Refinement of the timing-based estimator of pulsar magnetic fields

Abstract

Numerical simulations of realistic non-vacuum magnetospheres of isolated neutron stars have shown that pulsar spin-down luminosities depend weakly on the magnetic obliquity. This result provides the opportunity to estimate the surface magnetic field for a given radiopulsar with quite a high accuracy. In the current work, we present a refinement of the classical `magneto-dipolar' formula for pulsar magnetic fields $B_{\rm md} = (3.2\times 10^{19}\mbox{ G})\sqrt{P\dot P}$, where $P$ is the neutron star spin period. The new, robust timing-based estimator is introduced as $\log B = \log B_{\rm md} + Δ_{\rm B}(M, α)$, where the correction $Δ_{\rm B}$ depends on the equation of state (EOS) of dense matter, the individual pulsar obliquity $α$ and the mass $M$. Adopting state-of-the-art statistics for $M$ and $α$ we calculate the distributions of $Δ_{\rm B}$ for a representative subset of 22 EOSs that do not contradict observations. It has been found that $Δ_{\rm B}$ is distributed nearly normally, with the average in the range -0.5 to -0.25 dex and standard deviation $σ[Δ_{\rm B}] \approx$ 0.06 to 0.09 dex, depending on the adopted EOS. The latter quantity represents a formal uncertainty of the corrected estimation of $\log B$ because $Δ_{\rm B}$ is weakly correlated with $\log B_{\rm md}$. At the same time, if it is assumed that every considered EOS has the same chance of occurring in nature, then another, more generalized, estimator $B^* \approx 3B_{\rm md}/7$ can be introduced providing an unbiased value of the pulsar surface magnetic field with $\sim$30 per cent uncertainty with 68 per cent confidence. Finally, we discuss the possible impact of pulsar timing irregularities on the timing-based estimation of $B$ and review the astrophysical applications of the obtained results.