Showing papers in "Astrophysics and Space Science in 2018"

Structure scalars of spherically symmetric dissipative fluids with f ( G , T ) $f(G,T)$ gravity

Abstract: The aim of this study is to analyze the role of modification of gravity on some dynamical properties of spherically symmetric relativistic systems. In this settings, the mathematical modeling of scalar variables associated with the shearing viscous dissipative anisotropic spherical stars is explored. We assume that the non-static diagonally symmetric spherical structure is coupled with a relativistic matter content in the presence of $f(G,T)=\alpha G^{n}+\beta \ln [G]+\lambda T$ gravity. After using Misner–Sharp mass function, we have made correspondence between metric scale factors, tidal forces and structure parameters. We have adopted Herrera’s technique for the orthogonally breaking down of the Riemann tensor, in order to formulate modified forms of structure scalars. The role of these invariants is then explored in the evolutionary properties of radiating spheres. The parameters responsible for the outbreak of inhomogeneities are being examined in the presence and absence of constant $f(G,T)$ terms. It is inferred that the evolutionary phases of the spherical interiors can be well studied via extended versions of scalar variables.

Basins of attraction of equilibrium points in the planar circular restricted five-body problem

Abstract: We numerically explore the Newton-Raphson basins of convergence, related to the libration points (which act as attractors), in the planar circular restricted five-body problem (CR5BP). The evolution of the position and the linear stability of the equilibrium points is determined, as a function of the value of the mass parameter. The attracting regions, on several types of two dimensional planes, are revealed by using the multivariate version of the classical Newton-Raphson iterative method. We perform a systematic investigation in an attempt to understand how the mass parameter affects the geometry as well as the degree of fractality of the basins of attraction. The regions of convergence are also related with the required number of iterations and also with the corresponding probability distributions.

Bianchi-V string cosmological model with dark energy anisotropy

Abstract: The role of anisotropic components on the dark energy and the dynamics of the universe is investigated. An anisotropic dark energy fluid with different pressures along different spatial directions is assumed to incorporate the effect of anisotropy. One dimensional cosmic strings aligned along x-direction supplement some kind of anisotropy. Anisotropy in the dark energy pressure is found to evolve with cosmic expansion at least at late times. At an early phase, the anisotropic effect due to the cosmic strings substantially affect the dynamics of the accelerating universe.

Mathematical modelling of ionospheric TEC from Turkish permanent GNSS Network (TPGN) observables during 2009–2017 and predictability of NeQuick and Kriging models

Abstract: The present study examines the ionospheric Total Electron Content (TEC) variations in the lower mid-latitude Turkish region from the Turkish permanent GNSS network (TPGN) and International GNSS Services (IGS) observations during the years 2009 to 2017. The corresponding vertical TEC (VTEC) predicted by Kriging and NeQuick-2 models are evaluated to realize their efficacy over the country. We studied the diurnal, seasonal and spatial pattern of VTEC variation and tried to estimate by a new mathematical model using the long term of 9 years VTEC data. The diurnal variation of VTEC demonstrates a normal trend with its gradual enhancement from dawn to attain a peak around 09:00–14.00 UT and reaching the minimum level after 22.00 UT. The seasonal behavior of VTEC indicates a strong semi-annual variation of VTEC with maxima in September equinox followed by March equinox and minima in June solstice followed by December solstice. Also, the spatial variation in VTEC depicts a meaningful longitudinal/latitudinal pattern altering with seasons. It decreases longitudinally from the west to the east during March equinox and June solstice increases with latitude. The comparative analysis among the GNSS-VTEC, Kriging, NeQuick and the proposed mathematical model are evaluated with the help one way ANOVA test. The analysis shows that the null hypothesis of the models during storm and quiet days are accepted and suggesting that all models are statistically significantly equivalent from each other. We believe the outcomes from this study would complement towards a relatively better understanding of the lower mid-latitude VTEC variation over the Turkish region and analogous latitudes over the globe.

