Table of contents

The conference

This was the 11th gathering on neutron star physics in Saint Petersburg since 1988. The 2017 Conference was organized by the Theoretical Astrophysics Department of the Ioffe Institute and the Relativistic Astrophysics Department of the Sternberg Astronomical Institute. It commemorated the semicentenary of the discovery of pulsars.

During these 50 years, the field of astrophysics of neutron stars has become a broad field covering all ranges of physical scales, from microscales characteristic of strong interactions to macroscales of neutron star sizes strongly affected by General Relativity. All branches of contemporary physics are involved in the research of astrophysical phenomena related to neutron stars, and observations throughout the entire range of electromagnetic spectra have been used to constrain fundamental physical theories. Towards the end of the 20th century, electromagnetic observations were joined by direct neutrino detections from a newly born neutron star (Supernova 1987a) and, between the end of the Conference and the publication of this volume, a long-awaited gravitational wave signal from a binary neutron star merger was detected for the first time, thus promoting neutron stars to the first class of astrophysical sources observed in the electromagnetic band, neutrinos, and gravitational waves. With many more detections of binary neutron star mergers to come, the next decade of the neutron star research is expected to be developing under great influence of the gravitational wave astronomy. In addition, the launch of telescopes of the next generation (proposed, planned and already operating) will shed light on many of the current mysteries, but surely it will also unveil new ones, making the near future even more stimulating than the previous five decades.

List of Acknowledgments, Organizing committees, Invited speakers, Conference photo are available in this PDF

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

A brief, personal, and very incomplete account of 50 years of pulsar astronomy presented at the Conference Dinner for "Physics of Neutron Stars – 2017 – 50 Years After", held in Saint Petersburg, July 2017.

Pulsars are remarkably precise "celestial clocks" that can be used to explore many different aspects of physics and astrophysics. In this article I give a brief summary of pulsar properties and describe some of the applications of pulsar timing, including tests of theories of gravitation, efforts to detect low-frequency gravitational waves using pulsar timing arrays and establishment of a "pulsar timescale".

PSR J0337+1715 is a millisecond radio pulsar in a hierarchical stellar triple system containing two white dwarfs. The pulsar orbits the inner white dwarf every 1.6 days. In turn, this inner binary system orbits the outer white dwarf every 327 days. The gravitational influence of the outer white dwarf strongly accelerates the inner binary, making this system an excellent laboratory in which to test the strong equivalence principle (SEP) of general relativity – especially because the neutron star has significant gravitational self-binding energy. This system has been intensively monitored using three radio telescopes: Arecibo, Green Bank and Westerbork. Using the more than 25000 pulse times of arrival (TOAs) collected to date, we have modeled the system using direct 3-body numerical integration. Here we present our efforts to quantify the effects of systematics in the TOAs and timing residuals, which can limit the precision to which we can test the SEP in this system. In this work we describe Fourier-based techniques that we apply to the residuals in order to isolate the effects of systematics that could masquerade as an SEP violation. We also demonstrate that tidal effects are insignificant in the modeling.

We report preliminary results of the analysis of the proper motion of the bright radio pulsar B1727−47. Using archival Parkes timing data, as well as original and archival ATCA interferometry observations, we, for the first time, constrain the pulsar proper motion at the level of 148±11 mas yr⁻¹. The backward extrapolation of the proper motion vector to the pulsar birth epoch points at the center of the Galactic supernova remnant RCW 114 suggesting the genuine association between the two objects. We discuss the implications of the association and argue that the distance to the system is less than 1 kpc. This value is at least two times lower than the dispersion measure distance estimates. This suggests that the existing Galaxy electron density models are incomplete in the direction to the pulsar.

Ever since the discovery of the Crab and Vela pulsars in their respective Supernova Remnants, our understanding of how neutron stars manifest themselves observationally has been dramatically shaped by the surge of discoveries and dedicated studies across the electromagnetic spectrum, particularly in the high-energy band. The growing diversity of neutron stars includes the highly magnetized neutron stars (magnetars) and the Central Compact Objects shining in X-rays and mostly lacking pulsar wind nebulae. These two subclasses of high-energy objects, however, seem to be characterized by anomalously high or anomalously low surface magnetic fields (thus dubbed as 'magnetars' and 'anti-magnetars', respectively), and have pulsar characteristic ages that are often much offset from their associated SNRs' ages. In addition, some neutron stars act 'schizophrenic' in that they occasionally display properties that seem common to more than one of the defined subclasses.

I review the growing diversity of neutron stars from an observational perspective, then highlight recent and on-going theoretical and observational work attempting to address this diversity, particularly in light of their magnetic field evolution, energy loss mechanisms, and supernova progenitors' studies.

Central Compact Objects (CCOs) are a handful of sources located close to the geometrical center of young supernova remnants. They only show thermal-like, soft X-ray emission and have no counterparts at any other wavelength. While the first observed CCO turned out to be a very peculiar magnetar, discovery that three members of the family are weakly magnetised Isolated Neutron Stars (INSs) set the basis for an interpretation of the class. However, the phenomeology of CCOs and their relationship with other classes of INSs, possibly ruled by supernova fall-back accretion, are still far from being well understood.

