SMC X-3 the closest ultraluminous X-ray source powered by a neutron star with non-dipole magnetic field

Aims. The magnetic field of accreting neutron stars determinestheir overall behavior including the maximum possible luminosity. Somemodels require an above-average magnetic field strength(≳1013 G) in order to explain super-Eddington massaccretion rate in the recently discovered class of pulsatingultraluminous X-ray sources (ULX). The peak luminosity of SMC X-3 duringits major outburst in 2016-2017 reached 2.5 × 1039 ergs-1 comparable to that in ULXs thus making this source thenearest ULX-pulsar. Determination of the magnetic field of SMC X-3 isthe main goal of this paper.
Methods. SMC X-3 belongs to theclass of transient X-ray pulsars with Be optical companions, andexhibited a giant outburst in July 2016-March 2017. The source has beenobserved over the entire outburst with the Swift/XRT and Fermi/GBMtelescopes, as well as the NuSTAR observatory. Collected data allowed usto estimate the magnetic field strength of the neutron star in SMC X-3using several independent methods.
Results. Spin evolution of thesource during and between the outbursts, and the luminosity of thetransition to the so-called propeller regime in the range of (0.3-7)× 1035 erg s-1 imply a relatively weakdipole field of (1-5) × 1012 G. On the other hand,there is also evidence for a much stronger field in the immediatevicinity of the neutron star surface. In particular, transition fromsuper- to sub-critical accretion regime associated with the cease of theaccretion column and very high peak luminosity favor a field that is anorder of magnitude stronger. This discrepancy makes SMC X-3 a goodcandidate for possessing significant non-dipolar components of thefield, and an intermediate source between classical X-ray pulsars andaccreting magnetars which may constitute an appreciable fraction of ULXpopulation.