Isotropic compact stars model in Rastall theory admitting conformal motion

Abstract: We study a new solution for an isotropic compact star model admitting conformal motion in the background of Rastall theory. The conformal Killing vector ( $\mathit{CKV}$ ) and the equation of state (EoS) $p=\omega \rho $ , where $\omega $ satisfying $0<\omega <1$ is the $\mathit{EoS}$ parameter for normal matter distribution, are the main ingredients of our methodology. Several physical aspects of the model has been explored analytically to observe the behavior of compact stars such as $\mathit{TOV}$ equation, energy conditions, Buchdahl condition, stability analysis, compactness and surface redshift. Graphical analysis of the physical parameters have also been presented to support our analytical investigation. We observed that all the physical requirements are fulfilled and the presented model is quite acceptable.

Establishing the Galactic Centre distance using VVV Bulge RR Lyrae variables

Abstract: This study’s objective was to exploit infrared VVV (VISTA Variables in the Via Lactea) photometry for high latitude RRab stars to establish an accurate Galactic Centre distance. RRab candidates were discovered and reaffirmed ( $n=4194$ ) by matching $K_{s}$ photometry with templates via $\chi ^{2}$ minimization, and contaminants were reduced by ensuring targets adhered to a strict period-amplitude ( $\Delta K_{s}$ ) trend and passed the Elorietta et al. classifier. The distance to the Galactic Centre was determined from a high latitude Bulge subsample ( $|b|>4^{\circ}$ , $R_{\mathit{GC}}=8.30 \pm 0.36$ kpc, random uncertainty is relatively negligible), and importantly, the comparatively low color-excess and uncrowded location mitigated uncertainties tied to the extinction law, the magnitude-limited nature of the analysis, and photometric contamination. Circumventing those problems resulted in a key uncertainty being the $M_{K_{s}}$ relation, which was derived using LMC RRab stars ( $M_{K_{s}}=-(2.66\pm 0.06) \log {P}-(1.03\pm 0.06)$ , $(J-K_{s})_{0}=(0.31\pm 0.04) \log {P} + (0.35\pm 0.02)$ , assuming $\mu _{0,\mathit{LMC}}=18.43$ ). The Galactic Centre distance was not corrected for the cone-effect. Lastly, a new distance indicator emerged as brighter overdensities in the period-magnitude-amplitude diagrams analyzed, which arise from blended RRab and red clump stars. Blending may thrust faint extragalactic variables into the range of detectability.

Anisotropic new holographic dark energy model in Saez–Ballester theory of gravitation

Abstract: This work is devoted to the investigation of new holographic dark energy (infrared cutoff is the Hubble radius) in locally rotationally symmetric Bianchi type- $I$ universe within the framework of Saez–Ballester (Phys Lett A 113:467, 1986) scalar–tensor theory of gravitation We construct interacting and non-interacting dark energy models by solving the field equations using a relationship between the metric potentials This leads to a variable deceleration parameter model which exhibits a transition of the universe from deceleration to acceleration We evaluate various cosmological parameters of our models We have observed that the energy density parameters, equation of state and important cosmological planes ( $\omega _{\mathit{de}} - \omega _{\mathit{de}}'$ and $r - s$ ) yield the results compatible with the modern observational data We have, also, discussed the stability analysis of our models

FLRW cosmological models with quark and strange quark matters in f ( R , T ) $f(R,T)$ gravity

Abstract: In this paper, we have studied the magnetized quark matter (QM) and strange quark matter (SQM) distributions in the presence of $f(R,T)$ gravity in the background of Friedmann-Lemaitre-Robertson-Walker (FLRW) metric. To get exact solutions of modified field equations we have used $f(R,T ) = R + 2 f(T)$ model given by Harko et al. with two different parametrization of geometrical parameters i.e. the parametrization of the deceleration parameter $q $ , and the scale factor $a $ in hybrid expansion form. Also, we have obtained Einstein Static Universe (ESU) solutions for QM and SQM distributions in $f(R,T)$ gravity and General Relativity (GR). All models in $f(R,T)$ gravity and GR for FRW and ESU Universes with QM also SQM distributions, we get zero magnetic field. These results agree with the solutions of Aktas and Aygun in $f(R,T)$ gravity. However, we have also discussed the physical consequences of our obtained models.

