Article ID 1
Relation between solar flares and halo coronal mass ejections
V. KOTESWARA RAO K. RAMA GOPAL R. RAMAKRISHNA REDDY K. AMARESWARI K. SANKARASUBRAMANIAN
Solar flares and coronal mass ejections (CMEs) are two very important active events from Sun. Inspite of several theoretical and statistical analyses, the relation between solar flares and CMEs is so far not well established, and strong opinions and arguments still continue. Statistical approaches use a lot of dataavailable from many measurements by space and ground instruments. They try to map the measured parameters of one event to that of another event and try to establish the relation between them. Halo CMEs are a kind of special CMEs in the sense that they are directed towards Earth and hence can influence Earth’s atmosphere. For a scientist interested in Sun–Earth interactions and the effect on Earth’s atmosphere, study of Halo CMEs is extremely important. In this paper the relation between solar flares and Halo CMEs is studied. The data setsused are for the period from October 2006 to March 2017. For the first time, the Halo CMEs are categorized into four different groups based on the relative time of occurrence with respect to the flares and the relation between the flare and Halo CME parameters is studied. It is shown that: (a) there is a good correlation between certain flare parameters (like flare flux and peak intensity) and CME parameters (like kinetic energy, linear speed, and mass) especially when the Halo CME occurs during the flare; (b) For the same set of CMEs, the correlation is poor with flare duration; and (c) For CMEs before or after the flare, the correlation is lesser than the CMEs occurring during the flare.
Article ID 2
Transit cosmological models with perfect fluid and heat flow in Sáez-Ballester theory of gravitation
UMESH KUMAR SHARMA RASHID ZIA ANIRUDH PRADHAN
In this paper, the Bianchi-V universe has been applied to the transitional universe. Exact solutions of Einstein’s modified field equations in the framework of Sáez-Ballester theory are obtained with heat conduction and perfect fluid. We have applied the hybrid expansion law for the average scale factor $a = kt^{\alpha} e^{\beta t}$, (where $\alpha \geq 0$, $k$ > 0, and $\beta \geq 0$ are constants). This results into a new class of transit models from deceleratinguniverse to the current accelerating universe. The present work also elucidates some of the physical, geometric and kinematic properties of the universe and found them in good agreement with recent observations.
Article ID 3
The dynamic spectral signatures from lunar occultation: A simulation study
JIGISHA V. PATEL AVINASH A. DESHPANDE
Lunar occultation, which occurs when the Moon crosses sight-lines to distant sources, has been studied extensively through apparent intensity pattern resulting from Fresnel diffraction, and has been successfully used to measure angular sizes of extragalactic sources. However, such observations till date havebeen mainly over narrow bandwidth, or averaged over the observing band, and the associated intensity pattern in time has rarely been examined in detail as a function of frequency over a wide band. Here, we revisit the phenomenon of lunar occultation with a view to study the associated intensity pattern as a function of both time and frequency. Through analytical and simulation approach, we examine the variation of intensity across the dynamic spectra, and look for chromatic signatures which could appear as discrete dispersed signal tracks, when the diffraction pattern is adequately smoothed by a finite source size. We particularly explore circumstances in which such diffraction pattern might closely follow the interstellar dispersion law followed by pulsars and transients, such as the Fast Radio Bursts (FRBs), which remain a mystery even after a decade of their discovery. In this paper, we describe details of this investigation, relevant to radio frequencies at which FRBs have been detected, and discuss our findings, along with their implications. We also show how a band-averaged light curve suffers from temporal smearing, and consequent reduction in contrast of intensity variation, with increasing bandwidth. We suggest a way to recover the underlying diffraction signature, as well as the sensitivity improvement commensurate with usage of large bandwidths.
Article ID 4
Causes responsible for intense and severe storms during the declining phase of Solar Cycle 24
KALPANA PATEL ABHA SINGH S. B. SINGH A. K. SINGH
The occurrence of total 113 geomagnetic storms during declining phase of Solar Cycle 24 (2015–2017) subdivided as about 105 moderate storms (${\rm Dst} = −50$ nT to $−$100 nT), 6 intense storms (${\rm Dst} = −100$ nTto $−$200 nT) and 2 severe storms (Dst < $−$200 nT) has been diagnosed on the basis of 5 day active window through the CACTus (Computer aided CME tracking) software. A detailed study has been carried out for the 6intense and 2 severe storms. It is inferred that CMEs are the major source of geomagnetic storms to occur. Out of the 6 intense and 2 severe storms, only 1 has been observed with the origin of CIR. Thus, all analyzed intensegeomagnetic storms are due to coronal mass ejection at the Sun. Most of our results are in good accordance with other reported results.
