Showing papers in "Astrophysics and Space Science in 2021"

Charged strange star in f (R ,T ) gravity with linear equation of state

Abstract: Our present study involves the strange stars model in the framework of $f(R,T)$ theory of gravitation. We have taken a linear function of the Ricci scalar $R$ and the trace $T$ of the stress-energy tensor $T_{\mu u}$ for the expression of $f(R,T)$ , i.e., $f(R,T)=R+ 2 \gamma T $ to obtain the proposed model, where $\gamma $ is a coupling constant. Moreover, to solve the hydrostatic equilibrium equations, we consider a linear equation of state between the radial pressure $p_{r}$ and matter density $\rho $ as $p_{r}=\alpha \rho -\beta $ , where $\alpha $ and $\beta $ are some positive constants, Both $\alpha$ , $\beta$ depend on coupling constant $\gamma $ which have been also depicted in this paper. By employing the Krori-Barua ansatz already reported in the literature (J. Phys. A, Math. Gen. 8:508, 1975) we have found the solutions of the field equations in $f (R, T )$ gravity. The effect of coupling constant $\gamma $ have been studied on the model parameters like density, pressures, anisotropic factor, compactness, surface redshift, etc. both numerically and graphically. A suitable range for $\gamma $ is also obtained. The physical acceptability and stability of the stellar system have been tested by different physical tests, e.g., the causality condition, Herrera cracking concept, relativistic adiabatic index, energy conditions, etc. One can regain the solutions in Einstein gravity when $\gamma \rightarrow 0$ .

A model of a three-layered relativistic star

Abstract: We generate new exact solutions to the Einstein-Maxwell field equations for charged anisotropic stellar objects comprising three interior layers. We develop charged stellar models comprising three interior layers with a specified equation of state: the linear quark equation of state at the core layer, the quadratic equation of state at the intermediate layer, and the Chaplygin equation of state at the envelope layer. Earlier uncharged solutions, with different equations of state, are regained as special cases. We plot graphs for the geometrical and matter variables indicating that the matter, gravitational potentials, and other physical conditions are well behaved and consistent with astrophysical studies. A notable feature is that the outer layer satisfies the Chaplygin equation of state.

Secondary component of gravitational-wave signal GW190814 as an anisotropic neutron star

Abstract: The gravitational-wave signal GW190814 involves a compact object with mass $(2.50-2.67)~{\mathrm{M}}_{\odot }$ within the so-called low mass gap. As yet, a general consensus on its nature, being a black hole, a neutron star or an exotic star, has not been achieved. We investigate the possibility this compact object to be an anisotropic neutron star. Anisotropies in a neutron star core arise naturally by effects such as superfluidity, hyperons, strong magnetic fields and allow the maximum mass to exceed that of the ideally isotropic stars. We consider the Krori-Barua ansatz to model an anisotropic core and constrain the equation of state with LIGO/Virgo observations GW170817 and GW190814. We find that the GW190814 secondary component can be an anisotropic neutron star compatible with LIGO/Virgo constraints if the radius attains a value in the range $(13.2-14.0)~{\mbox{km}}$ with the anisotropic core’s boundary density in the range $(3.5-4.0)\cdot 10^{14}~{\mbox{g}}/{\mbox{cm}}^{3}$ .

Ionospheric TEC prediction using Long Short-Term Memory deep learning network

Abstract: In this paper, the prediction model for ionospheric total electron content (TEC) based on Long Short-Term Memory (LSTM) deep learning network and its performance are discussed. The input parameters of the model are previous values of daily TEC, solar radio flux at 10.7 cm parameter of 81 day moving average ( $\overline{F107\_81}$ ), sunspot number (SSN), geomagnetic Kp index, and disturbance storm time (Dst) index, and the outputs are TEC values for the target day. TEC data from January 1, 2001 to December 31, 2016 were used in this study. The dataset almost covers most of the years of the last two solar cycles (23, 24), and it is separated as 81.3% for training, 6.2% for validation, and 12.5% for testing. At BJFS IGS station (39.61° N, 115.89° E), LSTM yielded good TEC estimates with an RMSE of 4.07 TECU in 2001, it was 33% and 48% lower than the RMSE observed in TEC prediction using BP and IRI-2016 models, respectively. In the year of low solar activity (2016), the RMSE predicted by LSTM was 1.78 TECU, it provided 30% and 54% lower RMSE for TEC prediction than for BP and IRI-2016 models. Under the condition of magnetic storm, the LSTM TEC predictions are more consistent with the corresponding IGS Global Ionospheric Maps (GIMs) TEC than TEC predictions by BP and IRI-2016 models. LSTM can better grasp the influence of different external conditions on TEC. Seventeen grid points along 120° E meridian in latitude range from 80° S to 80° N were selected to further study the performance of LSTM model in different latitude. Results show that the prediction accuracy of LSTM is better than that of BP at different latitudes, especially at low latitudes. The performances of the two models are highly correlated with latitude and solar activity, and are both better than that of IRI-2016.