Classifying GRB 170817A/GW170817 in a Fermi duration–hardness plane

Abstract: GRB 170817A, associated with the LIGO-Virgo GW170817 neutron-star merger event, lacks the short duration and hard spectrum of a Short gamma-ray burst (GRB) expected from long-standing classification models. Correctly identifying the class to which this burst belongs requires comparison with other GRBs detected by the Fermi GBM. The aim of our analysis is to classify Fermi GRBs and to test whether or not GRB 170817A belongs—as suggested—to the Short GRB class. The Fermi GBM catalog provides a large database with many measured variables that can be used to explore gamma-ray burst classification. We use statistical techniques to look for clustering in a sample of 1298 gamma-ray bursts described by duration and spectral hardness. Classification of the detected bursts shows that GRB 170817A most likely belongs to the Intermediate, rather than the Short GRB class. We discuss this result in light of theoretical neutron-star merger models and existing GRB classification schemes. It appears that GRB classification schemes may not yet be linked to appropriate theoretical models, and that theoretical models may not yet adequately account for known GRB class properties. We conclude that GRB 170817A may not fit into a simple phenomenological classification scheme.

The surge-like eruption of a miniature filament associated with circular flare ribbon

Abstract: We present a study of a mini-filament erupting in association with a circular ribbon flare observed by NVST and SDO/AIA on 2014 March 17. The filament was located at one footpoint region of a large loops. The potential field extrapolation shows that it was embedded under a magnetic null point configuration. First, we observed a brightening of the filament at the corresponding EUV images, close to one end of the filament. With time evolution, a circular flare ribbon was observed around the filament at the onset of the eruption, which is regarded as a signature of reconnection at the null point. After the filament activation, its eruption took the form of a surge, which ejected along one end of a large-scale closed coronal loops with a curtain-like shape. We conjecture that the null point reconnection may facilitate the eruption of the filament.

Anisotropic model with decaying cosmological term

Abstract: A spatially homogeneous and anisotropic locally rotationally symmetric Bianchi type-I spacetime with cosmological term $\varLambda $ in $f(R,T) $ theory has been studied. The exact solution of the field equations is obtained under a variation law of the Hubble parameter $(H) $ which yields a time dependent deceleration parameter (Banerjee and Das in Gen. Relativ. Gravit. 37:10, 2005). The model presents a cosmological scenario which describes early deceleration and late time acceleration. The physical parameters of the model have been analysed.

On the photo-gravitational restricted four-body problem with variable mass

Abstract: This paper deals with the photo-gravitational restricted four-body problem (PR4BP) with variable mass. Following the procedure given by Gascheau (C. R. 16:393–394, 1843) and Routh (Proc. Lond. Math. Soc. 6:86–97, 1875), the conditions of linear stability of Lagrange triangle solution in the PR4BP are determined. The three radiating primaries having masses $m_{1}$ , $m_{2}$ and $m_{3}$ in an equilateral triangle with $m_{2}=m_{3}$ will be stable as long as they satisfy the linear stability condition of the Lagrangian triangle solution. We have derived the equations of motion of the mentioned problem and observed that there exist eight libration points for a fixed value of parameters $\gamma (\frac{m \ \text{at time} \ t}{m \ \text{at initial time}}, 0<\gamma\leq1 )$ , $\alpha$ (the proportionality constant in Jeans’ law (Astronomy and Cosmogony, Cambridge University Press, Cambridge, 1928), $0\leq\alpha\leq2.2$ ), the mass parameter $\mu=0.005$ and radiation parameters $q_{i}, (0< q_{i}\leq1, i=1, 2, 3)$ . All the libration points are non-collinear if $q_{2} eq q_{3}$ . It has been observed that the collinear and out-of-plane libration points also exist for $q_{2}=q_{3}$ . In all the cases, each libration point is found to be unstable. Further, zero velocity curves (ZVCs) and Newton–Raphson basins of attraction are also discussed.

