On the Magnetic Fields, Beaming Fractions, and Fastness Parameters of Pulsating Ultra-Luminous X-Ray Sources

TL;DR

This paper investigates the magnetic fields, beaming fractions, and fastness parameters of pulsating ultra-luminous X-ray sources (PULX), estimating neutron star magnetic field strengths and discussing implications for their magnetic nature and accretion processes.

Contribution

The study provides new estimates of magnetic field strengths and beaming fractions in PULX, considering super-critical accretion and magnetic effects, refining understanding of their magnetic properties.

Findings

Neutron star surface magnetic fields estimated between 10^11 and 10^13 G.

Fields could be as high as 10^15 G under subcritical conditions.

PULX do not necessarily require magnetar-strength fields when beaming is considered.

Abstract

The discovery of pulsating ultra-luminous X-ray sources (PULX) suggests that neutron stars are presumably common within the ultra-luminous X-ray source (ULX) population though the majority of the population members are currently lacking pulsations. These systems are likely to host neutron stars accreting mass at super-Eddington (super-critical) rates from their massive companion in high-mass X-ray binaries. Taking into account the spherization of the accretion flow in the super-critical regime, the beaming of X-ray emission, and the reduction of the scattering cross-section in a strong magnetic field, we infer the ranges for the neutron-star surface magnetic dipole field strengths, beaming fractions, and fastness parameters in the PULX M82 X-2, ULX NGC 5907, ULX NGC 7793 P13, NGC 300 ULX1, M51 ULX-7, NGC 1313 X-2, and Swift J0243.6+6124 from a set of conditions based on a variety of combinations of different spin and luminosity states. Using the observed spin-up rates under the critical luminosity condition, we estimate the surface-field strengths in the $\sim 10^{11}-10^{13}\,{\rm G}$ range for all PULX. In general, the results of our analysis under the subcritical luminosity condition indicate surface-field strengths in the $\sim 10^{11}-10^{15}\,{\rm G}$ range. We argue that the PULX do not require magnetar-strength surface dipole fields if beaming is taken into account; yet the fields are strong enough for the neutron stars in ULX to magnetically channel the accretion flow in super-critical accretion disks.