Determination of the magnetic fields of Magellanic X-ray pulsars

The 80 high-mass X-ray binary(HMXB) pulsars that are known to reside in the Magellanic Clouds(MCs) have been observed by the XMM-Newton and Chandra X-ray telescopes on a regular basis for 15 years,and the XMM-Newton and Chandra archives contain nearly complete information about the duty cycles of the sources with spin periods P_S 100 s.We have reprocessed the archival data from both observatories and we combined the output products with all the published observations of 31 MC pulsars with P_S 100 s in an attempt to investigate the faintest X-ray emission states of these objects that occur when accretion to the polar caps proceeds at the smallest possible rates.These states determine the so-called propeller lines of the accreting pulsars and yield information about the magnitudes of their surface magnetic fields.We have found that the faintest states of the pulsars segregate into five discrete groups which obey to a high degree of accuracy the theoretical relation between spin period and X-ray luminosity.So the entire population of these pulsars can be described by just five propeller lines and the five corresponding magnetic moments(0.29,0.53,1.2,2.9 and 7.3,in units of 10~(30) G cm~3).     

The nature of the bimodal luminosity distribution of ultraluminous X-ray pulsars

The mechanism that can be responsible for the bimodal luminosity distribution of super-Eddington X-ray pulsars in binary systems is pointed out. The transition from the high to low state of these objects is explained by accretion flow spherization due to the radiation pressure at certain (high) accretion rates. The transition between the states can be caused by a gradual change in the accretion rate. The complex behavior of the recently discovered ultraluminous X-ray pulsars M 82 X-2, NGC 5907 ULX-1, and NGC 7793 P13 is explained by the proposed mechanism. The proposed model also naturally explains the measured spinup of the neutron star in these pulsars, which is slower than the expected one by several times.     

Generation of the magnetic fields of pulsars

The influence of the effect of entrainment of superconducting protons by superfluid neutrons on the distribution of neutron vortices in a rotating neutron star is investigated. It is shown that the proton vortex clusters generated by entrainment currents create the magnetic structure of a neutron vortex. The average magnetic field induction in a neutron vortex is calculated. The presence of the magnetic field of a neutron vortex considerably alters the radius of the vortex zone. The width of the vortex-free zone at the surface of the neutron star’s core increases, reaching macroscopic values on the order of several meters. This result considerably changes earlier concepts of the distribution of neutron vortices in a neutron star. Translated from Astrofizika, Vol. 43, No. 3, pp. 377-386, July–September, 2000.     

Effects of dynamo magnetic fields on observational properties of accreting millisecond X-ray pulsars

In this paper,we have investigated accreting millisecond X-ray pulsars,which are rapidly rotating neutron stars in low-mass X-ray binaries.These systems exhibit coherent X-ray pulsations that arise when the accretion flow is magnetically channeled to the stellar surface.Here,we have developed the fundamental equations for an accretion disk around accreting millisecond X-ray pulsars in the presence of a dynamo generated magnetic field in the inner part of the disk.We have also formulated the numerical method for the structure equations in the inner region of the disk and the highest accretion rate is enough to form the inner region of the disk,which is overpowered by radiation pressure and electron scattering.Finally,we have examined our results with the effects of dynamo magnetic fields on accreting millisecond X-ray pulsars.     

X-ray observations of ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) are amongst the most intriguing of X-ray source classes. Their extreme luminosities—greater than 10³⁹ erg s^?1 in the 0.3–10 keV band alone—suggest either the presence of black holes larger than those regularly encountered in our own Galaxy (the Galactic centre excepted), or sources apparently radiating well above the Eddington limit. We review the insights afforded us by studies of their X-ray emission, focussing on what this reveals about the underlying compact object. In particular, we discuss recent deep observations of ULXs by the XMM-Newton observatory, and how the unprecedented data quality provided by this mission is starting to discriminate between the different physical models for these extraordinary X-ray emitters.     