Comparison of VTEC from GPS and IRI-2007, IRI-2012 and IRI-2016 over Sukkur Pakistan

Abstract: The comparative analysis of measured and modelled Vertical Total Electron Content (VTEC) in low, mid and high latitude assists in the correction of Global Positioning System (GPS) measurements. In this study, we compare VTEC from GPS and International Reference Ionosphere (IRI) models (2007, 2012 and 2016) over mid latitude Sukkur (Lat $27.7^{\circ}~\text{N}$ , Long $68.8^{\circ}~\text{E}$ ) region in Pakistan during the final phase of the solar cycle 24 (2019–2020). We investigated the comparison in the form of ionospheric VTEC deviation during diurnal, monthly and seasonal analyses in measured and modelled values over the mid latitude region. The diurnal analysis shows that the measured VTEC in low magnetic activity days’ correlates with IRI-2016 as compared to IRI-2007 and IRI-2012. Whereas, IRI-2016 matches more than IRI-2007 and IRI-2012 in monthly values with minimum deviation. Moreover, the seasonal variations depict maximum variability in the spring season between measured and modelled VTEC over Sukkur. Similarly, maximum correlation ( $R>9.0$ ) is noted between measured and modelled VTEC in low magnetic activity days than high solar activity days, whereas the correlation is $R<9.0$ between ionosphere measured and modelled VTEC during high solar activity days in 2019–2020. All these investigations aid in correcting the ionospheric errors over low latitude Sukkur region.

Charged analogues of isotropic compact stars model with buchdahl metric in general relativity

Abstract: In this work, we examine a spherically symmetric compact body with isotropic pressure profile. In this context we obtain a new class of exact solutions of Einstein-Maxwell field equation for compact stars with uniform charged distributions on the basis of Pseudo-spheroidal space-time with a particular form of electric field intensity and the metric potential $g_{11}$ . Taking these two parameters into account further examination has been done to decide unknown constants and to depict several compact strange star candidates. By the isotropic Tolman-Oppenhimer-Volkoff(TOV) equation, we explore the equilibrium among hydrostatic, gravitational and electric forces. Then, we analyze the stability of the model through adiabatic index( $\gamma $ ) and velocity of sound ( $0<\frac{dp}{c^{2}d\rho }<1$ ). We additionally talk about other physical features of this model e.g. pressure, redshift, density, energy conditions and mass-radius ratio of the stars in detail and demonstrated that our results satisfied all the basic prerequisites of a physically legitimate stellar model, showing density, pressure, pressure-density ratio, redshift and speed of sound are monotonically decreasing. The outcomes acquired are valuable in exploring the strength of other compact objects like white dwarfs, gravastars and neutron stars. Finally, we have shown that the obtained solutions are compatible with observational data for compact objects.

North–south asymmetry in solar activity and Solar Cycle prediction, V: prediction for the north–south asymmetry in the amplitude of Solar Cycle 25

Abstract: There exists a small but statistically significant north–south asymmetry in most of the solar activity indices and it has important implications on the solar dynamo mechanism. Here we analyzed the daily sunspot-group data reported by the Greenwich Photoheliographic Results (GPR) during the period 1874–1976, Debrecen Photoheligraphic Data (DPD) during the period 1977–2017, and the revised Version-2 of international sunspot number (ISSN) during the period 1874–2017. We determined the amplitudes (the largest 13-month smoothed monthly ISSN) of Solar Cycles 12–24 and the 13-month smoothed monthly mean corrected areas of the sunspot groups in the Sun’s whole-sphere (WSGA), northern hemisphere (NSGA), and southern hemisphere (SSGA) at the epochs of the maxima of Solar Cycles 12–24. Using all these we obtained the relations similar to that found in our earlier analyzes–i.e. the existence of a high correlation between the sum of the areas of sunspot groups in the southern-hemisphere near-equatorial band during a small (7–9 months) interval just after a maximum epoch of a solar cycle and the amplitude of next solar cycle–separately for the Sun’s whole-sphere and northern- and southern-hemispheres. By using these relations we predict ≈701 msh (millionth of solar hemisphere), ≈429 msh, and ≈366 msh for the values of WSGA, NSGA, and SSGA, respectively, at the maximum epoch of Solar Cycle 25. We predict $86 \pm 18$ for the amplitude of Solar Cycle 25. The 13-month smoothed monthly mean sunspot-group area highly correlate with that of ISSN. Using this relation and the predicted values of WSGA, NSGA, and SSGA we obtain $68 \pm 11$ for the amplitude of Solar Cycle 25, which is slightly lower than the aforementioned predicted value, and $39 \pm 4$ and $31 \pm 6$ for the values of northern- and southern-hemispheres’ sunspot numbers at the maximum epoch of Solar Cycle 25. The difference between the predicted NSGA and SSGA and also that between northern- and southern-hemispheres’ sunspot numbers at the maximum epoch of Solar Cycle 25 are considerably small. Overall, our results suggest that the amplitude of Solar Cycle 25 would be 25%–40% smaller, and the corresponding north–south asymmetry would be much smaller, than those of Solar Cycle 24.

