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      Volume 39, Issue 6

      December 2018

    • Non-singular coordinates of some Kiselev space-times


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      In this article, non-singular Kruskal-like coordinates of some Kiselev space-times are presented. Also, non-singular Carter-like coordinates are constructed for the extreme case of Kiselev space-time.

    • Long-term ultraviolet variability of Seyfert galaxies


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      Flux variability is one of the defining characteristics of Seyfert galaxies, a class of active galactic nuclei (AGN). Although these variations are observed over a wide range of wavelengths, results on their flux variability characteristics in the ultraviolet (UV) band are very limited.We present here the long-term UV fluxvariability characteristics of a sample of fourteen Seyfert galaxies using data from the International Ultraviolet Explorer acquired between 1978 and 1995. We found that all the sources showed flux variations with no statistically significant differences in the amplitude of UV flux variation between shorter and longer wavelengths. Also, the flux variations between different near-UV (NUV, 1850−3300 Å) and far-UV (FUV, 1150−2000 Å) pass bands in the rest frames of the objects are correlated with no time lag. The data show indications of (i) a mild negative correlation of UV variability with bolometric luminosity and (ii) weak positive correlation between UV variability and black hole mass. At FUV, about 50% of the sources show a strong correlation between spectral indices and flux variations with a hardening when brightening behaviour, while for the remaining sources the correlation is moderate. In NUV, the sources do show a harder spectrum when brighter, but the correlation is either weak or moderate.

    • Application of Chebyshev collocation method for relocating of spacecrafts in Hill’s frame


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      In this study, the Chebyshev collocation method is used for solving the spacecraft relative motion of equations in Hill’s frame. Three different models of governing equations of relative motion (M1, M2, and M3) are considered and the maneuver cost required moving the spacecraft from one state to another is computed in the form of delta velocity at the first terminal point as a function of time of flight (TOF) and inter-satellite distance (ISD). A quantitative as well as qualitative difference is observed in the maneuver cost with the inclusion ofradial and/or out of plane separation in along track separation of chaser. Also, a relative comparison of path profiles is made by considering M1, M2 and M3 models. Path profiles for M3 model are found close to M2 model for short intervals for a fixed ISD, whereas path profiles for M2 and M3 do not match even for smallvalues of ISD for a fixed but long TOF. Path profiles for M1 models match to M2 model for very low values of target orbit eccentricities.

    • Effect of rotation and self-gravity on the propagation of MHD waves


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      Magnetohydrodynamics waves and instabilities in rotating, self-gravitating, anisotropic and collision-less plasma were investigated. The general dispersion relation was obtained using standard mode analysis by constructing the linearized set of equations. The wave mode solutions and stability properties of the dispersion relations are discussed in the propagations transverse and parallel to the magnetic field. These special cases are discussed considering the axis of rotation to be in transverse and along the magnetic field. In the case of propagation transverse to the magnetic field with axis of rotation parallel to the magnetic field, we derived the dispersion relation modified by rotation and self-gravitation. In the case of propagation parallel to the magnetic field with axis of rotation perpendicular to the magnetic field, we obtained two separate modesaffected by rotation and self-gravitation. This indicates that the Slow mode and fire hose instability are not affected by rotation. Numerical analysis was performed for oblique propagation to show the effect of rotation and self-gravitation. It is found that rotation has an effect of reducing the value of the phase speeds on the fastand Alfven wave modes, but self-gravitation affect only on the Slow modes, thereby reducing the phase speed compare to the ideal magneto hydrodynamic (MHD) case.

    • Polarization and QPOs from jets in black hole systems


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      The historical observations of polarized jet emission for Blazars are reviewed and previous models are discussed. Motivated by this, a model for polarization of both steady and transient behavior using a helical magnetic field is presented. The variety of observed correlations and anti-correlations between the electricpolarization angle, the degree of polarization and optical flux can be explained by this model. In addition, the phenomena of quasi-periodic oscillations (QPO) behavior seen in jets in X-ray Binaries (XRBs) is also explained by a model based on helical trajectories of emitting blobs and the resulting time scales and harmonics of the QPO are derived. In both the models, the input parameters are the inclination angle, the Lorentz factor of the jet and pitch angle of the magnetic helix.

    • Existence of wormhole solutions and energy conditions in $f(R,T)$ gravity


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      This paper is devoted to investigate the spherically symmetricwormholemodels in $f(R,T)$ gravity, where $T$ and $R$ are trace of stress energy tensor and the Ricci scalar, respectively. In this context, we discuss three distinct cases of fluid distributions viz, anisotropic, barotropic and isotropic matter contents. After considering the exponential $f(R,T)$ model, the behavior of energy conditions are analyzed that will help us to explore the general conditions for wormhole geometries in this gravity. It is inferred that the usual matter in the throatcould obey the energy conditions but the gravitational field emerging from higher order terms of modified gravity favor the existence of the non-standard geometries of wormholes. The stability as well as the existence of wormholes are also analyzed in this theory.