Existence of Static Wormhole Solutions in $f(R,G)$ Gravity

Abstract: This work investigates some feasible regions for the existence of traversable wormhole geometries in $f(R,G)$ gravity, where $R$ and $G$ represent the Ricci scalar and the Gauss-Bonnet invariant respectively Three different matter contents anisotropic fluid, isotropic fluid and barotropic fluid have been considered for the analysis Moreover, we split $f(R,G)$ gravity model into Strobinsky like $f(R)$ model and a power law $f(G)$ model to explore wormhole geometries We select red-shift and shape functions which are suitable for the existence of wormhole solutions for the chosen $f(R,G)$ gravity model It has been analyzed with the graphical evolution that the null energy and weak energy conditions for the effective energy-momentum tensor are usually violated for the ordinary matter content However, some small feasible regions for the existence of wormhole solutions have been found where the energy conditions are not violated The overall analysis confirms the existence of the wormhole geometries in $f(R,G)$ gravity under some reasonable circumstances

A model of neutron-star-white-dwarf collision for fast radio bursts

Abstract: Fast radio bursts (FRBs) with unknown origin emit a huge luminosity (about 1 Jy at 1 GHz) with a duration of milliseconds or less at extragalactic distances estimated from their large dispersion measure (DM). We propose herein a scenario for a collision between a neutron star (NS) and a white dwarf (WD) as the progenitor of the FRBs by considering the burst duration scaling to the collision time and the radio luminosity proportional to the kinetic energy of the collision. The relations among the observed flux density, pulse width, and the DM are derived from the model and compared with the statistical results from the observed FRBs. Although the sample is quite small, we tentatively report a nearly inverse-square correlation between the observed peak flux density and the DM excess, which is an consequence of the assumption that the DM excess (i.e. that not due to our Galaxy) is dominated by the intergalactic medium. We also tentatively note a correlation among the duration of the FRB and the DM excess (possibly interpreted as due to the broadening of the signal in the intergalactic medium) and a correlation among the duration of the FRB and the flux density (shorter burst should be brighter), both roughly in agreement with the proposed model.

An interacting new holographic dark energy in the framework of fractal cosmology

Abstract: In this paper, we study an interacting holographic dark energy model in the framework of fractal cosmology. The features of fractal cosmology could pass ultraviolet divergencies and also make a better understanding of the universe in different dimensions. We discuss a fractal FRW universe filled with the dark energy and cold dark matter interacting with each other. It is observed that the Hubble parameter embraces the recent observational range while the deceleration parameter demonstrates an accelerating universe and a behavior similar to $\Lambda \mbox{CDM}$ . Plotting the equation of state shows that it lies in phantom region for interaction mode. We use $\mathit{Om}$ -diagnostic tool and it shows a phantom behavior of dark energy which is a condition of avoiding the formation of black holes. Finally we execute the StateFinder diagnostic pair and all the trajectories for interacting and non-interacting state of the model meet the fixed point $\Lambda \mbox{CDM}$ at the start of the evolution. A behavior similar to Chaplygin gas also can be observed in statefinder plane. We find that new holographic dark energy model (NHDE) in fractal cosmology expressed the consistent behavior with recent observational data and can be considered as a model to avoid the formation of black holes in comparison with the main model of NHDE in the simple FRW universe. It has also been observed that for the interaction term varying with matter density, the model generates asymptotic de-Sitter solution. However, if the interaction term varies with energy density, then the model shows Big-Rip singularity. Using our modified CAMB code, we observed that the interacting model suppresses the CMB spectrum at low multipoles $l<50$ and enhances the acoustic peaks. Based on the observational data sets used in this paper and using Metropolis-Hastings method of MCMC numerical calculation, it seems that the best value with $1\sigma $ and $2\sigma $ confidence interval are $\Omega _{m0}=0.278^{+0.008~+0.010} _{-0.007~-0.009}$ , $H_{0}=69.9^{+0.95~+1.57}_{-0.95~-1.57}$ , $r_{c}=0.08^{+0.02~+0.027}_{-0.002~-0.0027}$ , $\beta =0.496^{+0.005~+0.009} _{-0.005~-0.009}$ , $c= 0.691^{+0.024~+0.039}_{-0.025~-0.037}$ and $b^{2}=0.035$ according to which we find that the proposed model in the presence of interaction is compatible with the recent observational data.

