A method to constrain the neutron star magnetic field in Low Mass X-ray Binaries

Abstract

We describe here a method to put an upper limit to the strength of the magnetic field of neutron stars in low mass X‐ray binaries for which the spin period and the X‐ray luminosity during X‐ray quiescent periods are known. This is obtained using simple considerations about the position of the magnetospheric radius during quiescent periods. We applied this method to the accreting millisecond pulsar SAX J1808.4‐3658, which shows coherent X‐ray pulsations at a frequency of ∼ 400 Hz and a quiescent X‐ray luminosity of ∼ 5 × 1031 ergs/s, and found that B ⩽ 5 × 108 Gauss in this source. Combined with the lower limit inferred from the presence of X‐ray pulsations, this constrains the SAX J1808.4‐3658 neutron star magnetic field in the quite narrow range (1 – 5) × 108 Gauss. Similar considerations applied to the case of Aql X‐1 and KS 1731‐260 give neutron star magnetic fields lower than ∼ 109 Gauss.