Narrow Results

Gravitational wave astronomy opened dramatically in September 2015 with the LIGO discovery of a distant and massive binary black hole coalescence. The more recent discovery of a binary neutron star merger, followed by a gamma ray burst (GRB) and a kilonova, reinforces the excitement of this new era, in which we may soon see other sources of gravitational waves, including continuous, nearly monochromatic signals. Potential continuous wave (CW) sources include rapidly spinning galactic neutron stars and more exotic possibilities, such as emission from axion Bose Einstein “clouds” surrounding black holes. Recent searches in Advanced LIGO data are presented, and prospects for more sensitive future searches are discussed.

The detection potential of AMS for photon sources of different origin is assessed. In particular, galactic and extragalactic point sources detected by previous experiments are examined for their detectability at energies above a few GeV. In addition, gamma ray rates from a potential supersymmetric dark matter halo are estimated using different models. It is argued that the constant survey of the gamma ray sky by AMS with a high angular resolution will be an important complement to similar studies using other present and future instruments.

Diffusive TeV gamma-ray emissions have been recently discovered extending beyond the pulsar wind nebulae of a few middle-aged pulsars, implying that energetic electron/ positron pairs are escaping from the pulsar wind nebulae and radiating in the ambient interstellar medium. It has been suggested that these extended emissions constitute a distinct class of nonthermal sources, termed “pulsar halos”. In this paper, I will review the research progress on pulsar halos and discuss our current understanding on their physics, including the multiwavelength observations, different theoretical models, as well as implications for the origin of cosmic-ray positron excess and Galactic diffuse gamma-ray emission.

Dependences of the X-ray luminosity (L_x) of young single pulsars, due to ejection of relativistic particles, on electric field intensity, rate of rotational energy loss (Ė), magnetic field, period, and some other parameters of neutron stars are discussed. Influence of the magnetic field and effects of some other parameters of neutron stars on the L_x-Ė and the L_x-τ (characteristic time) dependences are considered. Evolutionary factors also play an important role in our considerations. Only the pulsars with L_{2–10 keV}>10³³erg/s have pulsar wind nebula around them. The pulsars from which γ-ray radiation has been observed have low X-ray luminosity in general.

In this work we have analysed various data on radio pulsars and we have shown that magnetic field decay of a factor about 10–20 is necessary to explain their evolution, in particular to remove the discrepancy between the characteristic and the real ages. The character of the field decay is exponential with a characteristic time of about 3×10⁶ year. Observational data on single X-ray pulsars which radiate due to cooling also support this result.

We briefly discuss a handful of topics in pulsar astrophysics, first some general well-known features, then an overview of the glitch phenomenon and the sort of information gathered about the internal structure and dynamics, and finally the quandary posed by the precession of PSR B1828-11 a very important clue pointing towards a novel paradigm for structure of the core regions. We point out that "exotic" solutions for the precession puzzle would force a consideration of exotic glitch mechanisms as well.

P-Stars are a new class of compact stars made of up and down quarks in β-equilibrium with electrons in an Abelian chromomagnetic condensate. We show that P-Stars are able to account for compact stars with R≲6 Km, as well as stars with radius comparable with canonical Neutron Stars. We find that cooling curves of P-Stars compare rather well with observational data. We suggest that P-Matter produced at the primordial deconfinement transition is a viable candidate for baryonic Cold Dark Matter. Finally, we show that P-Stars are able to overcome the gravitational collapse even for masses much greater than 10⁶ M_⊙.

It is shown that anomalous X-ray pulsars and soft gamma repeaters are neutron stars with mass less than 1 solar mass and with magnetic field about 3×10¹³–10¹⁴G. Their ages (t≤10⁵yr) are considerably larger than their characteristic times. The angle between the rotation axis and the axis of the magnetic field must be large for these objects. From time to time as a result of activities their value of Ṗ considerably increases because of the propeller mechanism. Using such an approach Guseinov et al.¹ have predicted the transient characteristic of these sources which has been confirmed recently.² We estimate the spatial densities and lifetimes of different types of isolated neutron star. Some of these sources must have relations with anomalous X-ray pulsars and soft gamma repeaters. In order to understand the locations of different types of isolated neutron star on the P–Ṗ diagram it is also necessary to take into account the differences in the mass and the magnetic field of neutron stars. We have also estimated the birth rates of different types of isolated neutron stars.

Some physical parameters and properties of neutron stars like the mass, moment of inertia, rotation and absence of stability in the atmosphere affect the evolution of pulsars on the P-Ṗ diagram. We have examined such possible influences which can enlighten the differences between various types of isolated neutron stars.

