field of 1.0e13 G. With reasonable confidence we were able to extend the timing solution back to archival XMM-Newton from 2002 and 2003, giving the same solution but with considerably more precision. This gives RX J0420 the lowest dipole magnetic field of the INSs. Our spectroscopy does not confirm the broad absorption feature at 0.3 keV hinted at in earlier observations, although difficulties in background subtraction near that energy make conclusions difficult. With this, all 6 of the INSs that have confirmed periodicities now have constrained spin-downs from coherent solutions. The evidence that the INSs are qualitatively different from rotation-powered pulsars now appears robust, with the likely conclusion that their characteristic ages are systematically older than their true ages, because their fields have decayed. The field decay probably also causes them to be systematically hotter than pulsars of the same (true) age.