Unified and bouncing cosmologies with inhomogeneous viscous fluid

Abstract: In this article, we examine the solution for expanding cosmology, which imitates properties similar to an interplay of dark matter and dark energy. The effective fluid of this toy model may behaves like dust matter at early epoch and as a cosmological constant at later epoch. Also, for this solution, the cosmological parameters are calculated and the model is able to explain dynamical universe evolution from decelerated to accelerated phase. Further, we investigate the bouncing solution and it’s relation with finite-time singularities in present model with inhomogeneous viscous fluid.

The Interstellar Medium in young supernova remnants: key to the production of cosmic X-rays and $\gamma$-rays

Abstract: We review recent progress in elucidating the relationship between high-energy radiation and the interstellar medium (ISM) in young supernova remnants (SNRs) with ages of ∼2000 yr, focusing in particular on RX J1713.7−3946 and RCW 86. Both SNRs emit strong nonthermal X-rays and TeV $\gamma $ -rays, and they contain clumpy distributions of interstellar gas that includes both atomic and molecular hydrogen. We find that shock–cloud interactions provide a viable explanation for the spatial correlation between the X-rays and ISM. In these interactions, the supernova shocks hit the typically pc-scale dense cores, generating a highly turbulent velocity field that amplifies the magnetic field up to 0.1–1 mG. This amplification leads to enhanced nonthermal synchrotron emission around the clumps, whereas the cosmic-ray electrons do not penetrate the clumps. Accordingly, the nonthermal X-rays exhibit a spatial distribution similar to that of the ISM on the pc scale, while they are anticorrelated at sub-pc scales. These results predict that hadronic $\gamma $ -rays can be emitted from the dense cores, resulting in a spatial correspondence between the $\gamma $ -rays and the ISM. The current pc-scale resolution of $\gamma $ -ray observations is too low to resolve this correspondence. Future $\gamma $ -ray observations with the Cherenkov Telescope Array will be able to resolve the sub-pc-scale $\gamma $ -ray distribution and provide clues to the origin of these cosmic $\gamma $ -rays.

Dynamical study of nonlinear ion acoustic waves in presence of charged space debris at Low Earth Orbital (LEO) plasma region

Abstract: We consider the nonlinear ion acoustic wave induced by orbiting charged space debris objects in the plasma environment generated at the Low Earth Orbital (LEO) region. The generated nonlinear ion acoustic wave is shown to be governed by a forced Korteweg-de Vries (fKdV) equation with the forcing function dependent on the charged space debris function. For a specific relationship between the forcing debris function and the nonlinear ion acoustic wave, the forced KdV equation turns out to be a completely integrable system; where the debris function obeys a definite nonholonomic constraint. A special exact accelerated soliton solution (velocity of the soliton changes over time whereas its amplitude remains constant) has been derived for the ion acoustic wave for the first time. On the other hand, the amplitude of the solitonic debris function varies with time, and its shape changes during the propagation. Approximate ion acoustic solitary wave solutions with time varying amplitudes and velocities have been derived for different types of weakly localized charged debris functions. Possible applications of the obtained results in space plasma physics are stated along with future directions for research.

Statistical analysis of soft X-ray solar flares during solar cycles 22, 23, and 24

Abstract: This paper presents a statistical analysis of soft X-ray flares during the period January 1986 to June 2017 covering the last three solar cycles (SCs) 22, 23, and 24. We have analyzed the distribution of flare number, the relationship between the occurrence rate of soft X-ray flares and solar activity, and the duration of different intensity class (B, C, M, and X) solar flares during the period of investigation. The total occurrence rates of M and X class flares are 10.33%, 6.77%, and 5.45% in SCs 22, 23, and 24, respectively, which indicates that SC 22 is an SC with frequent large flares. Meanwhile, the M and X class flares are mainly concentrated in solar maximum (71.13%) in SC 22. The correlation coefficients between the annual number of flares and solar F10.7 index, X-ray flux in 0.1–0.8 nm, and EUV flux in 0.1–50 nm in SC 24 (0.91, 0.88, and 0.96) are greater than that in SCs 23 (0.87, 0.85, and 0.86) and 22 (0.88 and 0.81). The median values of the duration of M and X class flares are 39.00 and 71.50 minutes in SC 22, 21.00 and 26.50 minutes in SC 23, and 19.00 and 24.00 minutes in SC 24. The duration of M and X class flares in SC 22 is obviously longer than that in SCs 23 and 24. In addition, the duration of flares increases with the increase of flare intensity.