Dim radio-quiet neutron star (DRQNS) 1E 1207.4-5209 is one of the most heavily examined isolated neutron stars. Wide absorption lines were observed in its spectrum obtained by both XMM-Newton and Chandra X-ray satellites. These absorption lines can be interpreted as a principal frequency centered at 0.7 keV and its harmonics at 1.4, 2.1 and possibly 2.8 keV. The principal line can be formed by resonant proton cyclotron scattering leading to a magnetic field which is two orders of magnitude larger than the perpendicular component of the surface dipole magnetic field (B) found from the rotation period (P) and the time rate of change in the rotation period of 1E 1207.4-5209. Besides, age of the supernova remnant (SNR) G296.5 + 10.0 which is physically connected to 1E 1207.4-5209 is two orders of magnitude smaller than the characteristic age of the neutron star. These huge differences between the magnetic field values and the ages can be explained based on a B-decay model. If the decay is assumed to be exponential, the characteristic decay time turns out to be several thousand years which is three orders of magnitude smaller than the characteristic decay time of radio pulsars represented in an earlier work. The lack of detection of radio emission from DRQNSs and the lack of point sources and pulsar wind nebulae in most of the observed SNRs can also be partly explained by such a very rapid exponential decay. The large difference between the characteristic decay times of DRQNSs and radio pulsars must be related to the differences in the magnetic fields, equation of states and masses of these isolated neutron stars.

In a previous work, we have predicted an isovector component of the light scalar meson sector by using the chiral symmetry transformation formalism. On the basis of this result, we study dense hadronic matter in a generalized relativistic mean field approach with σ, ω and ρ mesons as well as nonlinear self-couplings of the I = 1 component of a light scalar meson field and compare its predictions for neutron star properties and with results from different models for nuclear matter found in the literature.

In this paper, we analyze the available data on isolated X-ray pulsars, their wind nebulae, and the supernova remnants that are connected to some of these sources. It is shown that the electric fields of neutron stars tear off charged particles from the surface of the neutron star and trigger the acceleration of particles. The charged particles are accelerated mainly in the field of a magneto-dipole radiation wave. The power and energy spectra of the charged particles depend on the strength of the magneto-dipole radiation. Therefore, the X-ray radiation is strongly dependent on the rate of rotational energy loss and weakly dependent on the electric field intensity. Coulombic interaction between the charged particles is the main factor for the energy loss and the X-ray spectra of the charged particles.

In this work, the results of X-ray spectral analysis of radio-quiet X-ray pulsar 1E 1207.4–5209 and RX J0002+6246, RX J0822–4300, CXOU J185238.6+00402, which have the same observational characteristics as 1E 1207.4–5209, are represented based on XMM-Newton satellite observations. These four neutron stars with common physical properties may have similar physical evolution. It is shown that all of them have Pure blackbody spectra (two-blackbody fits in three cases) without a power law component. No pulsar wind nebula has been observed around any one of them, which strongly shows that they have low rotational energy loss despite their small ages. All of these neutron stars are physically connected to shell-type supernova remnants which are younger than (1–2) × 10⁴yr. None of these sources has been detected to emit at radiofrequencies. These common spectral properties (and possibly common timing characteristics) of these neutron stars are examined. Their possible evolution is discussed based on a model represented in Ref. 1. As a result, it is quite possible that these objects form a new class of neutron stars quite different than the other types.

The data collected by ATIC, PPB-BETS, FERMI-LAT and HESS all indicate that there is an electron/positron excess in the cosmic ray energy spectrum above ~100 GeV, although different instrumental teams do not agree on the detailed spectral shape. PAMELA also reported clearly the excessive feature of the fraction of positron above several GeV, but with no excess in antiprotons. Here we review the observational status and theoretical models of this interesting observational feature. We pay special attention to various physical interpretations proposed in the literature, including modified supernova remnant models for the e^± background, new astrophysical sources, and new physics (the dark matter models). We suggest that although most models can make a case to interpret the data, with the current observational constraints the dark matter interpretations, especially those invoking annihilation, require much more exotic assumptions than some other astrophysical interpretations. Future observations may present some "smoking-gun" observational tests to differentiate different models and to identify the correct interpretation of the phenomenon.