A detailed phase-resolved spectroscopy of archival XMM–Newton observations of X-ray Dim Isolated Neutron Stars (XDINSs) led to the discovery of narrow and strongly phase-dependent absorption features in two of these sources. The first was discovered in the X-ray spectrum of RX J0720.4-3125, followed by a new possible candidate in RX J1308.6+2127. Both spectral lines have similar properties: they are detected for only ∼ 20% of the rotational cycle and appear to be stable over the timespan covered by the observations. We performed Monte Carlo simulations to test the significance of these phase-variable features and in both cases the outcome has confirmed the detection with a confidence level > 4.6σ. Because of the narrow width and the strong dependence on the pulsar rotational phase, the most likely interpretation for these spectral features is in terms of resonant proton cyclotron absorption scattering in a confined high-B structure close to the stellar surface. Within the framework of this interpretation, our results provide evidence for deviations from a pure dipole magnetic field on small scales for highly magnetized neutron stars and support the proposed scenario of XDINSs being aged magnetars, with a strong non-dipolar crustal B-field component.

The observational manifestation of a neutron star is strongly connected with the properties of its magnetic field. During the star's lifetime, the field strength and its changes dominate the thermo-rotational evolution and the source phenomenology across the electromagnetic spectrum. Signatures of magnetic field evolution are best traced among elusive groups of X-ray emitting isolated neutron stars (INSs), which are mostly quiet in the radio and γ-ray wavelengths. It is thus important to investigate and survey INSs in X-rays in the hope of discovering peculiar sources and the long-sought missing links that will help us to advance our understanding of neutron star evolution. The Extended Röntgen Survey with an Imaging Telescope Array (eROSITA), the primary instrument on the forthcoming Spectrum-RG mission, will scan the X-ray sky with unprecedented sensitivity and resolution. The survey has thus the unique potential to unveil the X-ray faint end of the neutron star population and probe sources that cannot be assessed by standard pulsar surveys.

The mode-switching pulsar PSR B0943+10 has been extensively studied in the radio band for many years and, more recently, it has been found to vary also in X-rays, with a flux anticorrelated with the radio emission. Here we review the results of long observations of PSR B0943+10 carried out with XMM-Newton and the LOFAR, LWA and Arecibo radio telescopes in 2014. These results support a scenario in which both unpulsed non-thermal emission, likely of magnetospheric origin, and pulsed thermal emission from a small polar cap (∼1500 m²) with a strong non-dipolar magnetic field (∼ 10¹⁴ G), are present during both radio modes and vary in intensity in a correlated way.

We briefly report the results of XMM-Newton X-ray observations of the γ-ray pulsar J0633+0632. We detected, for the first time, X-ray pulsations from J0633+0632 with the pulsar period. The pulse profile is typical for spin-modulated thermal emission from the surface of a neutron star. The pulsed fraction in the 0.3–2 keV band is 23 ± 6%. We confirmed previous results that the X-ray spectrum of J0633+0632 contains a non-thermal power law and a thermal (described either by the blackbody or the neutron star atmosphere models) components. However, XMM-Newton observations do not confirm the absorption line in the pulsar spectrum at ≈ 0.8 keV found in the Chandra data (Danilenko et al 2015 PASA32 e038).Using the interstellar absorption–distance relations, we constrained the distance to the pulsar in the range of 0.7–2.2 kpc.

Pulsar radio emission involves a coherent emission mechanism involving relativistic particles, but there is no consensus on what the specific emission mechanism is. There are arguments for and against several suggested mechanisms of long-standing, including two, relativistic plasma emission and anomalous Doppler emission, that depend explicitly on the dispersive properties of waves in a pulsar plasma. Models that do not include dispersion due to the relativistic spread in energy of the plasma particles can be misleading, and including the effects of this spread on the wave dispersion leads to severe constraints on these two suggested mechanisms. It is argued that all suggested mechanisms are implausible. A variant of relativistic plasma emission involving large-amplitude oscillations needs to be explored as a possible alternative.

We show that the increasing of energy losses of radio pulsars W_tot with the inclination angle χ obtained numerically by many authors can be explained by the separatrix currents circulating in the pulsar magnetosphere which do not outflow into pulsar wind.

Studying electromagnetic field around neutron stars is one of the vital methods to understand the physics of pulsars. From the very beginning of the efforts made to understand these objects, most of the works have been based on the assumption of a standard centred dipolar electromagnetic field. However, lately some studies have been focussing on explaining that including higher multipolar field components could modify our current ideas regarding these objects. Also, quite recently a more generalized picture has been put forward for pulsars in which the magnetic dipole moment is shifted off from the geometrical centre of the star showing how a rotating off-centred dipole can be expanded into multipolar components. We discuss the effects of such off-centred rotating dipole on various characteristic features of pulsars in vacuum like shape of the polar caps, radio and high energy emission phase plots and light curves along with a comparison with the standard centred case.

