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

Astronomy & Astrophysics, Jun 2018

Aims. We study the temporal and spectral characteristics of SMC X-3 during its recent (2016) outburst to probe accretion onto highly magnetized neutron stars (NSs) at the Eddington limit.Methods. We obtained XMM-Newton observations of SMC X-3 and combined them with long-term observations by Swift. We performed a detailed analysis of the temporal and spectral behavior of the source, as well as its short- and long-term evolution. We have also constructed a simple toy-model (based on robust theoretical predictions) in order to gain insight into the complex emission pattern of SMC X-3.Results. We confirm the pulse period of the system that has been derived by previous works and note that the pulse has a complex three-peak shape. We find that the pulsed emission is dominated by hard photons, while at energies below ~1 keV, the emission does not pulsate. We furthermore 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. The results of our temporal study are supported by our spectroscopic analysis, which reveals a two-component emission, comprised of 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 the primary hard emission by optically thick material that partly obscures the central source. We also detect strong emission lines from highly ionized metals. The strength of the emission lines strongly depends on the phase.Conclusions. Our findings are in agreement with previous works. The energy and temporal evolution as well as 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 large reprocessing region is consistent with the analysis of previously studied X-ray pulsars observed at high accretion rates. This reprocessing region is consistent with recently proposed theoretical and observational works that suggested that highly magnetized NSs occupy a considerable fraction of ultraluminous X-ray sources.

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

A&A Astronomy & Astrophysics Filippos Koliopanos 1 2 Georgios Vasilopoulos 0 0 Max-Planck-Institut für Extraterrestrische Physik , Giessenbachstraße, 85748 Garching , Germany 1 Université de Toulouse , UPS-OMP, IRAP, Toulouse , France 2 CNRS, IRAP , 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4 , France Aims. We study the temporal and spectral characteristics of SMC X-3 during its recent (2016) outburst to probe accretion onto highly magnetized neutron stars (NSs) at the Eddington limit. Methods. We obtained XMM-Newton observations of SMC X-3 and combined them with long-term observations by Swift. We performed a detailed analysis of the temporal and spectral behavior of the source, as well as its short- and long-term evolution. We have also constructed a simple toy-model (based on robust theoretical predictions) in order to gain insight into the complex emission pattern of SMC X-3. Results. We confirm the pulse period of the system that has been derived by previous works and note that the pulse has a complex three-peak shape. We find that the pulsed emission is dominated by hard photons, while at energies below 1 keV, the emission does not pulsate. We furthermore 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. The results of our temporal study are supported by our spectroscopic analysis, which reveals a two-component emission, comprised of 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 the primary hard emission by optically thick material that partly obscures the central source. We also detect strong emission lines from highly ionized metals. The strength of the emission lines strongly depends on the phase. Conclusions. Our findings are in agreement with previous works. The energy and temporal evolution as well as 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 large reprocessing region is consistent with the analysis of previously studied X-ray pulsars observed at high accretion rates. This reprocessing region is consistent with recently proposed theoretical and observational works that suggested that highly magnetized NSs occupy a considerable fraction of ultraluminous X-ray sources. X-rays; binaries - accretion; accretion disks - stars; emission-line; Be - magnetic fields - radiation mechanisms; general - line; identification - Accreting, highly magnetized neutron stars at the Eddington limit: a study of the 2016 outburst of SMC X-3 1. Introduction X-ray pulsars (XRPs) are comprised of a highly magnetized (B > 109 G) neutron star (NS) and a companion star that ranges from low-mass white dwarfs to massive B-type stars (e.g., Caballero & Wilms 2012; Walter et al. 2015; Walter & Ferrigno 2017, and references therein) . XRPs are some of the most luminous (of-nuclear) Galactic X-ray point-sources (e.g., Israel et al. 2017a) . The X-ray emission is the result of accretion of material from the star onto the NS, which in the case of XRPs is strongly affected by the NS magnetic field: the accretion disk formed by the in-falling matter from the companion star is truncated at approximately the NS magnetosphere. From this point, the accreted material flows toward the NS magnetic poles, following the magnetic field lines, forming an accretion column, inside which material is heated to high energies (e.g., Basko & Sunyaev 1975, 1976; Meszaros & Nagel 1985) . The opacity inside the accretion column is dominated by scattering between photons and electrons. In the presence of a high magnetic field, the scattering cross-section is highly anisotropic (Canuto et al. 1971; Lodenquai et al. 1974) , the hard X-ray photons from the accretion column are collimated in a narrow beam (so-called pencil beam), which is directed parallel to the magnetic field axis (Basko & Sunyaev 1975) . The (possible) inclination between the pulsar beam and the rotational axis of the pulsar, combined with the NS spin, produce the characteristic pulsations of the X-ray light curve. During episodes of prolonged accretion, XRPs are known to reach and often exceed the Eddington limit for spherical accretion onto a NS ( 1:8 1038 erg s 1 for a 1:4 M NS). This is further complicated by the fact that material is accreted onto a very small area on the surface of the NS, and as a result, the Eddington limit is significantly lower. Therefore, even X-ray pulsars with luminosities of about a few 1037 erg s 1 persistently break the (local) Eddington limit. Basko & Sunyaev (1976) demonstrated that while the accreting material is indeed impeded by the emerging radiation and the accretion column becomes opaque along the magnetic field axis, the X (...truncated)


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Filippos Koliopanos, Georgios Vasilopoulos. Accreting, highly magnetized neutron stars at the Eddington limit: a study of the 2016 outburst of SMC X-3, Astronomy & Astrophysics, 2018, pp. A23, 614, DOI: 10.1051/0004-6361/201731623