Brice, Nabil; (2024) The Effect of the Magnetic Field Structure on the Spectral Properties of Accreting Neutron Stars. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The description of an accreting neutron star (NS) with a pure dipole magnetic field structure, allowing for a tilt from the spin-axis, is challenged when applied to Pulsating Ultra-Luminous X-ray sources (PULXs), which are X-ray bright ( $\gtrsim 10^{⁻³⁹} \text{erg s}^{⁻¹}$ ), accreting, magnetised NSs. One explanation of the apparent super-Eddington emission from PULXs involves the NS's ultra-strong ($\gtrsim 10^{¹³} \text{G}$) magnetic field reducing the opacity of the accreting plasma, consequently raising its Eddington limit (from $\sim 10^{³⁸} \text{erg s}^{⁻¹}$). However, NGC5907 ULX-1 presents an issue for the simple dipole topology, as its observed spin-period and inferred magnetic field strength would place it in the propeller regime. Relaxing the dipole topology assumption resolves this by decoupling the magnetic field strengths in the accretion column and magnetosphere. I developed an accretion column model, incorporating multipole components, and calculated the change to the maximum luminosity. Results demonstrated the need for a multipolar magnetic field in NGC5907 ULX-1 and likely in NGC7793 P13. PULXs' high accretion rates were theorised to sheathe the NS magnetosphere in an optically thick envelope. Whereas the luminosity amplification model was unsuccessful in explaining the observed pulsed fraction of PULXs, no similar test had been done for the optically thick envelope model. I developed a model for the time-resolved spectral emission from an optically thick envelope, accounting for tilt from the spin-axis, and calculated the pulsed fraction. This model successfully explained the high pulsed fractions seen in NGC7793 P13 and NGC5907 ULX-1. In conclusion, these results show the significance of the magnetic field structure in shaping the observed emission, especially for PULXs. Future missions that provide higher quality datasets of NS spectra will demand a more comprehensive understanding of the effects of the magnetic field structure. This thesis contributes to advancing that understanding.

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