A purely magnetospheric model is introduced for observed abrupt changes in pulsar radio profile. Motion of magnetospheric plasma is described by a drift frequency, ω_dr, that depends on a parameter 0 ≤ y ≤ 1, and a change in the magnetospheric state corresponds to a change in y. Emission is assumed to arise from m spots distributed uniformly around the magnetic axis, so that spots drift by at the rate mω_dr. Observable features, such as subpulses, appear to rotate as ω_R = ω_dr − mω_V. The motion of the visible point, ω_V, is ignored in a "standard" version of the viewing geometry that assumes a fixed line of sight (rather than a fixed line-of-sight direction), implying ω_V = 0. With ω_V ≠ 0, the apparent motion of subpulses is not constant. An abrupt (or more gradual) change in y implies a change in ω_R, which affects the observed pulse structure and the average profile. We apply the model for profile shifts observed with PSR B0919+06.

We calculate the maximum luminosity of the synchrotron radiation in X-ray and optical bands, which is originated from the electromagnetic cascade in the magnetosphere of rotation-powered pulsars. We find that for pulsars with spin-down luminosity L_sd ≤ 10³⁵ erg s⁻¹, even if the full allowed energy range of the γ-ray photon emitted by the accelerated particles is taken into account, the observed non-thermal luminosities are higher than the maximum luminosity of synchrotron radiation from pairs created by two-photon collision. The synchrotron radiation from the inner region could explain the observed non-thermal luminosities, so that the γ-ray pulsars with L_sd ≤ 10³⁵ erg s⁻¹ would have multiple particle accelerators in the magnetosphere.

A magnetar is one of variable sources, whose activity is supplied by huge magnetic energy. Magnetar magnetosphere is gradually twisted due to shearing motion at the surface. At the same time, the energy is stored there. When it exceeds a threshold, the energy is abruptly released on a dynamical timescale as observed in energetic flares. Here, static and axially symmetric force-free magnetospheres are calculated in the exterior of non-rotating relativistic star. The magnetic energy increases along a sequence characterized by the twist degree. In a highly twisted case, a magnetic flux rope, in which large amount of toroidal field is confined, is detached in vicinity of the star. It is found that larger amount of energy is capable to be stored in the relativistic magnetosphere. In an extreme case, the magnetic energy in presence of current flow increases up to a few times larger than that of current-free dipole. There is an upper limit in any model, and a catastrophic event like a giant flare would occur, when the field is further twisted.

The influence of the positronium photoionization rate on the polar cap X-ray luminosity of old radio pulsars is considered. It is assumed that the polar cap is heated only by reverse positrons accelerated in the pulsar diode. It is supposed that the pulsar diode is in a stationary state with the lower plate located near the star surface (polar cap model) occupies all the pulsar tube cross section and operates in the regime of steady space charge by the limited electron flow. The influence of a small-scale magnetic field on the electric field inside the pulsar diode is taken into account. The reverse positron current is calculated in the framework of two models: rapid and gradual screening. To calculate the production rate of electron-positron pairs we take into account only the curvature radiation of primary electrons and its absorption in the magnetic field. It is assumed that some fraction of electron-positron pairs is created in a bound state (positronium). Later, such positroniums are photoionized by thermal photons from the polar cap.

The wave propagation theory (Beskin and Philippov 2012 MNRAS425 814; Hakobyan et al 2017 MNRAS469 2704) allowed us to understand the main polarization properties of mean profiles. However, some radio pulsars indicate clear deviation from the theory predictions. Below we show how such anomalies can be explained within our theory without additional assumptions about orthogonal modes mixing.

We derive and describe analytically a new wide class of self-consistent magnetostatic structures with sheared field lines and arbitrary energy distributions of particles. To do so we analyze superpositions of two planar current sheets with orthogonal magnetic fields and cylindrically symmetric momentum distribution functions, such that the magnetic field of one of them is directed along the symmetry axis of the distribution function of the other. These superpositions satisfy the pressure balance equation and allow one to construct configurations with an almost arbitrarily sheared magnetic field. We show that most of previously known current sheet families with sheared magnetic field lines are included in this novel class.

A new mechanism of radiation emission in the polar gap of a pulsar is discussed. It is based on the curvature radiation which is emitted by positrons moving towards the surface of neutron star along field lines of the inclined magnetic field and reflects from the surface. This mechanism explains the mystery of the interpulse shift and appearance of additional components in the emission of Crab pulsar at high frequencies discovered by Moffett and Hankins twenty years ago. We discuss coherence, energy flux and spectrum of the reflected radiation, appearance and disappearance of the interpulse position shift with the frequency increase. It is also possible that a nonlinear reflection (stimulated scattering) from the star surface is observed in the form of HF components. The frequency drift of these components, discovered by Hankins, Jones and Eilek, is discussed. The nonlinear reflection is associated with "Wood's anomaly" at the diffracted waves grazing along the star surface. Two components can arise due to slow and fast waves which are present in the magnetospheric plasma. The possible scheme of their appearance due to birefringence at the reflection is also proposed.

