Monte-Carlo simulations on possible collimation effects of outflows to fan-beamed emission of ultraluminous accreting X-ray pulsars

TL;DR

This study uses Monte-Carlo simulations to assess whether outflows cause strong collimation in PULXs, concluding that their high luminosity is intrinsic and indicating very strong magnetic fields, with implications for understanding their emission mechanisms.

Contribution

The paper demonstrates through simulations that outflows do not significantly collimated PULX emission, supporting the idea of intrinsically high luminosity and strong magnetic fields in these sources.

Findings

Main pulsed emission is not strongly collimated despite outflows.

High luminosity of PULXs is intrinsic, implying strong magnetic fields.

Regular dipole fields may be sufficient if outflows are disk-driven.

Abstract

Pulsating ultraluminous X-ray sources (PULXs) are accreting pulsars with apparent X-ray luminosity exceeding $10^{39}\, \rm erg\ s^{-1}$. We perform Monte-Carlo simulations to investigate whether high collimation effect (or strong beaming effect) is dominant in the presence of accretion outflows, for the fan beam emission of the accretion column of the neutron stars in PULXs. We show that the three nearby PULXs (RX J0209.6$-$7427, Swift J0243.6+6124 and SMC X-3), namely the three musketeers here, have their main pulsed emission not strongly collimated even if strong outflows exist. This conclusion can be extended to the current sample of extragalactic PULXs, if accretion outflows are commonly produced from them. This means that the observed high luminosity of PULXs is indeed intrinsic, which can be used to infer the existence of very strong surface magnetic fields of $\sim10^{13-14}$ G, possibly multipole fields. However, if strong outflows are launched from the accretion disks in PULXs as a consequence of disk spherization by radiation pressure, regular dipole magnetic fields of $\sim10^{12}$ G may be required, comparable to that of the three musketeers, which have experienced large luminosity changes from well below their Eddington limit ($2\times10^{38}\, \rm erg\ s^{-1}$ for a NS) to super-Eddington and their maximum luminosity fills the luminosity gap between Galactic pulsars and extragalactic PULXs.