Article ID 5
JAGADISH SINGH TAJUDEEN OLUWAFEMI AMUDA
This paper examines the linear stability analysis around triangular equilibrium points of a test body in the gravitational field of a low-mass post-AGB binary system, enclosed by circumbinary disc and radiating with effective Poynting–Robertson (P–R) drag force. The equations of motion are derived and positions of triangular equilibrium points are located. These points are determined by; the circumbinary disc, radiation and P–R drag. In particular, for our numerical computations of triangular equilibrium points and the linear stability analysis, we have taken a pulsating star, IRAS 11472-0800 as the first primary, with a young white dwarf star; G29-38 as the second primary. We observe that the disc does not change the positions of the triangular points significantly, except on the y-axis. However, radiation, P–R drag and the mass parameter $\mu$ contributeeffectively in shifting the location of the triangular points. Regarding the stability analysis, it is seen that these points under the combined effects of radiation, P–R drag and the disc, are unstable in the linear sense due to at least a complex root having a positive real part. In order to discern the effects of the parameters on the stability outcome, we consider the range of the mass parameter to be in the region of the Routhonian critical mass (0.038520). It is seen that in the absence of radiation and the presence of the disc, when the mass parameter isless than the critical mass, all the roots are pure imaginary and the triangular point is stable. However, when $\mu = 0.038521$, the four roots are complex, but become pure imaginary quantities when the disc is present. This proofs that the disc is a stabilizing force. On introducing the radiation force, all earlier purely imaginary roots became complex roots in the entire range of the mass parameter. Hence, the component of the radiation force is strongly a destabilizing force and induces instability at the triangular points making it an unstable equilibriumpoint.
Article ID 6
On the unified scheme for high-excitation galaxies and quasars in 3CRR sample
F. C. ODO P. E. OKOBI J. A. ALHASSAN A. A. UBACHUKWU
In this paper, we use the distributions of luminosity ($P$) and radio size ($D$) to re-examine the consistency of the unified scheme of high-excitation radio galaxies and quasars in the recently updated 3CRR sample. Based on a standard cosmology, we derive theoretically and show from observed data, the luminosity limit above which the 3CRR objects are well-sampled. We find, on average, a quasar fraction $\sim$0.44 and galaxy-to-quasar size ratio $\approx$2. Assuming a relativistic outflow of jet materials, we find a mean angle to the line of sight in the range 35$^{\circ} \leq \phi \leq 44^{\circ}$ for the quasars. On supposition of luminosity and orientation-dependent linear size evolution, expressed in a general functional form $D_{\rm (P,z,\phi)} \approx P^{\pm q}(1+z)^{−w} \sin \phi$, we show that above the flux detection threshold of the 3CRR sample, high-excitation galaxies and quasars undergo similar evolutionwith $q = −0.5$; $w = −0.27$ and luminosity independent evolution parameter $x = 2.27$, when orientation effect is accounted for. The results are consistent with orientation-based unified scheme for radio galaxies and quasars.
Article ID 7
Theoretical $Sigma$-$D$ relations for shell-type galactic supernova remnants
YA-PENG HU HONG-AN ZENG JUN FANG JUN-PENG HOU JIAN-WEN XU
Relations between radio surface brightness ($\Sigma$) and diameter ($D$) of supernova remnants (SNRs) are important in astronomy. In this paper, following the work Duric and Seaquist (ApJ 301:308, 1986) at adiabaticphase, we carefully investigate shell-type supernova remnants at radiative phase, and obtain theoretical $\Sigma$-$D$ relation at radiative phase of shell-type supernova remnants at 1 GHz. By using these theoretical $\Sigma$-$D$ relations at adiabatic phase and radiative phase, we also roughly determine phases of some supernova remnant from observation data.
Article ID 8
Stellar models with generalized pressure anisotropy
JEFTA M. SUNZU ALBERTO K. MATHIAS SUNIL D. MAHARAJ
New models for a charged anisotropic star in general relativity are found. We consider a linear equation of state consistent with a strange quark star. In our models a new form of measure of anisotropy is formulated; our choice is a generalization of other pressure anisotropies found in the past by other researchers. Our results generalize quark star models obtained from the Einstein–Maxwell equations.Well-known particular charged models are also regained.We indicate that relativistic stellar masses for several stars are obtained using the general mass function found in our model.
Current Issue
Volume 40 | Issue 1
February 2019
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.
© 2017 Indian Academy of Sciences, Bengaluru.