NICER timing of the X-ray thermal isolated neutron star RX J0806.4--4123

TL;DR

This study used NICER X-ray timing data from 2019 to 2023 to accurately measure the spin-down rate of the isolated neutron star RX J0806.4--4123, revealing its exceptionally low energy loss compared to similar objects.

Contribution

It introduces a universal method for estimating frequency and its derivative uncertainties, and provides the first precise measurement of RX J0806.4--4123's spin-down rate.

Findings

Spin-down rate $ u$ dot measured as -7.3(1.2)×10^{-17} Hz/s.

Spin-down power $ ext{dot}E$ is 2.6×10^{29} erg/s, lowest among XTINSs.

RX J0806.4--4123's $ ext{dot}E$ is 60 times lower than its X-ray luminosity.

Abstract

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 dataset. We also developed a simple and universal method for estimating the uncertainties of frequency $\nu$ and its derivative $\dot{\nu}$ 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 $\dot{\nu} = -7.3(1.2)\times 10^{-17}\,{\rm Hz\, s}^{-1}$. The resulting spin-down power $\dot{E}=2.6\times 10^{29}$ 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 $\dot{E}$ in general.