The ultra luminous X-ray source NuSTAR J095551+6940.8: A magnetar in a high mass X-ray binary

TL;DR

This paper analyzes the magnetic field strength of the neutron star in the ULX NuSTAR J095551+6940.8, suggesting it is a magnetar with a magnetic field around 6.7 x 10^{13} G, supporting the magnetar-HMXB connection.

Contribution

It provides a revised estimate of the neutron star's magnetic field, indicating it is a magnetar, and discusses implications for magnetar formation from HMXBs.

Findings

Magnetic field estimated at 6.7 x 10^{13} G, higher than previous estimates.

Supports the hypothesis that magnetars originate from HMXBs with magnetic field decay.

Indicates the presence of higher multipole magnetic fields.

Abstract

The recent detection of pulsations from the ultra luminous X-ray source (ULX) NuSTAR J095551+6940.8 in M82 by Bachetti et al. indicates that the object is an accreting neutron star in a high mass X-ray binary (HMXB) system. The super-Eddington luminosity of the object implies that the magnetic field is sufficiently strong to suppress the scattering cross-section unless its beam is viewed at a favourable angle. We show that the torque equilibrium condition for the pulsar indicates the dipole magnetic field of the neutron star is $6.7 \times 10^{13}$ G, two orders of magnitude higher than that estimated by Bachetti et al., and further point to the possibility that even stronger magnetic fields could well be in the higher multipoles. This supports the recent view that magnetars descent from HMXBs if the magnetic field decays an order of magnitude during the process of transition.