Non-thermal quiescent X-ray emission extending between 10 keV and around 150 keV has been seen in about 10 magnetars by RXTE, INTEGRAL, Suzaku, NuSTAR and Fermi-GBM. For inner magnetospheric models of such hard X-ray signals, inverse Compton scattering is anticipated to be the most efficient process for generating the continuum radiation, because the scattering cross section is resonant at the cyclotron frequency. We present hard X-ray upscattering spectra for uncooled monoenergetic relativistic electrons injected in inner regions of pulsar magnetospheres. These model spectra are integrated over bundles of closed field lines and obtained for different observing perspectives. The spectral turnover energies are critically dependent on the observer viewing angles and electron Lorentz factor. We find that electrons with energies less than around 15 MeV will emit most of their radiation below 250 keV, consistent with the turnovers inferred in magnetar hard X-ray tails. Electrons of higher energy still emit most of the radiation below around 1 MeV, except for quasi-equatorial emission locales for select pulse phases. Our spectral computations use a new state-of-the-art, spin-dependent formalism for the QED Compton scattering cross section in strong magnetic fields.

We present the results of the systematic study of all magnetar outbursts observed to date through a reanalysis of data acquired in about 1100 X-ray observations. We track the temporal evolution of the luminosity for all these events, model empirically their decays, and estimate the characteristic decay time-scales and the energy involved. We study the link between different parameters (maximum luminosity increase, outburst peak luminosities, quiescent X-ray and bolometric luminosities, energetics, decay time-scales, magnetic field, spin-down luminosity and age), and reveal several correlations between different quantities. We discuss our results in the framework of the models proposed to explain the triggering mechanism and evolution of magnetar outbursts. The study is complemented by the Magnetar Outburst Online Catalog (http://www.magnetars.ice.csic.es), an interactive database where the user can plot any combination of the parameters derived in this work and download all reduced data.

Bursts and flares are among the distinctive observational manifestations of magnetars, isolated neutron stars endowed with an ultra-strong magnetic field (B ≈ 10¹⁴–10¹⁵ G). It is believed that these events arise in a hot electron-positron plasma, injected in the magnetosphere, due to a magnetic field instability, which remains trapped within the closed magnetic field lines (the "trapped-fireball" model). We have developed a simple radiative transfer model to simulate magnetar flare emission in the case of a steady trapped fireball. We assume that magnetic Thomson scattering is the dominant source of opacity in the fireball medium, and neglect contributions from second-order radiative processes. The spectra we obtained in the 1–100 keV energy range are in broad agreement with those of available observations. The large degree of polarization (≳ 80%) predicted by our model should be easily measured by new-generation X-ray polarimeters, like IXPE, XIPE and eXTP, allowing one to confirm the model predictions.

Recent optical polarimetry observations of an X-ray dim isolated neutron star, RX J1856.5−3754, showed a first evidence for QED vacuum birefringence induced by a strong magnetic field. This important result can be confirmed by performing systematically polarimetry observations in the X-ray band for other strongly magnetized neutron stars, such as transient or persistent magnetars. We computed the phase averaged polarization fraction (PF) and polarization angle (PA) expected in the thermal emission from transient magnetars in the soft X-ray energy band. We found that the detection of a PF higher than 60% is a strong evidence for vacuum birefringence. We also found that a steady change in the PA measured from transient magnetars during their outburst decay (up to 23 degrees for a magnetospheric untwisting of 0.5 rad) is a strong signature of vacuum birefringence. This latter detection would also provide an independent check of the magnetospheric untwisting model for these sources. Simulations show that these measurements are achievable by future polarimetric missions such as XIPE and IXPE with 20−380 ks of observational time, and with eXTP with 3−60 ks.

The discovery of the 'Lorimer Burst', a little over a decade ago, ignited renewed interest in searching for short-duration radio transients (Lorimer et al 2007 Science318 777). This event is now considered to be the first established Fast Radio Burst (FRB), which is a class of millisecond-duration radio transients (Thornton et al 2013 Science341 53). The large dispersive delays observed in FRBs distinguish them from the individual bright pulses from Galactic pulsars, and suggests that they originate deep in extragalactic space. Amazingly, FRBs are not rare: the implied event rate ranges up to many thousands of events per sky, per day (Champion et al 2016 MNRAS460 L30). The fact that only two dozen FRBs have been discovered to date is a consequence of the limited sensitivity and field of view of current radio telescopes (Petroff et al 2016 PASA33 e045). The precise localization of FRB 121102, the first and currently only FRB observed to repeat (Spitler et al 2014 ApJ790 101; Spitler et al 2016 Nature531 202; Scholz et al 2016 ApJ833 177), has led to the unambiguous identification of its host galaxy and thus proven its extragalactic origin and large energy scale (Chatterjee et al 2017 Nature541 58; Tendulkar et al 2017 ApJL834 L7; Marcote et al 2017 ApJL834 L8). It remains unclear, however, whether all FRBs are capable of repeating [many appear far less active (Petroff et al 2015 MNRAS454 457)] or whether FRB 121102 implies that there are multiple sub-classes. Regardless, the repetitive nature of FRB 121102 and its localization to within a star-forming region in the host galaxy (Bassa et al 2017 ApJL843 L8) imply that the bursts might originate from an exceptionally powerful neutron star – one necessarily quite unlike any we have observed in the Milky Way. In these proceedings, I give a very brief introduction to the FRB phenomenon and focus primarily on the insights that FRB 121102 has provided thus far.