Super resolution for astronomical observations

Abstract: In order to obtain detailed information from multiple telescope observations a general blind super-resolution (SR) reconstruction approach for astronomical images is proposed in this paper. A pixel-reliability-based SR reconstruction algorithm is described and implemented, where the developed process incorporates flat field correction, automatic star searching and centering, iterative star matching, and sub-pixel image registration. Images captured by the 1-m telescope at Yunnan Observatory are used to test the proposed technique. The results of these experiments indicate that, following SR reconstruction, faint stars are more distinct, bright stars have sharper profiles, and the backgrounds have higher details; thus these results benefit from the high-precision star centering and image registration provided by the developed method. Application of the proposed approach not only provides more opportunities for new discoveries from astronomical image sequences, but will also contribute to enhancing the capabilities of most spatial or ground-based telescopes.

Accretion flow dynamics during 1999 outburst of XTE J1859+226—modeling of broadband spectra and constraining the source mass

Abstract: We examine the dynamical behavior of accretion flow around XTE J1859+226 during the 1999 outburst by analyzing the entire outburst data (similar to 166 days) from RXTE Satellite. Towards this, we study the hysteresis behavior in the hardness intensity diagram (HID) based on the broadband (3-150 keV) spectral modeling, spectral signature of jet ejection and the evolution of Quasi-periodic Oscillation (QPO) frequencies using the two-component advective flow model around a black hole. We compute the flow parameters, namely Keplerian accretion rate ( (d)), sub-Keplerian accretion rate ( (h)), shock location (r(s)) and black hole mass (M-bh) from the spectral modeling and study their evolution along the q-diagram. Subsequently, the kinetic jet power is computed as L-jet(obs) similar to 3-6 x 10(37) erg s(-1) during one of the observed radio flares which indicates that jet power corresponds to 8-16% mass outflow rate from the disc. This estimate of mass outflow rate is in close agreement with the change in total accretion rate (similar to 14%) required for spectral modeling before and during the flare. Finally, we provide a mass estimate of the source XTE J1859+226 based on the spectral modeling that lies in the range of 5.2-7.9 M-circle dot with 90% confidence.

Quasi-periodicities in cosmic rays and time lag with the solar activity at a middle latitude neutron monitor: 1982–2017

Abstract: Galactic cosmic rays (GCRs) after entering the heliosphere are continuously modulated by the change in solar wind and the associated heliosphericmagnetic field. GCRs also follow about 11 year sunspot cycle in an inverse way with some time lag of few months to years, which usually varies from cycle to cycle Badruddin et al. 2007; Kane 2014; Aslam and Badruddin 2015 (e.g. Badruddin et al. in Astron. Astrophys. 466(2):697–704, 2007; Kane in Sol. Phys. 289:2669–2675, 2014; Aslam and Badruddin in Sol. Phys. 290:2333, 2015). In this work, we have investigated solar modulation of GCRs measured at three mid cut-off rigidity neutron monitor stations for the period of 1982 to 2017, covering two complete solar cycles and a major part of the recent cycle 24. We study the time lag between GCRs intensity and various solar, geomagnetic and interplanetary parameters separately during the solar cycles under investigation. Further, we have investigated the presence and temporal evolution of mid term quasi-periodicities of GCRs time series during the above mentioned phases utilizing both Morlet wavelet transform and Scargle periodogram technique. Power spectral analysis indicates the presence of several mid term quasi-periodicities including Rieger type and Quasi-biennial oscillations. We discuss the findings and compare them with results of other authors.