Five-hundred-meter Aperture Spherical radio Telescope (FAST) is a Chinese mega-science project to build the largest single dish radio telescope in the world. Its innovative engineering concept and design pave a new road to realize a huge single dish in the most effective way. FAST also represents Chinese contribution in the international efforts to build the square kilometer array (SKA). Being the most sensitive single dish radio telescope, FAST will enable astronomers to jump-start many science goals, such as surveying the neutral hydrogen in the Milky Way and other galaxies, detecting faint pulsars, looking for the first shining stars, hearing the possible signals from other civilizations, etc.

The idea of sitting a large spherical dish in a karst depression is rooted in Arecibo telescope. FAST is an Arecibo-type antenna with three outstanding aspects: the karst depression used as the site, which is large to host the 500-meter telescope and deep to allow a zenith angle of 40 degrees; the active main reflector correcting for spherical aberration on the ground to achieve a full polarization and a wide band without involving complex feed systems; and the light-weight feed cabin driven by cables and servomechanism plus a parallel robot as a secondary adjustable system to move with high precision. The feasibility studies for FAST have been carried out for 14 years, supported by Chinese and world astronomical communities. Funding for FAST has been approved by the National Development and Reform Commission in July of 2007 with a capital budget ~ 700 million RMB. The project time is 5.5 years from the commencement of work in March of 2011 and the first light is expected to be in 2016.

This review intends to introduce the project of FAST with emphasis on the recent progress since 2006. In this paper, the subsystems of FAST are described in modest details followed by discussions of the fundamental science goals and examples of early science projects.

Twin kHz quasi-periodic oscillations (QPOs) have been found in the emission spectra of about thirty X-ray neuron star systems, where the upper ones of twin QPOs are often interpreted as the Keplerian frequencies of the accreting matter orbiting around the central stars. The observations of the kHz QPOs can be used to constrain the masses of neutron stars (NSs), and in particular to imply the properties of the matters at nuclear densities. By the way, we discuss the preferences of some kHz QPO models that constrain the NS parameters.

In order to study constraints imposed on kinematics of the Crab pulsar's jet, we consider motion of particles along co-rotating field lines in the magnetosphere of the Crab pulsar. It is shown that particles following the co-rotating magnetic field lines may attain velocities close to observable values. In particular, we demonstrate that if the magnetic field lines are within the light cylinder (LC), the maximum value of the velocity component parallel to the rotation axis is limited by 0.5c. This result in the context of the X-ray observations performed by Chandra X-ray Observatory seems to be quite indicative and useful to estimate the density of field lines inside the jet. Considering the three-dimensional (3D) field lines crossing the LC, we found that for explaining the force-free regime of outflows the magnetic field lines must asymptotically tend to the Archimedes spiral configuration. It is also shown that the 3D case may explain the observed jet velocity for appropriately chosen parameters of magnetic field lines.

What is the real nature of pulsars? This is essentially a question of the fundamental strong interaction between quarks at low-energy scale and hence of the non-perturbative quantum chromo-dynamics, the solution of which would certainly be meaningful for us to understand one of the seven millennium prize problems (i.e., "Yang-Mills Theory") named by the Clay Mathematical Institute. After a historical note, it is argued here that a pulsar is very similar to an extremely big nucleus, but is a little bit different from the gigantic nucleus speculated 80 years ago by L. Landau. The paper demonstrates the similarity between pulsars and gigantic nuclei from both points of view: the different manifestations of compact stars and the general behavior of the strong interaction.

The state of cold quark matter at supra-nuclear density, depending on the non-perturbative nature of quantum chromo-dynamics (QCD), is essential for modelling pulsars. It is conventionally thought that the state equation of dense matter softens and thus cannot result in high maximum mass if pulsars are quark stars. However, this standard point of view would be revisited and updated if the strong coupling between quarks at realistic baryon densities of compact stars could render quarks grouped in quark-clusters, because the state equation of clustering quark matter stiffs to support compact stars with maximum mass M_max > 2M_⊙. Although the hypothetical quark-clusters has not been confirmed due to the lack of both theoretical and experimental evidence, the astrophysical observations of pulsars (e.g. pulsar-mass) could help us to explore the properties of cold quark matter.

Neutron star parameters, e.g. mass, radius and magnetic field, can be constrained by the observations of kHz quasi-periodic oscillations (QPOs), which show the information close to the surface of neutron stars (NSs) in the accreting binary systems. The high QPO frequency will leak out the physics around the inner disk boundary, where the physics in a strong gravity regime will effect. By means of the twin kHz QPO detections, which are interpreted as the Keplerian orbital motions near NSs, the methods of determining NS mass and radius are introduced.