• Volume 39, Issue 3

June 2018

• Non-collinear libration points in ER3BP with albedo effect and oblateness

In this paper we establish a relation between direct radiations (generally called radiation factor) and reflected radiations (albedo) to show their effects on the existence and stability of non-collinear libration points in the elliptic restricted three-body problem taking into account the oblateness of smaller primary. It isdiscussed briefly when $\alpha = 0$ and $\sigma = 0$, the non-collinear libration points form an isosceles triangle with the primaries and as e increases the libration points $L_{4,5}$ move vertically downward ($\alpha$, $\sigma$ and $e$ represents the radiation factor, oblateness factor and eccentricity of the primaries respectively). If $\alpha = 0$ but $\sigma \neq 0$, the libration points slightly displaced to the right-side from its previous location and form scalene triangle with the primaries and go vertically downward as $e$ increases. If $\alpha \neq 0$ and $\sigma\neq 0$, the libration points $L_{4,5}$ form scalene triangle with the primaries and as e increases $L_{4,5}$ move downward and displaced to the left-side. Also, the libration points $L_{4,5}$ are stable for the critical mass parameter $\mu \leq \mu_c$.

• Trajectory of asteroid 2017 SB20 within the CRTBP

Regular monitoring the trajectory of asteroids to a future time is a necessity, because the variety of known probably unsafe near-Earth asteroids are increasing. The analysis is perform to avoid any incident or whether they would have a further future threat to the Earth or not. Recently a new Near Earth Asteroid (2017 SB20) has been observed to cross the Earth orbit. In view of this we obtain the trajectory of Asteroid in the circular restricted three body problem with radiation pressure and oblateness.We examine nature of Asteroid’sorbit with Lyapunov Characteristic Exponents (LCEs) over a finite intervals of time. LCE of the system confirms that the motion of asteroid is chaotic in nature. With the effect of radiation pressure and oblateness the length of curve varies in both the planes. Oblateness factor is found to be more perturbative than radiation pressure. To see the precision of result obtain from numerical integration we show the error propagation and the numerical stability is assured around the singularity by applying regularized equations of motion for precise long-term study.

• Multiband optical–IR variability of the blazar PKS 0537–441

We have reconsidered the simultaneous and homogeneous optical–IR light curves and the corresponding spectral indices curve of the blazarPKS0537–441 from January 2011 toMay2015. All the curves show significant fluctuations on various timescales, and the flux variations seem to be more pronounced towards the IR bands. The relation between average fluxes and spectral indices reveals the existence of redder-when-brighter (RWB) and bluer-when-brighter (BWB) trends at different flux levels, along with a long-term achromatic trend and a mild RWB trend on short-term timescales. Cross-correlation analyses present an energy-dependent time delay that the lower-frequency variations follow higher-frequency ones by a few weeks and a hysteresis pattern between spectra and fluxes. Our analysis reveals some potential coherence between low-energy-peaked BL Lacs (LBLs) and FSRQs, and indicates that the observed flux variability and spectral changes could be due to the superposition of a dominant jet emission, an underlying thermal contribution from a more slowly varyingdisk and/or other geometric effects under the shock-in-jet scenario.

• Interactions of galaxies outside clusters and massive groups

We investigate the dependence of physical properties of galaxies on small- and large-scale density environment. The galaxy population consists of mainly passively evolving galaxies in comparatively lowdensity regions of Sloan Digital Sky Survey (SDSS). We adopt (i) local density, $\rho_{20}$, derived using adaptive smoothing kernel, (ii) projected distance, $r_p$, to the nearest neighbor galaxy and (iii) the morphology of the nearest neighbor galaxy as various definitions of environment parameters of every galaxy in our sample. In orderto detect long-range interaction effects, we group galaxy interactions into four cases depending on morphology of the target and neighbor galaxies. This study builds upon an earlier study by Park and Choi (2009) by including improved definitions of target and neighbor galaxies, thus enabling us to better understand the effect of “the nearest neighbor” interaction on the galaxy. We report that the impact of interaction on galaxy properties is detectable at least up to the pair separation corresponding to the virial radius of (the neighbor) galaxies. Thisturns out to be mostly between 210 and 360 $h^{-1}$ kpc for galaxies included in our study.We report that early type fraction for isolated galaxies with $r_p \ge r_{vir,nei}$ is almost ignorant of the background density and has a very weakdensity dependence for closed pairs. Star formation activity of a galaxy is found to be crucially dependent on neighbor galaxy morphology. We find star formation activity parameters and structure parameters of galaxies to be independent of the large-scale background density.We also exhibit that changing the absolute magnitude of the neighbor galaxies does not affect significantly the star formation activity of those target galaxies whose morphology and luminosities are fixed.