MCP detector development for UV space missions

Abstract: We are developing imaging and photon counting UV-MCP detectors, which are sensitive in the wavelength range from far ultraviolet to near ultraviolet. A good quantum efficiency, solar blindness and high spatial resolution is the aim of our development. The sealed detector has a Cs-activated photoactive layer of GaN (or similarly advanced photocathode), which is operated in semitransparent mode on (001)-MgF2. The detector comprises a stack of two long-life MCPs and a coplanar cross strip anode with advanced readout electronics. The main challenge is the flawless growth of the GaN photocathode layer as well as the requirements for the sealing of the detector, to prevent a degradation of the photocathode. We present here the detector concept and the experimental setup, examine in detail the status in the production and describe the current status of the readout electronics development.

Multi constellation GNSS precise point positioning and prediction of propagation errors using singular spectrum analysis

Abstract: The present paper investigates GPS, GLONASS, and Beidou observables from the Multi-GNSS Experiment (MGEX) network of stations in the East Asia region. Precise Point Positioning (PPP) estimates, Dilution of Precision (DOP), Slant Total Electron Content (STEC), and Zenith Total Delay (ZTD), along with their prediction from the Singular Spectrum Analysis (SSA), were studied to analyze and understand their effectiveness on the overall positioning accuracy over the region. The analysis confirms that the PPP accuracy does not solely depend on the DOP values, but is also affected by seasonal changes or partial corrections of the ionospheric and tropospheric delays. The accuracy is improved by approximately 70% for the GPS (G), GPS+GLONASS (GR), and GPS+GLONASS+Beidou (GRC) constellations after applying the SSA method. The GNSS-derived STEC and ZTD values were also predicted with the SSA methods to evaluate the trustworthiness of the approach for mitigation of atmospheric errors. The results show that the STEC and ZTD seem to be in perfect agreement with the SSA model. The technique could be applicable for improved navigational measurements augmenting other available constellations such as the Galileo, IRNSS, and QZSS, as well as satellite-based augmentation systems across the globe.

An efficient computational method for the approximate solution of nonlinear Lane-Emden type equations arising in astrophysics

Abstract: The key purpose of this article is to introduce an efficient computational method for the approximate solution of the homogeneous as well as non-homogeneous nonlinear Lane-Emden type equations. Using proposed computational method given nonlinear equation is converted into a set of nonlinear algebraic equations whose solution gives the approximate solution to the Lane-Emden type equation. Various nonlinear cases of Lane-Emden type equations like standard Lane-Emden equation, the isothermal gas spheres equation and white-dwarf equation are discussed. Results are compared with some well-known numerical methods and it is observed that our results are more accurate.

Relativistic star in higher dimensions with Finch and Skea geometry

Abstract: We study relativistic solutions of compact objects with Finch and Skea (FS) metric of a hydrodynamical stable star in four and in higher dimensions. The solutions obtained in the usual four and in higher dimensions will be employed to construct stellar models. We study variation of different physical parameters inside the star. It is noted that a compact star in 4-dimensions with Finch and Skea geometry is always isotropic here which however is anisotropic if the space time is higher dimensional. The plausibility of such stars are studied here.

Comparison of GPS TEC with modelled values from IRI 2016 and IRI-PLAS over Istanbul, Turkey

Abstract: The Total Electron Content (TEC) prediction performances of the empirical IRI model and IRI-PLAS model were investigated by comparing the GPS-based TEC values provided by the IONOLAB group. TEC values were obtained on equinox (March 21 and September 23) and solstice (June 21, and December 21) days in low (2009), medium (2012) and high (2015) solar activity periods at Istanbul, Turkey. The prediction performances of the models were statistically analyzed based on the differences between the GPS-TEC and the empirical models, considering the maximum and minimum deviations, the correlation analysis and the root mean square error (RMSE). As a result of the investigation, it is seen that the empirical models have similar predictive performances when the plasmaspheric effects are neglected, and the IRI-PLAS estimations are generally a little closer to the observed GPS-TEC values than all options of IRI-2016 model. Also, it can be said that “IRI2001”, one of the IRI-2016’s “topside” options, can make better predictions than other options and “IG” solar proxy option of IRI-PLAS model is a more appropriate option than the others in TEC calculations over Istanbul, Turkey.

