Abstract:
A sufficiently
large star will collapse to form a Black Hole Singularity due to Gravitational Pressure
beyond Neutron Degeneracy. A Black Hole exhibits extremely strong Gravitational
attraction that no particle or electromagnetic radiation can escape from it. The
boundary of the region from which no escape is possible is called Event Horizon.
In this work it is proposed that there exists a Neutron star smaller than Event
Horizon, which is termed as Black Neutron Star. Furthermore an alternative method
is proposed to ascertain the maximum permissible mass limit of the Neutron Star
and the minimum mass limit of the naturally occurring gravitationally collapsed
Black hole.

Abstract:
This
paper initially reviews types of deep space navigation methods. Then, it
studies the use of pulsars as one of sources emitting electromagnetic waves in
navigation; hence more details regarding the pulsar physics and the history of
navigation using pulsars are presented. The various methods of navigation
(including radio method), their advantages and disadvantages—in comparison with
navigation using pulsars in spacecraft—are discussed. Then, the equations
necessary for calculating position and velocity of a spacecraft (such as the
arrival time of pulse from pulsar to the receiver) are introduced, and the
methods of calculating position and velocity are dealt with. Finally, two
algorithms are presented for positioning, and one for velocity. Attitude
determination follows the same simple methods presented in various articles.

Abstract:
In the present article, we explore a new static, spherically symmetric charged anisotropic fluid model of compact star in curvature coordinates. We consider metric potential g_{44} of Durgapal’s fifth solution [1] with a specific choice of electric field intensity E and physically acceptable expression of anisotropy factor Δ , which involve parameters K (charge) and Δ (anisotropy) respectively. The solution so obtained is utilized to construct the model for superdense star like neutron star. We have analysed that corresponding to X=0.1, K=2.8, a=1.6 and by assuming surface density , the mass of the compact star comes out to be with radius 14.51 kms, which closely resembles to that of PSRJ0348 + 0432. The solution is well behaved for the values of K satisfying ？1≤K<5. Our model is described analytically as well as with the help of graphical representations. Our solution is well behaved and free from any central singularity. It also satisfies all the energy conditions as well as the causality condition thus reconfirming the stability of our model.

Abstract:
In accreting neutron star (NS) low-mass X-ray binaries (LMXBs), the turbulent flow in accretion disk may show magnetic structures. Its emission will vary in time due to inhomogeneous motions through and with the accretion flow. These emissions contribute to considerable X-ray variability on a wide range of timescales in all wavelengths, and down to milliseconds. In this article, we give a brief review for quasi-periodic oscillations (QPOs), one of a periodic X-ray variability, in NS/ LMXBs. Firstly, we give a brief introduction to NS/LMXBs and the fruitful QPO components. As an example, the energy dependence of normal branch oscillations in Scorpius X-1 is discussed. We mostly focus on the properties and mechanism of kilohertz QPOs—the fastest variability components that have the same order as the dynamical timescales of the innermost regions of accretion flow. Finally, we discuss the success and questions for theoretical interpretations and present the possible entry for investigation of nature of QPOs.

Abstract:
The attempt has been taken to calculate the density of stars possessing quark matter core using sphere packing concept of crystallography. The quark matter has been taken as solid in nature as predicted in references 36 and 37, and due to immense gravitational pressure at the core of the star the densest packing of quarks as spheres has been assumed to calculate the packing fraction Φ, thus the density ρ of the matter. Three possible types of pickings—mono-sized sphere packing, binary sphere packing and ternary sphere packing, have been worked out using three possible types of quark matter. It has been concluded that no value about the ρ of quark matter can be calculated using binary and ternary packing conditions and for mono-sized packing condition different flavor quark matters of different values in the density have been calculated using results from the experiments done by HI, ZEUS, L3 and CDF Collaborations about the radius limit of quark. For example, for u quark matter ρ ranges from 4.0587 × 10^{48} - 7.40038 × 10^{48} MeV/c^{2} cm^{3} using results of L3 Collaboration, for s quark matter 15.91794 × 10^{48} - 17.6866 × 10^{48} MeV/c^{2} cm^{3}, etc.

Abstract:
Using the data from the Proportional Counter Array on board theRossi X-Ray Timing Explorersatellite, we study the orbital modulation for the spectrum and mass accretion rate of Cir X-1 during its two orbital periods. We use a model consisting of a blackbody, a multicolor disk blackbody, and a line component to fit the spectrum and find that the spectrum is obviously modulated by the orbital phase. It is shown that the disk accretion rate in Cir X-1 undergoes three states during the orbital period. At the periastron with orbital phase 0-0.1, the disk accretion rate is sup-Eddington, then from phase 0.1 to the apastron (phase 0.5) it decreases dramatically and becomes near-Eddington, and from the apastron to the next periastron (phase 1) the disk accretion rate approximates Eddington and tends to be steady. We argue that the evolution of the disk accretion rate is attributed to the high orbital eccentricity of this source. The mass accretion rate onto the neutron star is much less than that onto the inner disk, indicating significant outflows in this source.

Abstract:
βdecay in the strong magnetic field of the crusts of neutron stars is analysed by an improved method. The reactions 67 Ni(β-)67 Cu and 62 Mn\beta -62 Fe are investigated as examples. The results show that a weak magnetic field has little effect on βdecay but a strong magnetic field (B>1012G) increases β decay rates obviously. The conclusion derived may be crucial to the research of late evolution of neutron stars and nucleosynthesis in r-process.

Abstract:
The imprints of the neutron star crust on the gravitational waves emitted from the axial w-modes are investigated by adopting two typical equations of state (EOSs) of the crust matter and two representative EOSs of the core matter. It is shown that there is a significant effect of the crust EOSs on the gravitational waves from the axial w-mode oscillation for a stiff core EOS.

The metrics of the compact
objects should be the continuous function of coordinates. The metrics inside
every object is set by its internal structure. The metrics in the adjacent
empty space is described by the outer Schwarzschild or Kerr solution of the
Einstein field equations. It appears that the linkup of both object-interior
and empty-space metrics is not continuous at the physical surfaces of the
objects for the common, generally (by convention) accepted set of assumptions.
We suggest the new way of how to achieve the success in the linkup, which does
not assume the higher value of the relativistic speed limit in the empty space
governed by the object, in contrast to our previous suggestion. We also give a
more detailed explanation of the existence of inner physical surface of compact
objects and suggest the way of the linkup of metrics in this surface. To achieve
the continuous linkup, we assume a lower value of the speed limit in the
object’s interior as well as a new gauging of the outer Schwarzschild solution
for the inner empty space of the object. Newly established gauging constants
are calculated and the success of the linkup is shown in several examples. The
new gauging implies a lower gravitational attraction (lower gravitational
constant) in the inner empty space in comparison with that in the outer space,
which is measured in the common, observed, gravitational interactions of
material objects.