Large-scale peculiar velocity fields: Newtonian vs relativistic treatment

Abstract: We employ a perturbative analysis to study the evolution of large-scale peculiar velocity fields within the framework of Newtonian gravity and then compare our results to those of the corresponding relativistic treatment. In so doing, we use the same mathematical formalism and apply the same physical approach. This facilitates a direct and transparent comparison between the two treatments. Our study recovers and extends the familiar Newtonian results on the one hand, while on the other it shows that the Newtonian analysis leads to substantially weaker growth-rates for the peculiar velocity field, compared to the relativistic approach. This implies that, by using Newton’s rather than Einstein’s theory, one could seriously underestimate the overall kinematic evolution of cosmological peculiar motions. We are also in the position to identify the reason the two theories arrive at such considerably different results and conclusions.

Redshift of light emitted by particles orbiting a black hole immersed in a strong magnetic field

Abstract: In this paper we analyze the frequency shifts of the light emitted by particles describing stable circular geodesics around a static black hole immersed in an external magnetic field of arbitrary strength. This system is represented by the Ernst solution of the Einstein-Maxwell equations. The presence of the magnetic field and its magnitude affects both the geodesics and the red-blueshifts of the light emitted by neutral or charged particles orbiting the black hole. When the magnetic field is turned off we recover the characteristic redshifts coming from particles orbiting a Schwarzschild black hole.

Wavelength dependent transit depth of HATS-5b: a haze dominant atmosphere?

Abstract: The wavelength-dependent transit depth indicates the atmosphere composition of an exoplanet. We analyze the transit depth of HATS-5b using the data from the Transiting Exoplanet Survey Satellite (TESS) and compare the radius ratio of the planet to the star with the different band result in previous work. We generate a photometric pipeline to obtain the TESS light curve. The fitting of the HATS-5b light curve derives similar bandpass independent parameters compared to the reference work, e.g., the differences in the inclination and the semi-major axis within 1 $\sigma$ . We fix the bandpass-independent parameters to values from the previous work for comparison purposes. The wavelength-dependent $R_{p}/R_{\ast}$ obtained is 2.5% (1.9 $\sigma$ ) smaller compared to the joint band result from the referenced work. The difference of $R_{p}$ / $R_{\ast}$ suggests a haze-dominant model preferred by observation (with $\chi^{2}=1.68$ ) when fitting with different atmospheric models. The opaque featureless atmospheric model has a $\chi^{2}\sim4$ , while the other models are ruled out with $\chi^{2}> 7$ . We also predict an $R_{p}/R_{\ast}$ difference of ∼1 $\%$ , correlating the presence of water at $z$ -band observation. This difference is detectable at 3 $\sigma$ if the photometry precision reaches 500 ppm with 2 minute exposure for one-night observation.

Existence of periodic solutions and their stability for a sextic galactic potential function

Abstract: The periodic solutions for the Hamiltonian function governing the sextic galactic potential function in accordance with two different methods are investigated. The first method is applied using the averaging theory of first order. The sufficient conditions on the parameters for the stability are given and analyzed. The numerical examples of families of periodic orbits are introduced. Meanwhile, the second method is presented using Lyapunov’s theorem for the holomorphic integral, where the periodic solutions depend on the type of the equilibrium points.

Stability analysis in the perturbed CRR3BP finite straight segment model under the effect of viscosity

Abstract: In this paper Robe-finite straight segment model is analyzed under the effects of viscosity and perturbations in the Coriolis and centrifugal forces. We have taken the first primary $P_{1}$ as a rigid spherical shell $m_{1}$ filled with viscous, homogeneous incompressible fluid of density $\rho _{1}$ , and the second primary $P_{2}$ as a finite straight segment of length $2l$ . A third body of mass $m_{3}$ , moving inside $m_{1}$ is a small solid sphere of density $\rho _{3}$ . We prove how the locations of equilibrium points $L_{i}, i=1,2,\ldots ,5$ are affected by the presence of perturbation in the centrifugal force. However, these remain unaffected by the viscosity and perturbation in the Coriolis force. The stability criteria for $L_{1,2}$ are investigated and it has been observed that their stability is affected by the viscosity and perturbation of the centrifugal force. It is prominently observed that the viscosity changes their nature of stability from being stable to asymptotically stable. The equilibrium points $L_{3,4,5}$ are unstable irrespective of the perturbation and viscosity.