On the creation of magnetic monopoles in pulsars

The production of pairs of magnetic monopoles-antimonopoles should be expected in the interactions of the high energy particles accelerated by pulsars. In the frame of the Sturrock model, the interactions of the very high energy protons emitted from the polar caps with the secondary electrons can be a source of magnetic monopoles. It may be the dominating process in very young pulsars such as the Crab pulsar. In the polar gap model of Ruderman and Sutherland, magnetic monopoles can be created by the electrons accelerated across the cap and interacting with the neutron star crust or by the negatons and positrons interacting head-on inside the sparks.Half of these monopoles are accelerated towards the interstellar medium by the pulsar magnetic field and the others are likely to be trapped inside the neutron star crust. This leads to a decrease in the pulsar magnetic field which would imply that the characteristic age may not give the true age of the pulsar This can be related to the discrepancy between and the real age of the Crab pulsar and the kinematical ages obtained from the measurement of the proper motion of some pulsars. Furthermore, the trapping of magnetic monopoles close under the surface of the neutron star perturbates the pulsar electrodynamics. To have such observable effects, it is shown that the cross-sections for the magnetic monopoles production can be several orders of magnitude smaller than the upper limits so far derived from cosmic rays or accelerator data.The possibility that the magnetic monopoles, accelerated outwards, are responsible for the highest energy extensive air showers, is considered.The production of an avalanche of secondary monopoles, due to acceleration by the magnetic field in the neutron star crust, is possible and the consequences of this process are considered.     

On the mechanism of X-ray emission from radio pulsars

We discuss the correlations between the luminosities of radio pulsars in various frequency ranges and the magnetic fields on the light cylinder. These correlations suggest that the observed emission is generated in outer layers of the pulsar magnetospheres by the synchrotron mechanism. To calculate the distribution functions of the relativistic particles in the generation region, we use a model of quasilinear interactions between the waves excited by cyclotron instability and particles of the primary beam and the secondary electron—positron plasma. We derive a formula for calculating the X-ray luminosity L _x of radio pulsars. A strong correlation was found between L _x and the parameter \(\dot P_{ - 15} /P^{3.5}\), where P is the neutron-star rotation period, in close agreement with this formula. The latter makes it possible to predict the detection of X-ray emission from more than a hundred (114) known radio pulsars. We show that the Lorentz factors of the secondary particles are small (γ _p = 1.5–8.5), implying that the magnetic field near the neutron-star surface in these objects is multipolar. It follows from our model that almost all of the millisecond pulsars must emit X-ray synchrotron radiation. This conclusion differs from predictions of other models and can be used to test the theory under consideration. The list of potential X-ray radiators presented here can be used to search for X-ray sources with existing instruments.     

Propagation of γ-radiation in strong magnetic fields of pulsars

The propagation, decay and absorption of electromagnetic waves in magnetospheres of pulsars are considered with emphasis on the resonant effects in the vacuum polarization.     

An accreting low magnetic field magnetar for the ultraluminous X-ray source in M82

One ultraluminous X-ray source in M82 has recently been identified as an accreting neutron star(named Nu STAR J095551+6940.8).It has a super-Eddington luminosity and is spinning up.An aged magnetar is more likely to be a low magnetic field magnetar.An accreting low magnetic field magnetar may explain both the superEddington luminosity and the rotational behavior of this source.Considering the effect of beaming,the spin-up rate is understandable using the traditional form of accretion torque.The transient nature and spectral properties of M82 X-2 are discussed.The theoretical range of periods for accreting magnetars is provided.Three observational appearances of accreting magnetars are summarized.     

Spectral variability of ultraluminous X-ray sources

We study spectral variability of 11 ultraluminous X-ray sources (ULX) using archived XMM–Newton and Chandra observations. We use three models to describe the observed spectra: a power law, a multicolour disc (MCD) and a combination of these two models. We find that seven ULXs show a correlation between the luminosity L _X and the photon index Γ. Furthermore, four out of these seven ULXs also show spectral pivoting in the observed energy band. We also find that two ULXs show an   L _X–Γ  anticorrelation. The spectra of four ULXs in the sample can be adequately fitted with a MCD model. We compare these sources to known black hole binaries (BHB) and find that they follow similar paths in their luminosity–temperature diagrams. Finally, we show that the 'soft excess' reported for many of these ULXs at ∼0.2 keV seems to roughly follow a trend   L _soft∝ T ^−3.5  when modelled with a power law plus a 'cool' MCD model. This is contrary to the   L ∝ T ⁴  relation that is expected from theory and what is seen for many accreting BHBs. The observed trend could instead arise from disc emission beamed by an outflowing wind around a  ∼10 M_⊙  black hole.     