Multi-rendezvous low-thrust trajectory optimization using costate transforming and homotopic approach

Abstract: This paper investigates a method for optimizing multi-rendezvous low-thrust trajectories using indirect methods. An efficient technique, labeled costate transforming, is proposed to optimize multiple trajectory legs simultaneously rather than optimizing each trajectory leg individually. Complex inner-point constraints and a large number of free variables are one main challenge in optimizing multi-leg transfers via shooting algorithms. Such a difficulty is reduced by first optimizing each trajectory leg individually. The results may be, next, utilized as an initial guess in the simultaneous optimization of multiple trajectory legs. In this paper, the limitations of similar techniques in previous research is surpassed and a homotopic approach is employed to improve the convergence efficiency of the shooting process in multi-rendezvous low-thrust trajectory optimization. Numerical examples demonstrate that newly introduced techniques are valid and efficient.

Distributions of pseudo-redshifts and durations (observed and intrinsic) of Fermi GRBs

Abstract: Ever since the insightful analysis of the durations of gamma-ray bursts (GRBs) by Kouveliotou et al. (Astrophys. J. Lett. 413:101, 1993), GRBs have most often been classified into two populations: “short bursts” (shorter than 2.0 seconds) and “long bursts” (longer than 2.0 seconds). However, recent works have suggested the existence of an intermediate population in the bursts observed by the Swift satellite. Moreover, some researchers have questioned the universality of the 2.0-second dividing line between short and long bursts: some bursts may be short but actually result from collapsars, the physical mechanism behind normally long bursts, and some long ones may originate from mergers, the usual progenitors of short GRBs. In this work, we focus on GRBs detected by the Fermi satellite (which has a much higher detection rate than Swift and other burst-detecting satellites) and study the distribution of their durations measured in the observer’s reference frame and, for those with known redshifts, in the bursts’ reference frames. However, there are relatively few bursts with measured redshifts, and this makes an accurate study difficult. To overcome this problem, we follow Zhang and Wang (Astrophys. J. 852:1, 2018) and determine a “pseudo-redshift” from the correlation relation between the luminosity $L_{p}$ and the energy $E_{p}$ , both of which are calculated at the peak of the flux. Interestingly, we find that the uncertainties in the quantities observed and used in the determination of pseudo-redshifts, do affect the precision of the individual results significantly, but they keep the distribution of pseudo-redshifts very similar to that of the actual ones and thus allow us to use pseudo-redshifts for our statistical study. We briefly present the advantages and disadvantages of using pseudo-redshifts in this context. We use the reduced chi-square and the maximization of the log-likelihood to statistically analyze the distribution of Fermi GRB durations. Both methods show that the distribution of the observed (measured) and the intrinsic (source/rest frame) bursts durations are better represented by two groups/populations, rather than three.

The Pleiades apex and its kinematical structure

Abstract: A study of cluster characteristics and internal kinematical structure of the middle-aged Pleiades open star cluster is presented. The individual star apexes and various cluster kinematical parameters including the velocity ellipsoid parameters are determined using both Hipparcos and Gaia data. Modern astrometric parameters were taken from the Gaia Data Release 1 (DR1) in combination with the Radial Velocity Experiment Fifth Data Release (DR5). The necessary set of parameters including parallaxes, proper motions and radial velocities are used for $n=17$ stars from Gaia DR1+RAVE DR5 and for $n=19$ stars from the Hipparcos catalog using SIMBAD data base. Single stars are used to improve accuracy by eliminating orbital movements. RAVE DR5 measurements were taken only for the stars with the radial velocity errors not exceeding $2~\mbox{km}/\mbox{s}$ . For the Pleiades stars taken from Gaia, we found mean heliocentric distance as $136.8 \pm 6.4$ pc, and the apex position is calculated as: $A_{CP}=92^{\circ }.52\pm 1^{\circ }.72$ , $D_{CP}=-42^{\circ }.28\pm 2^{\circ }.56$ by the convergent point method and $A_{0}=95^{\circ }.59\pm 2^{\circ }.30$ and $D_{0}=-50^{\circ }.90\pm 2^{\circ }.04$ using AD-diagram method ( $n=17$ in both cases). The results are compared with those obtained historically before the Gaia mission era.