The spectral index study for Fermi blazars

Abstract: In this work, we compiled a sample of 1172 4FGL blazars with available spectral information and broadband intensities to the study the spectral indices. Based on this sample, we studied the correlations of spectral indices, and further studied BCU classification by using multi-band spectral indices. Among these sources, there are 196 BCUs, we manage to classify these sources into either FSRQs or BL Lacs through three methods: 1) for the $\alpha _{\mathrm{ro}}$ – $\alpha _{\mathrm{ox}}$ plane, we classified 135 sources into BL Lac candidates; 2) for the $\alpha _{\mathrm{ro}}$ – $\alpha _{\upgamma}$ plane, we classified them into 40 FSRQ candidates and 156 BL Lac candidates; and 3) for the $\alpha _{\upgamma}$ – ${\mathrm{log}} F$ plane, we classified them into 45 FSRQ candidates and 151 BL Lac candidates. From those above results, we see that 36 BCUs were consistently classified as FSRQ candidates by the latter two methods and that 128 BCUs were consistently classified as BL Lac candidates by all the three methods. The completeness is 92.9%, 88.1% and 88.1% for the corresponding result from three methods through comparing with the 4FGL updated catalogue. The correlations of spectral indices were investigated individually for FSRQs and BL Lacs. FSRQs showed very weak anti-correlations for $\alpha _{\mathrm{ro}}$ vs $\alpha _{\upgamma}$ and $\alpha _{\mathrm{ox}}$ vs $\alpha _{\upgamma}$ , positive correlation for $\alpha _{\mathrm{x}\upgamma}$ vs $\alpha _{\upgamma}$ . On the contrary, BL Lacs showed positive correlations for $\alpha _{\mathrm{ro}}$ vs $\alpha _{\upgamma}$ and $\alpha _{\mathrm{ox}}$ vs $\alpha _{\upgamma}$ , anti-correlation for $\alpha _{\mathrm{x}\upgamma}$ vs $\alpha _{\upgamma}$ .

A study of the Czernik 2 and NGC 7654 open clusters using CCD UBV photometric and Gaia EDR3 data

Abstract: We analysed the open clusters Czernik 2 and NGC 7654 using CCD UBV photometric and Gaia Early Data Release 3 (EDR3) photometric and astrometric data. Structural parameters of the two clusters were derived, including the physical sizes of Czernik 2 being $r=5^{\prime}$ and NGC 7654 as $8^{\prime}$ . We calculated membership probabilities of stars based on their proper motion components as released in the Gaia EDR3. To identify member stars of the clusters, we used these membership probabilities taking into account location and the impact of binarity on main-sequence stars. We used membership probabilities higher than $P=0.5$ to identify 28 member stars for Czernik 2 and 369 for NGC 7654. The mean proper motion components ( $\mu _{\alpha}\cos \delta $ , $\mu _{\delta}$ ) of Czernik 2 were derived as ( $-4.03 \pm 0.04$ , $-0.99 \pm 0.05$ ) mas yr−1 and for NGC 7654 as ( $-1.89 \pm 0.03$ , $-1.20 \pm 0.03$ ) mas yr−1. We estimated colour-excesses and metallicities separately using $(U-B) \times (B-V)$ two-colour diagrams to derive homogeneously determined parameters. The derived $E(B-V)$ colour excess is $0.46 \pm 0.02$ mag for Czernik 2 and $0.57 \pm 0.04$ mag for NGC 7654. [Fe/H] metallicities were obtained for the first time for both clusters, $-0.08 \pm 0.02$ dex for Czernik 2 and $-0.05 \pm 0.01$ dex for NGC 7654. Keeping the reddening and metallicity as constant quantities, we fitted PARSEC models using $V\times (B-V)$ and $V\times (U-B)$ colour-magnitude diagrams, resulting in estimated distance moduli and ages of the two clusters. We obtained the distance modulus for Czernik 2 as $12.80 \pm 0.07$ mag and for NGC 7654 as $13.20\pm 0.16$ mag, which coincide with ages of $1.2\pm 0.2$ Gyr and $120\pm 20$ Myr, respectively. The distances to the clusters were calculated using the Gaia EDR3 trigonometric parallaxes and compared with the literature. We found good agreement between the distances obtained in this study and the literature. Present day mass function slopes for both clusters are comparable with the value of Salpeter (1955), being $X=-1.37\pm 0.24$ for Czernik 2 and $X=-1.39\pm 0.19$ for NGC 7654.

The transfers from lunar DROs to Earth orbits via optimization in the four body problem

Abstract: The distant retrograde orbits (DROs) can serve as the parking orbits for a long-term cis-lunar space station. This paper gives a comprehensive study on the transfer problem from DROs to Earth orbits, including low Earth orbits (LEOs), medium Earth orbits (MEOs), and geosynchronous orbits (GSOs), in the bicircular restricted four-body problem (BR4BP) via optimizations within a large solution space. The planar transfer problem is firstly solved by grid search and optimization techniques, and two types of transfer orbits, direct ones and low-energy ones, are both constructed. Then, the nonplanar transfer problem to Earth orbits with inclinations between 0 and 90 degrees are solved via sequential optimizations based on the planar transfers. The transfer characteristics in the cases of different destination orbit inclinations are discussed for both the direct and the low-energy transfer orbits. The important role of the lunar gravity in the low-energy transfers is also discussed, which can overcome the increase of transfer cost caused by the high inclination of Earth orbits. The distinct features of different transfer scenarios, including multiple revolutions around the Earth and Moon, the exterior phase, and the lunar flyby, are discovered. The energy of transfer orbits is exploited to discuss the effects of close lunar flybys. The results will be helpful for the transfer design in future manned or unmanned return missions, and can also provide valuable information for selecting proper parking DROs for cis-lunar space stations.