    • A new technique for understanding magnetosphere–ionosphere coupling using directional derivatives of SuperDARN convection flow


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      The purpose of this paper is to present and evaluate a new technique to better understand ionospheric convection and it’s magnetospheric drivers using convection maps derived from the Super Dual Auroral Radar Network (SuperDARN). We postulate that the directional derivative of the SuperDARN ionospheric convection flow can be used as a technique for understanding solar wind–magnetosphere–ionosphere coupling by identifying regions of strong acceleration/deceleration of plasma flow associated with drivers of magnetosphericconvection such as magnetic reconnection. Thus, the technique may be used to identify the open–closed magnetic field line boundary (OCB) in certain circumstances. In this study, directional derivatives of the SuperDARN ionospheric convection flow over a four and a half hour interval on Nov. 04, 2001, is presented during which the interplanetary magnetic field was predominantly southward. At each one-minute time point in the interval the positive peak in the directional derivative of flow is identified and evaluated via comparison with known indicators of the OCB including the poleward boundary of ultraviolet emissions from three FUV detectors onboard the IMAGE spacecraft as well as the SuperDARN spectral widths. Good comparison is found between the location of the peak in the directional derivative of SuperDARN flow and the poleward boundary of ultraviolet emissions confirming that acceleration of ionospheric plasma flow is associated with magnetic reconnection and the open–closed boundary.

    • Editorial

      Ram Sagar

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    • Lissajous motion near Lagrangian point $L_2$ in radial solar sail


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      An attempt was made to study the dynamics close to the collinear libration point $L_2$ of the radial solar sail circular-restricted three-body problem (RSCRTBP) in the Sun–Jupiter System, where the third massless body is a solar sail.We analyse the qausi-periodic (Lissajous solutions) orbits about the libration point $L_2$. The Lindstedt–Poincaré approximation for the qausi-periodic orbitswas used for numerical simulations.We utilized linear quadratic regulator (LQR) to stabilize the full nonlinear model, and linear state-feedback controller was designed to stabilize the trajectory.

    • Interstellar communication: The colors of optical SETI


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      It has recently been argued from a laser engineering point of view that there are only a few magic colors for optical SETI. These are primarily the Nd:YAG line at 1,064 nm and its second harmonic (532.1 nm). Next best choices would be the sum frequency and/or second harmonic generation of Nd:YAG and Nd:YLFlaser lines, 393.8 nm (near Fraunhofer CaK), 656.5 nm (H$\alpha$) and 589.1 nm (NaD2). In this paper, we examine the interstellar extinction, atmospheric transparency and scintillation, as well as noise conditions for these laserlines. For strong signals, we find that optical wavelengths are optimal for distances $d \lesssim {\rm kpc}$. Nd:YAG at $\lambda = 1,064$ nm is a similarly good choice, within a factor of two, under most conditions and out to $d \lesssim 3$ kpc.For weaker transmitters, where the signal-to-noise ratio with respect to the blended host star is relevant, the optimal wavelength depends on the background source, such as the stellar type. Fraunhofer spectral lines, while providing lower stellar background noise, are irrelevant in most use cases, as they are overpowered byother factors. Laser-pushed spaceflight concepts, such as “Breakthrough Starshot”, would produce brighter and tighter beams than ever assumed for OSETI. Such beamers would appear as naked eye stars out to kpc distances.If laser physics has already matured and converged on the most efficient technology, the laser line of choice for a given scenario (e.g., Nd:YAG for strong signals) can be observed with a narrow filter to dramatically reducebackground noise, allowing for large field-of-view observations in fast surveys.

    • Interstellar communication: Short pulse duration limits of optical SETI


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      Previous and ongoing searches for extraterrestrial optical and infrared nanosecond laser pulses and narrow line-width continuous emissions have so far returned null results. At the commonly used observation cadence of $\sim$10$^{−9}$ s, sky-integrated starlight is a relevant noise source for large field-of-view surveys. This can be reduced with narrow bandwidth filters, multipixel detectors, or a shorter observation cadence. We examine the limits of short pulses set by the uncertainty principle, interstellar scattering, atmospheric scintillation,refraction, dispersion and receiver technology. We find that optimal laser pulses are time-bandwidth limited Gaussians with a duration of $\lambda_t \approx 10^{−12}$ s at a wavelength $\lambda_0 \approx 1$ $\mu$m, and a spectral width of $\Delta \lambda\approx 1.5$ nm. Shorter pulses are too strongly affected through Earth’s atmosphere. Given certain technological advances, survey speed can be increased by three orders of magnitude when moving from ns to ps pulses. Faster (and/or parallel) signal processing would allow for an all-sky-at-once survey of lasers targeted at Earth

    • Observations and physics of prompt emission of gamma ray bursts


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      Gamma ray bursts (GRBs) are the brightest explosions known to occur in the Universe. For the last several decades, they have been extensively observed and studied using both space as well as ground based observatories. In this review, the observational breakthroughs made till date, the techniques of observation andanalyses of obtained data, temporal and spectral properties of the observed prompt emission of GRBs including polarisation, as well as the various theoretical models adopted to explain them are discussed.

  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

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