• Higher-speed coronal mass ejections and their geoeffectiveness

We have attempted to examine the ability of coronal mass ejections to cause geoeffectiveness. To that end, we have investigated total 571 cases of higher-speed (>1000 km/s) coronal mass ejection events observed during the years 1996–2012. On the basis of angular width (W) of observance, events of coronal mass ejection were further classified as front-side or halo coronal mass ejections (W $=$ 360$^{\circ}$); back-side halo coronal mass ejections (W = 360$^{\circ}$); partial halo (120$^{\circ}$ < W < 360$^{\circ}$) and non-halo (W < 120$^{\circ}$). From further analysis, we found that front halo coronal mass ejections were much faster and more geoeffective in comparison of partial halo and non-halo coronal mass ejections. We also inferred that the front-sided halo coronal mass ejections were 67.1% geoeffective while geoeffectiveness of partial halo coronal mass ejections and non-halo coronal mass ejections were found to be 44.2% and 56.6% respectively. During the same period of observation, 43% ofback-sided CMEs showed geoeffectiveness. We have also investigated some events of coronal mass ejections having speed >2500 km/s as a case study. We have concluded that mere speed of coronal mass ejection and their association with solar flares or solar activity were not mere criterion for producing geoeffectiveness but angular width of coronal mass ejections and their originating position also played a key role.

• A technique to detect periodic and non-periodic ultra-rapid flux time variations with standard radio-astronomical data

We demonstrate that extremely rapid and weak periodic and non-periodic signals can easily be detected by using the autocorrelation of intensity as a function of time. We use standard radio-astronomical observations that have artificial periodic and non-periodic signals generated by the electronics of terrestrial origin. The autocorrelation detects weak signals that have small amplitudes because it averages over long integration times. Another advantage is that it allows a direct visualization of the shape of the signals, while it isdifficult to see the shape with a Fourier transform. Although Fourier transforms can also detect periodic signals,a novelty of this work is that we demonstrate another major advantage of the autocorrelation, that it can detect non-periodic signals while the Fourier transform cannot. Another major novelty of our work is that we use electric fields taken in a standard format with standard instrumentation at a radio observatory and therefore no specialized instrumentation is needed. Because the electric fields are sampled every 15.625 ns, they therefore allow detection of very rapid time variations. Notwithstanding the long integration times, the autocorrelationdetects very rapid intensity variations as a function of time. The autocorrelation could also detect messages from Extraterrestrial Intelligence as non-periodic signals.

• OH megamasers: dense gas & the infrared radiation field

To investigate possible factors related to OH megamaser formation (OH MM, $L_{{\rm H}_2{\rm O}} > 10L_{\odot}$), we compiled a large HCN sample from all well-sampled HCN measurements so far in local galaxies and identifiedwith the OH MM, OH kilomasers ($L_{{\rm H}_2{\rm O}} > 10L_{\odot}$, OH kMs), OH absorbers and OH non-detections (non-OH MM). Through comparative analysis on their infrared emission, CO and HCN luminosities (good tracers for the low-density gas and the dense gas, respectively), we found that OH MM galaxies tend to have stronger HCN emission and no obvious difference on CO luminosity exists between OH MM and non-OH MM. This implies that OH MM formation should be related to the dense molecular gas, instead of the low-density molecular gas. It can be also supported by other facts: (1) OH MMs are confirmed to have higher mean molecular gas density and higher dense gas fraction ($L_{\rm HCN}/L_{\rm CO}$) than non-OH MMs. (2) After taking the distance effect into account, the apparent maser luminosity is still correlated with the HCN luminosity, while no significant correlation can be found at all between the maser luminosity and the CO luminosity. (3) The OH kMs tend to have lower values than those of OH MMs, including the dense gas luminosity and the dense gas fraction. (4) From analysis of known data of another dense gas tracer HCO$^+$, similar results can also be obtained. However, from our analysis,the infrared radiation field can not be ruled out for the OH MM trigger, which was proposed by previous works on one small sample (Darling in ApJ 669:L9, 2007). On the contrary, the infrared radiation field should play one more important role. The dense gas (good tracers of the star formation) and its surrounding dust are heated by the ultra-violet (UV) radiation generated by the star formation and the heating of the high-density gas raises the emission of the molecules. The infrared radiation field produced by the re-radiation of the heated dust inturn serves for the pumping of the OH MM.

