Speaker
Description
Just over a decade ago the first X-ray pulsations were discovered in an Ultraluminous X-ray source (ULX), revolutionizing our understanding of these systems. The detection of pulsations showed at least some ULXs offer us ideal laboratories to study the most extreme manifestation of accretion onto neutron stars (NSs), even if the number of NS-ULXs within the ULX population remains uncertain. Most intriguing is the mechanism responsible for their extreme luminosities, upwards of 10$^39$ erg/s in the X-ray band, which remains unclear. Some theories suggest most of the emission emanates from the accretion column, where magnetar-like magnetic fields ($B > 10^{13}$ G) reduce electron scattering opacities, permitting above-Eddington luminosities. Others argue ULXs are lowly-magnetised NSs ($B \lesssim 10^{12}$G) with most of the emission emanating from a the disc, whereby radiation-driven outflows collimate the emission towards the observer.
In order to address these fundamental question about the nature of NSs in ULXs, we have carried out numerical calculations of the spin period and magnetic field evolution of NSs under extreme accretion. Self-consistently accounting for the accretion disk physics, the accretion torques onto the NS, and the decay of the magnetic field due to material pile-up onto the NS surface, our calculations predict dipole magnetic field strengths $B \lesssim 10^{12}$ G and spin periods $P \lesssim 1$ s after an accretion period of $t > 10^4$ yr. This suggest there might be a population of rapidly-rotating NS-ULXs with severely suppressed magnetic fields, hard to identify through pulsations. I will also discuss the evolution of some notable NS-ULXs, which I will argue cannot be accommodated under a model with low magnetic field ($B < 10^{12}$ G). Nevertheless, our results suggest the accreting age of the system plays a key role in shaping the properties of the NS.
| Talk category | NOVA Network 3 |
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