We present preliminary results of systematic temporal and spectral analysis of the Konus-Wind observational data on short and intermediate SGR bursts. We conclude that the burst energy spectra are equally well described, in the 20 − 200 keV range, by both power-law with an exponential cutoff, and double black-body functions. We also discuss energetics and durations of the bursts, distributions of the spectral parameters and correlations between them.

We briefly report first results of high time resolution optical multi-band panoramic photo-polarimetric observations of the eclipsing binary millisecond redback pulsar J1023+0038 obtained in February 2017 with the 6 m BTA telescope. The time resolution was varied from 10 to 120 ms depending on observational mode. Our data show that the pulsar still remained in the low-mass X-ray binary stage, characterised by rapid flaring at time scales of 10–100 s with amplitudes of 0.2–0.5 mag. We resolved a fine structure of the flares at time scales of 0.1–10 s. The polarimetry at the time scale of 0.1 s shows no polarization with an upper limit of 2%–4% for the linear polarisation degree in flaring and quiet stages, while at a 10 minute scale averaging it is about 1.5% at 3σ significance. We shortly outline implications of the results.

We perform MHD simulations of a thin resistive and viscous accretion disk around a neutron star with the surface dipolar magnetic field of 10⁸ Gauss. The system evolution is followed during the interval of 500 millisecond pulsar rotations. Matter is accreted through a stable accretion column from the disk onto the star. We also show propagation of the stellar wind through the corona. Analysis of the mass accretion flux and torques on the star shows that the disk reaches the quasi-stationary state.

The cataclysmic variable AE Aquarii is a low-mass close binary system containing a red dwarf and a 33 s rotation period magnetic white dwarf which operates as a rotation-powered pulsar. The 33 s pulsations are detected in the optical, UV and X-rays. The 16.5 s harmonic is also present in the optical and UV. This pulsing emission comes from two hot spots (T_p ∼ 26000 K) located in the regions of magnetic poles on the white dwarf surface. The nature of the X-ray pulsations of AE Aqr is still under discussion. No 16.5 s harmonic in X-ray is observed and the luminosity of the pulsing component in X-rays is significantly smaller than the luminosity of the pulsing component in the optical and UV. We suggest that the source of pulsing X-ray emission is also located in the magnetic pole region at the surface of the white dwarf and can be associated with a hot spot (T ∼ 10⁶ − 10⁷ K). This spot is heated by the backflowing charged particles. However, the source of particles responsible for the heating the X-ray spot differs from the source of particles responsible for the heating the area emitting in the optical an UV.

The interaction between the hypersonic plasma jet from the accreting neutron star and the ambient interstellar medium is studied. It is assumed that the jet is launched from the accretion disk via the open magnetic field anchored in the disk. The analytical investigation for the structure of the working surface of the jet is carried out. The estimates of the volume stream functions in the region of the interaction between the jet and the interstellar medium are derived. The obtained results allow to examine the distribution of the plasma velocity fields in the interaction region.

A question about the age of accretion-powered X-ray pulsars has recently been reopened by a discovery of the X-ray pulsar SXP 1062 in the SMC. This High Mass X-ray Binary (HMXB) contains a neutron star rotating with the period of 1062 s and is associated with a supernova remnant of the age ∼ 10⁴ yr. An attempt to explain the origin of this young long-period X-ray pulsar within the traditional scenario of three basic states (ejector, propeller and accretor) encounters difficulties. Even if this pulsar were born as a magnetar the spin-down time during the propeller stage would exceed 10⁴ yr. Here we explore a more circuitous way of the pulsar spin evolution in HMXBs, in which the propeller stage in the evolutionary track is avoided. We find this way to be possible if the stellar wind of the massive companion to the neutron star is magnetized. The geometry of plasma flow captured by the neutron star in this case differs from spherically symmetrical and the magnetospheric radius of the neutron star is smaller than that evaluated in the convention accretion scenarios. We show that the age of an accretion-powered pulsar in this case can be as small as ∼ 10⁴ years without the need of invoking initial magnetic field in excess of 10¹³ G.