• Color–magnitude relations in nearby galaxy clusters

The rest-frame $(g–r) /M_r$ color–magnitude relations of 12 Abell-type clusters are analyzed in the redshift range ($0.02\lesssim z \lesssim 0.10$) and within a projected radius of 0.75 Mpc using photometric data from SDSS-DR9. We show that the color–magnitude relation parameters (slope, zero-point, and scatter) do not exhibit significant evolution within this low-redshift range. Thus, we can say that during the look-back time of $z \sim 0.1$ all red sequence galaxies evolve passively, without any star formation activity.

• Effect of geomagnetic storms on VHF scintillations observed at low latitude

A geomagnetic storm affects the dynamics and composition of the ionosphere and also offers an excellent opportunity to study the plasma dynamics. In the present study, we have used the VHF scintillations data recorded at low latitude Indian station Varanasi (Geomag. latitude $=$ 14$^{\circ}$55$'$N, long. $=$ 154$^{\circ}$E) which is radiated at 250 MHz from geostationary satellite UFO-02 during the period 2011–2012 to investigate the effects of geomagnetic storms on VHF scintillation.Various geomagnetic and solar indices such as Dst index, Kpindex, IMF Bz and solar wind velocity (Vx) are used to describe the geomagnetic field variation observed during geomagnetic storm periods. These indices are very helpful to find out the proper investigation and possible interrelation between geomagnetic storms and observed VHF scintillation. The pre-midnight scintillation is sometimes observed when the main phase of geomagnetic storm corresponds to the pre-midnight period. It is observed that for geomagnetic storms for which the recovery phase starts post-midnight, the probability ofoccurrence of irregularities is enhanced during this time and extends to early morning hours.

• Numerical simulation of inertial alfven waves to study localized structures and spectral index in auroral region

In the present paper, the numerical simulation of Inertial Alfven wave (IAW) in low-$\beta$ plasma applicable to the auroral region at 1700 km was studied. It leads to the formation of localized structures when the nonlinearity arises due to ponderomotive effect and Joule heating. The effect of perturbation and magnitude of pump IAW, formed the localized structures of magnetic field, has been studied. The formed localized structures at different times and average spectral index scaling of power spectrum have been observed. Resultsobtained from simulation reveal that spectrum steepens with power law index $\sim −$3.5 for shorter wavelength. These localized structures could be a source of particle acceleration and heating by pump IAW in low-$\beta$ plasma.

• The gravitational redshift of a optical vortex being different from that of an gravitational redshift plane of an electromagnetic wave

A hypothesis put forward in late 20th century and subsequently substantiated experimentally posited the existence of optical vortices (twisted light). An optical vortex is an electromagnetic wave that in addition to energy and momentum characteristic of flatwaves also possesses angular momentum. In recent yearsoptical vortices have found wide-ranging applications in a number of branches including cosmology. The main hypothesis behind this paper implies that the magnitude of gravitational redshift for an optical vortex will differ from the magnitude of gravitational redshift for flat light waves. To facilitate description of optical vortices, we have developed the mathematical device of gravitational interaction in seven-dimensional time-space that we apply to the theory of electromagnetism. The resulting equations are then used for a comparison of gravitational redshift in optical vortices with that of normal electromagnetic waves. We show that rotating bodies creating weak gravitational fields result in a magnitude of gravitational redshift in optical vortices that differs from themagnitude of gravitational redshift in flat light waves. We conclude our paper with a numerical analysis of the feasibility of detecting the discrepancy in gravitational redshift between optical vortices and flat waves in the gravitational fields of the Earth and the Sun.

• Kelvin–Helmholtz instability of two finite-thickness fluid layers with continuous density and velocity profiles

The effect of density and velocity gradients on the Kelvin–Helmholtz instability (KHI) of two superimposed finite-thickness fluid layers are analytically investigated. The linear normalized frequency and normalized growth rate are presented. Then, their behavior as a function of the density ratio of the lightfluid to the heavy one $(r)$ was analyzed and compared to the case of two semi-infinite fluid layers. The results showed that the values of normalized frequency of KHI for two finite-thickness fluid layers are less than their counterparts for two semi-infinite fluid layers. The behavior of normalized growth rate as a functionof the velocity and density gradients capitulates to the effect of velocity gradient at the large values of $(r)$.

• # Journal of Astrophysics and Astronomy

Current Issue
Volume 40 | Issue 2
April 2019

• # 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.