Predicting the maximum sunspot number and the associated geomagnetic activity indices a a $aa$ and A p $Ap$ for solar cycle 25

Abstract: Over the last three solar cycles, the precursor method has proven to be a very reliable technique for forecasting ensuing solar cycles. In the present work, we have used the geomagnetic activity indices: $aa$ and $Ap$ measured during the last 4 years of the descending phase of solar activity cycles as precursors for predicting the strength of the next solar cycle 25. From the indices $aa$ and $Ap$ , we estimate the maximum sunspot number (SSN) amplitude of the upcoming solar cycle $i$ . $e$ ., solar activity cycle 25 to be $119.42\pm 28.41$ and $113.70\pm 23.51$ , respectively. This suggests that the activity of the next solar cycle 25 is likely to be comparable to that of cycle 24. The results further suggest that this forecast of the maximum SSN will be associated with the values $aa = 16.67 \pm 3.11$ nT and $Ap= 8.78 \pm 2.34$ nT. This indicates that the maximum solar cycle 25 is expected to be accompanied by higher geomagnetic activity levels than in cycle 24, but less than those in the previous cycles 22 and 23. Predicting the level of solar activity is of particular interest because it enhances understanding on establishing a consistent picture of the solar dynamo process and changes in the magnetic field therein; and also helps us in taking decisions having implications in our operational technological capability in space.

Statistics of GPS TEC at the northern EIA crest region of the Indian subcontinent during the solar cycle 24 (2013-2018): comparison with IRI-2016 and IRI-2012 models

Abstract: In this study the statistics of ionospheric total electron content (TEC), derived from a GSV4004B dual-frequency Global Positioning System (GPS) receiver at Agartala station (23.450°N, 91.150°E) located in northern equatorial ionization anomaly (EIA) crest region of the Indian subcontinent, is reported with a performance analysis of IRI-2016 and IRI-2012 models during the ascending, maxima, declining and minima phases (2013-2018) of the solar cycle 24. Variations of model total electron content, as obtained from the IRI-2016 and IRI-2012 for the three options of topside electron density namely NeQuick, IRI 2001 and IRI 01-corr, are compared with the observed total electron content during different periods of interest viz. monthly, seasonal, annual and the correlations with solar activity parameters viz. sunspot number (SSN), 10.7 cm solar radio flux (F10.7), solar EUV flux, are also investigated. All the three options of IRI-2016 and IRI-2012 models show an earlier occurrence of diurnal maximum total electron content, as compared to the observed diurnal maximum GPS total electron content, throughout all the months during the complete period of observation. As the solar activity decreases (from 2015 to 2018), the model starts underestimating GPS total electron content, which becomes significantly high during the very low solar activity period of 2017-18 for all the months. IRI-2016 model underestimates the GPS total electron content before the hours of diurnal maximum and overestimates after the hours of diurnal maximum in the years from 2013-2018. IRI-2012 model underestimates the GPS total electron content before the hours of diurnal maximum and overestimates after the hours of diurnal maximum in the years from 2013-17 but overestimate during the whole day in the year of 2018. Overestimation by IRI-2012 is much more than that by IRI-2016 in the year of 2018. Predictions given by IRI-2016 are better than that given by IRI-2012 for our region. The seasonal mean maximum total electron content values are highest during the spring equinox months and lowest during the winter months except the year of 2014 and 2013. The correlation analysis, between the GPS total electron content and solar indices, show that the correlation coefficient is higher for the solar EUV flux, as compared to the sunspot number (SSN) and 10.7 cm solar radio flux (F10.7).

The deceleration parameter in “tilted” Friedmann universes: Newtonian vs relativistic treatment

Abstract: Although bulk peculiar motions are commonplace in the universe, most theoretical studies either bypass them, or take the viewpoint of the idealised Hubble-flow observers. As a result, the role of these peculiar flows remains largely unaccounted for, despite the fact that relative-motion effects have led to the misinterpretation of the observations in a number of occasions. Here, we examine the implications of large-scale peculiar flows for the interpretation of the deceleration parameter. We compare, in particular, the deceleration parameters measured by the Hubble-flow observers and by their bulk-flow counterparts. In so doing, we use Newtonian theory and general relativity and employ closely analogous theoretical tools, which allows for the direct and transparent comparison of the two studies. We find that the Newtonian relative-motion effects are generally too weak to make a difference between the two measurements. In relativity, however, the deceleration parameters measured in the two frames differ considerably, even at the linear level. This could deceive the unsuspecting observers to a potentially serious misinterpretation of the universe’s global kinematic status. We also discuss the implications and the observational viability of the relativistic study.

