Accreting, highly magnetized neutron stars at the Eddington limit: A study of the 2016 outburst of SMC X-3

TL;DR

This study analyzes the 2016 outburst of SMC X-3, revealing complex emission patterns, phase-dependent spectral features, and accretion behaviors of highly magnetized neutron stars at the Eddington limit, supported by combined XMM-Newton and Swift observations.

Contribution

It provides new insights into the emission geometry and spectral components of highly magnetized neutron stars during super-critical accretion, using a combined observational and modeling approach.

Findings

Pulse profile is complex with three peaks and dominated by hard photons.

Soft thermal emission is mainly from reprocessing of hard X-rays by surrounding material.

Emission lines from ionized metals are phase-dependent and indicative of accretion environment.

Abstract

We study the temporal and spectral characteristics of SMC X-3 during its 2016 outburst. To probe accretion onto highly magnetized neutron stars (NSs), at the Eddington limit. We obtained XMM-Newton observations of SMC X-3 and combined them with long-term observations by Swift. We studied the temporal and spectral behavior of the source, and its short- and long-term evolution. We constructed a simple toy-model to gain insight into the complex emission pattern of SMC X-3. We confirm the pulse period of the system, derived by previous works and note that the pulse has a complex, three peaked shape. We find that the pulsed emission is dominated by hard photons, while at energies below ~1 keV, the emission is virtually non-pulsating. We further find that the shape of the pulse profile and the short and long-term evolution of the source light-curve can be explained by invoking a combination of a "fan" and a "polar" beam. Our spectroscopic analysis, reveals a two-component emission: a hard power law and a soft thermal component. We find that the latter produces the bulk of the non-pulsating emission and is most likely the result of reprocessing of the primary hard emission, by optically thick material that partially obscures the central source. We also detect strong emission lines from highly ionized metals. The strength of the emission lines are strongly phase-depended. The energy and temporal evolution and the shape of the pulse profile and the long-term spectra evolution of the source are consistent with the expected emission pattern of the accretion column in the super-critical regime, while the presence of a large reprocessing region is consistent with the analysis of previously studied X-ray pulsars observed at high accretion rates. The presence of this region is consistent with recently proposed works that suggest that high-B NSs occupy a considerable fraction of ULXs.