NICER Timing of the X-Ray Thermal Isolated Neutron Star RX J0806.4-4123

The X-ray thermal isolated neutron star (XTINS) RX J0806.4-4123 shows interesting multiwavelength properties that seemingly deviate from those of similar neutron stars. An accurate determination of the spin frequency change over time can assist in interpreting RX J0806.4-4123's properties in comparison to those of other XTINSs and the wider pulsar population. From 2019 to 2023 we carried out a tailored X-ray timing campaign of RX J0806.4-4123 with the NICER instrument. We used statistical properties of the Fourier coefficients and the Z_K ² test for phase connecting separate observations and finding a timing solution for the entire data set. We also developed a simple and universal method for estimating the uncertainties of the frequency ν and its derivative ν ̇ from the empirical dependencies of Z_K ² on trial values of these parameters, with account of all significant harmonics of the frequency. Applying this method, we determined a spin-down rate ν ̇ = − 7.3 ( 1.2 ) × 10 − 17 Hz s − 1 . The resulting spin-down power E ̇ = 2.6 × 10 29 erg s⁻¹ is the lowest among the XTINSs, and it is a factor of 60 lower than the X-ray luminosity of this neutron star. RX J0806.4-4123 is also among the pulsars with the lowest measured E ̇ in general.