Bouncing scenario with causal cosmology

Abstract: We present the cosmological model of a bouncing universe with causal fluid in the universe. The dissipative effects of the causal cosmic fluid is probed with both truncated Israel–Stewart theory and full Israel–Stewart theory. The following classes of non-vanishing scale factors of bouncing cosmology are considered with a viscous fluid: (i) the power-law bouncing model, (ii) the exponential bouncing model and (iii) the matter bouncing model in the Einstein gravity. The cosmological parameters of the Hubble parameter, deceleration parameter, jerk parameter, energy density and bulk viscous pressure of the universe are determined in the theories to obtain realistic cosmologies. The observational constraints of the parameters of bouncing cosmological models are estimated making use of the observed cosmological data. The physical and geometrical aspects of the cosmological models based on the imposed constraints are also discussed.

Analysis of TEC values predicted by OKSM amongst low, mid and high latitude GPS stations during X 9.3 solar flare

Abstract: The primary descriptive quantity of the ionospheric regions of the Earth is the Total Electron Content (TEC). This value is affected by erratic solar events such as solar flares and Coronal Mass Ejections (CME), which affect satellite signal communication. This paper aims to utilize the Ordinary Kriging-based Surrogate model (OKSM) to predict the TEC values for Low, Mid, and High- latitude GPS satellite signals receiving stations across the globe during X 9.3 solar flare that occurred on 6 September 2017. The GPS TEC values that are obtained from IONOLAB are down sampled into steps of 24 values per day for 10 days between 1 September 2017 to 10 September 2017. The relative solar parameters such as Sun Spot Number (SSN), F10.7, Kp, Ap, and Dst are considered for analyzing OKSM response. The effects of the solar flares on TEC values during these days were analyzed and validated with the TEC values from internationally acclaimed models like IRI 2016 and IRI PLAS 2017. It is found that the OKSM predicted better TEC values during extreme solar disturbance conditions when compared with its respective counterparts. The comparative results with the IRI-2016 and IRI PLAS-2017 model indicate that OKSM follows the same TEC. The performance indicator for the proposed model considered as Root Mean Square Error (RMSE) for low latitude Global Positioning System (GPS) station is estimated as 4.60 TECU. Similarly, the RMSE calculated for Mid and High latitude stations is computed as 1.55 and 1.29 TECU. It is evident from the results that OKSM values are closer to the original TEC values and precisely following the response of the solar flare days.

Bifurcation analysis and approximate analytical periodic solution of ER3BP with radiation and albedo effects

Abstract: The nonlinear dynamics of an elliptic restricted three-body system with perturbations such as radiation and albedo are studied in this paper. The effects of different perturbation parameters on the nonlinear dynamic behavior of the third-body are analyzed through the system’s bifurcation diagram. The second- and third-order approximate periodic solutions near the collinear libration points in the plane are obtained using the Lindstedt-Poincare perturbation method.

Radioactive isotopes in the interstellar medium

Abstract: Radioactive components of the interstellar medium provide an entirely-different and new aspect to the studies of the interstellar medium. Injected from sources of nucleosynthesis, unstable nuclei decay along their trajectories. Measurements can occur through characteristic gamma rays that are emitted with the decay, or in cosmic material samples through abundances of parent and daughter isotopes as they change with decay. The dynamics and material flows within interstellar medium are thus accessible to measurement, making use of the intrinsic clock that radioactive decay provides. We describe how measurements of radioactive decay have obtained a break-through in studies of the interstellar medium, after first summarizing the characteristics of radioactivity and the sources of unstable nuclei.

Probing the evolution of the EBL photon density out to z ∼ 1 $z\sim 1$ via γ $\gamma $ -ray propagation measurements with Fermi

Abstract: The redshift ( $z$ ) evolution of the Extragalactic Background Light (EBL) photon density is very important to understand the history of cosmological structure formation of galaxies and stars since the epoch of recombination. The EBL photons with the characteristic spectral energy distribution ranging from ultraviolet/optical to far-infrared provide a major source of opacity of the Universe to the GeV-TeV $\gamma $ -rays travelling over cosmological distances. The effect of the EBL is very significant through $\gamma \gamma \rightarrow e^{-} e^{+}$ absorption process on the propagation of the $\gamma $ -ray photons with energy $E >$ 50 GeV emitted from the sources at $z \sim 1$ . This effect is characterized by the optical depth ( $\tau $ ) which strongly depends on $E$ , $z$ and density of the EBL photons. The proper density of the EBL photons increases with $z$ due to expansion of the Universe whereas evolution of radiation sources contributing to the EBL leads to a decrease in the density with increasing $z$ . Therefore, the resultant volumetric evolution of the EBL photon density is approximated by a modified redshift dependence. In this work, we probe evolution of the EBL photon density predicted by two prominent models using cosmic gamma-ray horizon ( $\tau (E,z)=$ 1) determined by the measurements from the Fermi-Large Area Telescope (LAT) observations. The modified redshift dependence of the EBL photon density is optimized for a given EBL model by estimating the same gamma-ray horizon as predicted by the Fermi-LAT observations. We further compare the optical depth estimates in the energy range $E =$ 4 GeV-1 TeV and redshift range $z =0.01-1$ from the Fermi-LAT observations with the values derived from the two EBL models to further constrain the evolution of the EBL photon density in the $z~\sim 1$ Universe.

