An Alternative Model for the Orbital Decay of M82 X-2: The Anomalous Magnetic Braking of a Bp Star

Recently, the first pulsating ultraluminous X-ray source M82 X-2 was reported to be experiencing a rapid orbital decay at a rate of P ̇ = − ( 5.69 ± 0.24 ) × 10 − 8 s s − 1 based on 7 yr NuSTAR data. To account for the observed orbital-period derivative, it requires a mass transfer rate of ∼ 200 M ̇ edd ( M ̇ edd is the Eddington accretion rate) from the donor star to the accreting neutron star. However, other potential models cannot be completely excluded. In this work, we propose an anomalous magnetic braking (AMB) model to interpret the detected orbital decay of M82 X-2. If the donor star is an Ap/Bp star with an anomalously strong magnetic field, the magnetic coupling between strong surface magnetic field and irradiation-driven wind from the surface of the donor star could cause an efficient angular-momentum loss, driving a rapid orbital decay observed in M82 X-2. The AMB mechanism of an Ap/Bp star with a mass of 5.0–15.0 M ⊙ and a surface magnetic field of 3000–4500 G could produce the observed P ̇ of M82 X-2. We also discuss the possibility of other alternative models including the companion star expansion and a surrounding circumbinary disk.