The strong magnetic fields of the X-ray pulsars Hercules X-1 and 4U 1626–67 and their evolutionary scenarios

We consider the magnetic and spin evolution of the X-ray binary pulsars Her X-1 and 4U 1626–67, assuming that their magnetic fields are of crustal origin. We adopt the standard evolutionary model which implies that the neutron star passes through several phases in a binary system ('isolated pulsar' – propeller – wind accretion – Roche lobe overflow). In the framework of the model under consideration, the strong magnetic fields of relatively old pulsars like Her X-1 and 4U 1626–67 can naturally be understood if, at their birth, they had a sufficiently strong magnetic field, ∼3 × 10¹³ G, comparable to the maximal field observed in radio pulsars.     

Spectroscopy of optical counterparts of ultraluminous X-ray sources

Here we present the results of panoramic and long-slit observations of eight ULX nebular counterparts performed with the 6m SAO telescope. In two ULX nebulae (ULXNe) we detected for the first time signatures of high excitation ([O III]λ5007 / Hβ > 5). Two of the ULXs were identified with young (T ～ 5–10 Myr) massive star clusters. Four of the eight ULXNe show bright high-excitation lines. This requires existence of luminous (～ 10³⁸ ÷ 10⁴⁰ erg s^?1) UV/EUV sources coinciding with the X-ray sources. The other 4 ULXNe require shock excitation of the gas with shock velocities of 20–100 km s ^?1. However, all the studied ULXNe spectra show signatures of shock excitation, but even those ULXNe where the shocks are prevailing show presence of a hard ionizing source with a luminosity of at least ～ 10³⁸ erg s^?1. Most likely shock waves, X-ray and EUV ionization act simultaneously in all the ULXNe, but they may be roughly separated in two groups: shock-dominated and photoionization-dominated ULXNe. The ULXs have to produce strong winds and/or jets (～ 10³⁹ erg s^?1) for powering their nebulae. Both the wind/jet activity and the existence of a bright UV source are consistent with the suggestion that ULXs are high-mass X-ray binaries with supercritical accretion disks of the SS433 type.     

The X-ray luminosity of isolated pulsars

We obtained a theoretical relation between the X-ray luminosity and the rotational energy loss of isolated pulsars, which is in agreement with the observed X-ray data.     

Spectroscopic studies of X-ray binary pulsars

Several new features of X-ray binary pulsars are revealed from recent observations with ASCA, RXTE, BeppoSAX and other X-ray observatories. Among these, I will review in this paper some recent progress in spectroscopic studies of accreting X-ray pulsars in binary systems (XBPs). First, I will discuss soft excess features observed in the energy spectra of XBPs and propose that it is a common feature for various subclasses of XBPs. Next I will present some recent results of high resolution spectroscopy with ASCA and Chandra.     

Formation of accreting millisecond X-ray pulsars

Accreting millisecond X-ray pulsars(AMXPs) are an important subclass of low-mass X-ray binaries(LMXBs), in which coherent millisecond X-ray pulsations can be observed during outburst states.They have dual characteristics of LMXBs and millisecond pulsars, providing a direct confirmation for the recycling scenario. However, their formation is not well understood. In this work, we simulate the evolution of LMXBs with the MESA code to explore the formation and evolution of AMXPs. Based on the binary evolutionary model of LMXBs and the model of accretion disk instability, we find that most of the observed AMXPs can be produced from LMXBs with orbital periods at the onset of Roche lobe overflow close to the bifurcation period and their observed properties can be explained by our models. The AMXPs with main sequence(MS) donors ultimately evolve into AMXPs with extremely low-mass He white dwarf donors.Moreover, our results indicate that these AMXPs with MS donors are likely to have donor stars near the terminal-age main sequence.