On the magnetic fields of ultraluminous X-ray pulsars

So far quite a few ultraluminous X-ray(ULX) pulsars have been discovered.In this work,we construct a super-Eddington,magnetic accretion disk model to estimate the dipole magnetic field of eight ULX pulsars based on their observed spin-up variations and luminosities.We obtain two branches of dipole magnetic field solutions.They are distributed in the range of B-(0.156-64.5) × 10^(10) G and-(0.275-79.0) × 10^(13) G corresponding to the low-and high-B solutions respectively.The low magnetic field solutions correspond to the state that the neutron stars are far away from the spin equilibrium,and the high magnetic field solutions are close to the spin equilibrium.The ultra-strong magnetic fields derived in Be-type ULX pulsars imply that the accretion mode in Be-type ULX pulsars could be more complicated than in the persistent ULX pulsars and may not be accounted for by the magnetized accretion disk model.We suggest that the transition between the accretor and the propeller regimes may be used to distinguish between the low-and high-B magnetic field solutions in addition to the detection of the cyclotron resonance scattering features.

Flaring activity from quiescent states in neutron-star X-ray binaries

We examine systematically the observed X-ray luminosity jumps(or flares) from quiescent states in millisecond binary pulsars(MSBPs) and high-mass X-ray binary pulsars(HMXBPs). We rely on the published X-ray light curves of seven pulsars: four HMXBPs, two MSBPs and the ultraluminous X-ray pulsar M82 X-2. We discuss the physics of their flaring activities or lack thereof, paying special attention to their emission properties when they are found on the propeller line, inside the Corbet gap or near the light-cylinder barrier. We provide guiding principles for future interpretations of faint X-ray observations, as well as a method of constraining the propeller lines and the dipolar surface magnetic fields of pulsars using a variety of quiescent states. In the process, we clarify some disturbing inaccuracies that have made their way into the published literature.

The Great Pretenders Among the ULX Class

The recent discoveries of pulsed X-ray emission from three ultraluminous X-ray(ULX)sources have finally enabled us to recognize a subclass within the ULX class:the great pretenders,neutron stars(NSs) that appear to emit X-ray radiation at isotropic luminosities L^(39)X= 7 × 10 erg s^(-1)-1 × 10^(41) erg s^(-1) only because their emissions are strongly beamed toward our direction and our sight lines are offset by only a few degrees from their magnetic-dipole axes.The three known pretenders appear to be stronger emitters than the presumed black holes of the ULX class,such as Holmberg II& IX X-1,IC10 X-1 and NGC 300 X-1.For these three NSs,we have adopted a single reasonable assumption,that their brightest observed outbursts unfold at the Eddington rate,and we have calculated both their propeller states and their surface magnetic-field magnitudes.We find that the results are not at all different from those recently obtained for the Magellanic Be/X-ray pulsars:the three NSs reveal modest magnetic fields of about 0.3–0.4 TG and beamed propeller-line X-ray luminosities of ~10^(36)-10^(37) erg s^(-1),substantially below the Eddington limit.