Radio-loud and radio-quiet CMEs: solar cycle dependency, influence on cosmic ray intensity, and geo-effectiveness

Abstract: Coronal mass ejections (CMEs) largely influence the space weather and cause geomagnetic perturbations. Hence, the statistical studies pertaining to the occurrence of CMEs over the solar cycles and their consequence at the near-Earth region are extremely important. For an in-depth understanding, such studies need to be carried out considering various observational aspects of CMEs. With this motivation, here we present a statistical study of radio-loud (RL) and radio-quiet (RQ) CMEs during solar cycles 23 and 24. We also assess their geo-effectiveness and analyze their influence on cosmic ray intensity (CRI). The RL and RQ CMEs constitute 40% and 60% cases, respectively, of the total population of CMEs that arrive the near-Earth region at 1 AU. The mean speed of RL CMEs (≈1170 km s−1) was found to be significantly higher (almost twice) than the mean speed of RQ CMEs (≈519 km s−1) in the low corona while their speed became comparable (≈536 km s−1 for RL and ≈452 km s−1 for RQ CMEs) at near-Earth region. The yearly-averaged speeds of Earth-reaching CMEs follow solar cycle variations. The CRI and geomagnetic Dst index are found to have good negative correlation with speed of Earth-reaching CMEs. RL CMEs were found to be more effective in producing CRI depressions and geomagnetic storms (GSs) in comparison to RQ CMEs; in about 70% cases RL CMEs produced CRI depression and GSs earlier than the RQ CMEs. Superposed epoch analysis suggests strongest depression in CRI occurs 2-5 days and 4-9 days after the onset of RL and RQ CMEs, respectively. Further, GS events show a time-lag of 1-5 days and 3-8 days, respectively, with respect to RL and RQ CMEs.

Non-equilibrium ionisation plasmas in the interstellar medium

Abstract: Obtaining astrophysical information from diffuse cool, warm and hot plasmas in interstellar and intergalactic media by electromagnetic radiation is based on highly non-linear heating and cooling processes, which are largely determined by atomic physical time scales and reaction rates. To calculate spectra is further complicated by gas dynamical interactions and processes, such as e.g. shock waves, fast adiabatic expansion and catastrophic cooling. In essence this leads to a non-linear coupling between atomic physics and hydro- or magnetohydrodynamics, which renders radiative cooling to become time- and space-dependent, contrary to the often conveniently used assumption of collisional ionisation equilibrium for optically thin plasmas. Computing power and new algorithms for high performance computing have made it possible to trace the dynamical and thermal evolution of a sufficiently large section of interstellar space over an appreciable time scale to derive characteristic quantities like temperature and density distribution as well as spectra, which can be compared to X-ray, UV and optical observations. In this review we describe diffuse interstellar plasma simulations, the physical processes which drive the temporal and spatial evolution, and present high resolution numerical simulations, including time-dependent cooling, which further our understanding of the state and evolution of interstellar (magnetised) plasmas. We also discuss briefly the role of cosmic rays and their interaction with the plasma.

Optimization of bi-impulsive Earth-Moon transfers using periodic orbits

Abstract: Low-energy bi-impulsive Earth-Moon transfers are investigated by using periodic orbits. Two Earth-Moon transfer design strategies in the CR3BP are proposed, termed direct and indirect design strategy. In the direct design strategy, periodic orbits which approach both vicinities of the Earth and Moon are selected as candidate periodic orbits, which can provide an initial guess of bi-impulsive Earth-Moon transfers. In the indirect design strategy, new bi-impulsive Earth-Moon transfers can be designed by patching together a bi-impulsive Earth-Moon transfer and a candidate periodic orbit which can approach the vicinity of the Moon. Optimizations in the CR3BP are undertaken based on the Gradient Descent method. Finally, bi-impulsive Earth-Moon transfer design and optimizations in the Sun-Earth-Moon bi-circular model (BCM) are carried out, using bi-impulsive Earth-Moon transfers in the CR3BP as initial guesses. Results show that the bi-impulsive Earth-Moon transfer in the CR3BP can serve as a good approximation for the BCM. Moreover, numerical results indicate that the optimal transfers in the BCM have the potential to be of lower cost in terms of velocity impulse than optimal transfers in the CR3BP.