Supergiant fast X-ray transients (SFXTs) are a sub-class of wind-fed High Mass X-ray Binaries (HMXB) in which the normal companion is a supergiant. These systems were collected in a sub-class because of short flares (a few hours duration) in which the X-ray luminosity increases by a few orders of magnitude. One of the members of SFXTs is the X-ray 1212 s pulsar IGR J16418−4532, which is characterized by a high quiescent X-ray luminosity and flaring on a short timescale. We show that the degenerate component of the system is either a magnetar which accretes matter from a Keplerian disk of quasi-spherical flow, or a regularly magnetized neutron star which rotates near spin equilibrium and accretes matter from a non-Keplerian magnetic disk.

A majority of accretion-powered X-ray pulsars in wind-fed High Mass X-ray Binaries (HMXBs) located in the Magellanic Clouds are observed to be transient X-ray sources. They are characterized by short luminous outbursts, while spending most of the time in quiescence. The quiescent states of the pulsars in the diagram "Pulsar Period vs. X-ray Luminosity" fall on a line with the slope −0.43. The same slope is expected for the propeller line which separates stars in the accretor state from stars in the propeller state. We show, however, that a line with the same slope would also be expected if rotation of the pulsars is close to equilibrium.

Calculations using the chiral effective field theory (ChEFT) indicate that the four-body force contribution to the equation of state (EOS) of pure neutron matter (PNM) at the nuclear density n₀ is negligibly small. However, the overall uncertainty in the EOS of PNM at n₀ remains ∼ 20%. Relativistic mean field (RMF) calculations with in-medium scaling, and including hyperons and Δ resonances, can be made consistent with recent nuclear and astrophysical constraints. Dirac-Brueckner-Hartree-Fock calculations with some medium dependence of the nuclear interaction yield neutron star (NS) models with hyperonic cores consistent with 2 M_⊙ stars and agreeing with the saturation parameters of nuclear matter. Many unified EOS for the NS crust and core were calculated, and are reviewed here. The effect of the finite size of baryons on the EOS, its treatment via the excluded-volume approximation, and its relevance for the hypothetical hybrid-star twins at ∼ 2 M_⊙ are dicussed.

The new generation of X-ray telescopes will provide simultaneous measurements of the radius and mass of a number of neutron stars (NSs), potentially constraining their equation of state (EoS). However the use of non-unified EoSs, that are based on different nuclear models for the crust and core, can introduce an uncertainty on the NS radius of the order of the precision expected from these instruments. Two solutions to this problem are presented. First a large number of unified EoSs and second an approximate approach to the NS crust that allows to obtain with a very large precision the relation between the NS mass and radius with no need to use an EoS for the crust. Correlations between the NS radius and nuclear parameters, which are properties of the EoS that can be indirectly measured in laboratory, are discussed. These open the possibility of constraining the NS radius with experiments on Earth. Finally, the appearance of hyperons in the core of NSs is examined. They are included in the EoS models consistently with available measurements on the hyperon properties. Because of the scarcity of these experimental data one can not exclude that hyperons appear at high densities in NSs in spite of the observations of massive NSs.

The two component model of neutron star dynamics describing the behaviour of the observed crust coupled to the superfluid interior has so far been applied to radio pulsars for which the external torques are constant on dynamical timescales. We recently solved this problem under arbitrary time dependent external torques. Our solutions pertain to internal torques that are linear in the rotation rates, as well as to the extremely non-linear internal torques of the vortex creep model. Two-component models with linear or nonlinear internal torques can now be applied to magnetars and to neutron stars in binary systems, with strong variability and timing noise. Time dependent external torques can be obtained from the observed spin-down (or spin-up) time series, $\dot \Omega \left( t \right)$.

Glitches, sudden spin-up of pulsars with subsequent recovery, provide us with a unique opportunity to investigate various physical processes, including the crust-core coupling, distribution of reservoir angular momentum within different internal layers, spin-up in neutral and charged superfluids and constraining the equation of state of the neutron star (NS) matter. In this work, depending on the dynamic interaction between the vortex lines and the nuclei in the inner crust, and between the vortex lines and the magnetic flux tubes in the outer core, various types of relaxation behavior are obtained and confronted with the observations. It is shown that the glitches have strong potential to deduce information about the cooling behavior and interior magnetic field configuration of NSs. Some implications of the relative importance of the external spin-down torques and the superfluid internal torques for recently observed unusual glitches are also discussed.

We consider the pulsar rotation assuming that the neutron star consists of crust component (which rotation is observed) and two core components. One of the core components contains pinned superfluid which can, for some reasons, suddenly inject small fraction of stored angular momentum in it. In the framework of this simple model the star can demonstrate glitch-like events together with long period precession (with period 1 − 10⁴ years).

The outer crust of an accreted neutron star is thought to contain a large distribution of different nuclear species resulting from the burying of ashes of X-ray bursts and superbursts. By analysing the stability of multicomponent Coulomb crystals against phase separation, it is found that various binary and ternary ionic compounds could be formed.

