Electromagnetic spin down of a transient accreting millisecond pulsar during quiescence

TL;DR

This paper revises the method for estimating magnetic moments of transient accreting millisecond pulsars during quiescence, accounting for residual accretion effects that distort the magnetosphere and affect spin-down measurements.

Contribution

It introduces an alternative spin-down formula that considers magnetospheric mass loading and crushing, improving magnetic moment estimates for pulsars during quiescence.

Findings

Standard magnetic dipole braking overestimates magnetic moments.

Residual accretion can distort the magnetosphere, affecting spin-down torque.

The new formula provides more accurate magnetic moment estimates.

Abstract

The measured spin-down rates in quiescence of the transient accreting millisecond pulsars IGR J00291+5934, XTE J1751-305, SAX J1808.4-3658, and Swift J1756.9-2508 have been used to estimate the magnetic moments of these objects assuming standard magnetic dipole braking. It is shown that this approach leads to an overestimate, if the amount of residual accretion is enough to distort the magnetosphere away from a force-free configuration, through magnetospheric mass loading or crushing, so that the lever arm of the braking torque migrates inside the light cylinder. We derive an alternative spin-down formula and calculate the residual accretion rates where the formula is applicable. As a demonstration, we apply the alternative spin-down formula to produce updated magnetic moment estimates for the four objects above. We note that, based on current uncertain observations of quiescent accretion rates, magnetospheric mass loading and crushing are neither firmly indicated nor ruled out in these four objects. Because quiescent accretion rates are not measured directly (only upper limits are placed), it is impossible to be confident without more data whether the thresholds for magnetospheric mass loading or crushing are reached or not.