A new probability-one homotopy method for solving minimum-time low-thrust orbital transfer problems

Abstract: Homotopy methods have been widely utilized to solve low-thrust orbital transfer problems, however, it is not guaranteed that the optimal solution can be obtained by the existing homotopy methods. In this paper, a new homotopy method is presented, by which the optimal solution can be found with probability one. Generalized sufficient conditions, which are derived from the parametrized Sard’s theorem, are first developed. A new type of probability-one homotopy formulation, which is custom-designed for solving minimum-time low-thrust trajectory optimization problems and satisfies all these sufficient conditions, is then constructed. By tracking the continuous zero curve initiated by an initial problem with known solution, the optimal solution of the original problem is guaranteed to be solved with probability one. Numerical demonstrations in a three-dimensional time-optimal low-thrust orbital transfer problem with 43 revolutions is presented to illustrate the applications of the method.

Nonlinear kinetic Alfven waves in space plasmas with generalized (r,q) distribution

Abstract: In this paper, we have investigated linear and nonlinear propagation of kinetic Alfven waves in which the electrons have been assumed to follow generalized ( $r,q$ ) distribution. We have shown that ( $r,q$ ) distribution gives us most of the distributions observed in space plasmas. We have varied the flatness parameter $r$ and the tail parameter $q$ to explore the linear and nonlinear propagation characteristics of kinetic Alfven waves. We have also discussed the limiting cases. It has been shown that our results agree well with Fast and Freja observations of the nonlinear kinetic Alfven waves. An important feature of our study is the formation of rarefactive solitary structures. It has been shown that this result cannot be obtained with Maxwellian distribution and that it agrees well with the observations of Fast and Freja satellites.

New holographic dark energy model with constant bulk viscosity in modified $$ gravity theory

Abstract: The aim of this paper is to study new holographic dark energy (HDE) model in modified $f(R,T)$ gravity theory within the framework of a flat Friedmann-Robertson-Walker model with bulk viscous matter content. It is thought that the negative pressure caused by the bulk viscosity can play the role of dark energy component, and drive the accelerating expansion of the universe. This is the motive of this paper to observe such phenomena with bulk viscosity. In the specific model $f(R,T)=R+\lambda T$ , where $R$ is the Ricci scalar, $T$ the trace of the energy-momentum tensor and $\lambda $ is a constant, we find the solution for non-viscous and viscous new HDE models. We analyze new HDE model with constant bulk viscosity, $\zeta =\zeta _{0}= \text{const.}$ to explain the present accelerated expansion of the universe. We classify all possible scenarios (deceleration, acceleration and their transition) with possible positive and negative ranges of $\lambda $ over the constraint on $\zeta _{0}$ to analyze the evolution of the universe. We obtain the solutions of scale factor and deceleration parameter, and discuss the evolution of the universe. We observe the future finite-time singularities of type I and III at a finite time under certain constraints on $\lambda $ . We also investigate the statefinder and $\mathit{Om}$ diagnostics of the viscous new HDE model to discriminate with other existing dark energy models. In late time the viscous new HDE model approaches to $\varLambda \mathit{CDM}$ model. We also discuss the thermodynamics and entropy of the model and find that it satisfies the second law of thermodynamics.

Charged isotropic model with conformal symmetry

Abstract: We investigate the behaviour of a charged isotropic model with conformal symmetry. The relationship between the gravitational potentials arising from the conformal condition is used to generate a new class of exact solutions to the Einstein-Maxwell equations. A specific form of the electric field intensity and the metric potential is required to avoid a singularity at the centre. We can find simple elementary functions for the matter variables and the potentials with realistic profiles. The causality conditions, stability conditions and energy conditions are satisfied. Masses, radii, central densities and surface redshifts are generated, and the values are consistent with the compact stars 4U 1538-52 and PSR J1614-2230.