We study the equation of state of cold and dense baryon matter within the relativistic mean-field framework with hadron masses and coupling constants dependent on the mean scalar field. We include Δ(1232) isobars into previously developed models with hyperons and consider the possibility of charged ρ-meson condensation. The Δ-isobars, being included with realistic values of the attractive in-medium potential, do not lead to a strong decrease of the maximum predicted neutron star (NS) mass, and our models thus resolve the "Δ-resonance puzzle". The charged ρ⁻-meson condensation leads to a substantial maximum NS mass decrease. However, the observational constraint on the minimal value of the maximum NS mass remains fulfilled in all our models under consideration. The decrease of NS mass can be lowered, if one assumes a limited decrease of the ρ-meson effective mass at high densities.

The momentum dependence of single-particle potential (SPP) and effective masses of nuclei in asymmetric nuclear matter are studied in the framework of the lowest order constrained variational (LOCV) method at zero temperature. The Av18 interactions including two-body interactions and Urbana type three-body force (TBF) are considered as the input for the nucleon-nucleon potential. We investigate the TBF effect on the momentum-dependence of neutron and proton SPP. Also, we calculate the isospin splitting of the neutron and proton effective masses in neutron-rich nuclear matter.

Thermal conductivity and shear viscosity of npeµ matter in non-superfluid neutron star cores are considered in the framework of Brueckner-Hartree-Fock many-body theory. We extend our previous work (Shternin et al 2013 PRC88 065803) by analysing different nucleon-nucleon potentials and different three-body forces. We find that the use of different potentials leads up to one order of magnitude variations in the values of the nucleon contribution to transport coefficients. The nucleon contribution dominates the thermal conductivity, but for all considered models the shear viscosity is dominated by leptons.

We study the evolution of the field on the surface of proto-neutron stars in the immediate aftermath of stellar core collapse by analyzing the results of self-consistent, axisymmetric simulations of the cores of rapidly rotating high-mass stars. To this end, we compare the field topology and the angular spectra of the poloidal and toroidal field components over a time of about one seconds for cores. Both components are characterized by a complex geometry with high power at intermediate angular scales. The structure is mostly the result of the accretion of magnetic flux embedded in the matter falling through the turbulent post-shock layer onto the PNS. Our results may help to guide further studies of the long-term magneto-thermal evolution of proto-neutron stars. We find that the accretion of stellar progenitor layers endowed with low or null magnetization bury the magnetic field on the PNS surface very effectively.

Neutron stars are extremely strong cosmic magnets which fields are expected to decay with time. Here we report on the simple test of this process. Adopting a novel approach, we have estimated surface magnetic fields B for 76 radiopulsars (the most numerous subclass of the known isolated neutron stars) which ages t are known independently. Focusing on the accurate evaluation of the precision of both quantities, we determined a significant power-law trend B(t) ∝ t^−β with index $\beta = 0.19_{ - 0.06}^{ + 0.05}$ at 95% C.L. The effects of the observational selection turn this value into the upper limit for the intrinsic field decay rate. If so, then neutron star crusts are close to the "impurity-free crystals", which results in a relatively slow magnetic fields decay.

We discuss the magnetic field enhancement by unstable r-modes (driven by the gravitational radiation reaction force) in rotating stars. In the absence of a magnetic field, gravitational radiation exponentially increases the r-mode amplitude α, and accelerates differential rotation (secular motion of fluid elements). For a magnetized star, differential rotation enhances the magnetic field energy. Rezzolla et al (2000–2001) argued that if the magnetic energy grows faster than the gravitational radiation reaction force pumps energy into the r-modes, then the r-mode instability is suppressed. Chugunov (2015) demonstrated that without gravitational radiation, differential rotation can be treated as a degree of freedom decoupled from the r-modes and controlled by the back reaction of the magnetic field. In particular, the magnetic field windup does not damp r-modes. Here we discuss the effect of the back reaction of the magnetic field on differential rotation of unstable r-modes, and show that it limits the generated magnetic field and the magnetic energy growth rate preventing suppression of the r-mode instability by magnetic windup at low saturation amplitudes, α ≪ 1, predicted by current models.

We discuss the effect of the chemical composition of heat blanketing envelopes of neutron stars (NSs) on the interpretation of the observations of these stars. First we analyze the diffusive fluxes of ions in non-isothermal and non-ideal Coulomb plasmas. Then we outline models of diffusively-equilibrated heat blanketing envelopes composed of binary ionic mixtures and finally we study their effect on the cooling of isolated NSs.

We model heat propagation and the thermal surface luminosity $L_{\rm{s}}^\infty \left( t \right)$ of a neutron star after an internal outburst in its crust. Simulations take into account superfluidity of free neutrons and the thickness of the outbursting layer (heater) in the crust. Crustal superfluidity can shorten and intensify variations of $L_{\rm{s}}^\infty \left( t \right)$.

Recently, numerical calculations of the magnetic field evolution in neutron stars demonstrated the possible existence of a Hall attractor, a stage at which the evolution of the field driven by the Hall cascade ends. The existence of such a stage in neutron star magnetic evolution is very important, and can be potentially probed by observations. Here we discuss three types of objects which could have reached this stage. First, we briefly describe the evolution of normal radio pulsars with ages about a few hundred thousand years. Then we analyse in more detail observations of RX J1856.5-3754, one of the Magnificent Seven, focusing on the surface temperature distribution and comparing model predictions with the temperature map inferred from X-ray observations. Finally, we discuss the necessity of the Hall attractor stage to explain the hypothetical existence of accreting magnetars. We conclude that at the moment there is no direct confirmation of the Hall attractor stage in known sources. However, more detailed observations in the near future can demonstrate existence (or absence) of this stage of the crustal magnetic field evolution.

We briefly review a simple method to analyse observations of cooling neutron stars (developed in our previous works) and discuss its applications to XMMU J173203.3–244518, RX J1856.5–3754 and the Vela pulsar.

We discuss the observational properties of pulsar wind nebulae (PWNe) linking them to the injected (at the termination shock) electron spectral energy distribution and parameters of pulsar magnetospheres. In particular, we (1) present spatially-resolved Chandra ACIS spectral maps of twelve PWNe and measure the slopes of the uncooled PWN spectra just downstream of the termination shock obtained from these maps, (2) consider the connections between PWN morphologies and predictions of the magnetospheric emission models and (3) discuss the limits on the maximum energies of particles in PWNe from X-ray and γ-ray observations.

Young supernova remnants are the sources of a broadband nonthermal synchrotron continuum emission from radio to X-rays produced by accelerated electrons. Diffusive shock acceleration (DSA) of relativistic particles is accompanied by strong amplification of the turbulent magnetic field. This is an intrinsic feature of an efficient DSA. Synchrotron X-rays are produced by the multi-TeV electrons which are concentrated in a narrow region near the shock due to the strong energy losses. The confinement of the X-ray emitting electrons in this region filled with highly fluctuating magnetic field results in a complex time dependent clumpy structure of the observed X-ray images of supernova shells. The lack of Faraday depolarization in X-rays makes X-ray polarimetry a powerful tool to study synchrotron structures in the young galactic shell type SNRs Cas A, Tycho, SN 1006, RX J1713.7–3946. We present here simulated synchrotron images of SNR shells for different models of isotropic and anisotropic magnetic turbulence in a shell and argue that polarized X-ray imaging can be constructed for Tycho and Cas A SNRs with the expected sensitivity and the angular resolution of the upcoming missions XIPE (ESA) and IXPE (NASA).

The standard model of the pulsar wind nebula states that the pulsar wind should be low-magnetization just upstream the termination shock, called the 'sigma-problem'. Low-magnetization is required in order to confine the pulsar wind nebula inside the slowly expanding supernova remnant whose expansion velocity is much smaller than the speed of light. Although the standard model is based on the ideal magnetohydrodynamic approximation, the recent studies indicate that the non-ideal magnetohydrodynamic effects would be important to resolve the sigma-problem. In this study, we extend the standard model including the turbulent magnetic field and the magnetic dissipation. The conversion of the toroidal magnetic field to the turbulent magnetic field and the magnetic dissipation terms are treated phenomenologically. We find that the conversion of the toroidal magnetic field to the turbulent magnetic field decelerates the nebular flow to fit the observed value of the expansion velocity. On the other hand, the magnetic dissipation hardly contributes the flow deceleration, although the magnetic dissipation is important to reproduce the observed emission properties of the PWNe.

G1.9+0.3 is the youngest known Galactic supernova remnant (SNR) and dominated by X-ray synchrotron emission. Synchrotron X-rays can be a useful tool to study the electron acceleration in young SNRs. The X-ray spectra of young SNRs give us information about the particle acceleration at the early stages of evolution of SNRs. In this work, we investigate the time evolution of roll-off frequency of the synchrotron spectrum from SNR G1.9+0.3 using Suzaku. For this analysis, we use ∼101 ks (2011) and ∼92 ks (2015) observations with the X-ray Imaging Spectrometer. We find that there is no significant differences in the spectral parameters and interpret our results.

The energetic and fast-moving radio and γ-ray pulsar B1951+32 is associated with the supernova remnant CTB 80. It powers a complex pulsar wind nebula detected in the radio, Hα and X-rays (Moon et al 2004 ApJ610 L33). A puzzling optical knot was detected about 0''.5 from the pulsar in the optical and near-IR (Moon et al 2004 ApJ610 L33; Hester 2000 Bulletin of the AAS32 1542). It is reminiscent of the unique "inner optical knot" located 0''.6 from the Crab pulsar. Until now there has been no evidence that B1951+32 knot is indeed associated with the pulsar.

We observed the pulsar field with the Gemini-North telescope in 2016 to check the association. We performed first near-IR high spatial resolution imaging in the K_s band using the NIRI+Altair instrument and deep optical imaging in the gr bands using the GMOS instrument. Our observations showed that the current knot position is shifted by ≈ 0''.6 from the position measured with the HST in 1997. This is consistent with the known pulsar proper motion and is direct evidence of the pulsar–knot connection. We compared the spectral energy distribution of the knot emission with that of the Crab knot. Possible implications of the results are discussed.