Editorial
Article ID 1 Research Article
Abdullrahnan Maghrabi Mohammed Alanazi A. Aldosari M. Almuteri
Solar disturbances modulate primary cosmic rays on different time scales. Studying cosmic ray variation is an important subject that attracts scientists from different disciplines. We have constructed and installed (in Riyadh, Saudi Arabia, Rc =14.4 GV) a three-layer small (20 × 20 cm^{2}) MultiWire Chamber (MWC) telescope to study cosmic ray variations and investigate their influence on various atmospheric and environmental processes. Preliminary results obtained from the developed detector are given. The influence of both atmospheric pressure and temperature was studied. Both the temperature and pressure coefficients were calculated and were consistent with those previously obtained. Short-term cosmic ray periodicities, such as the 27-day period, and its two harmonics, have been identified. Sporadic variations caused by some solar activity processes have been inspected. The obtained results from this detector have been compared to the existing 1 m^{2} scintillator detector, as well as to some of the neutron monitors, showing comparable results.
Article ID 2 Research Article
We studied plane symmetric cosmological model in the presence of quark and strange quark matter with the help of ${f(R, T)}$ theory. To decipher solutions of plane symmetric space-time, we used power law relation between scale factor and deceleration parameter. We considered the special law of variation of Hubble’s parameter proposed by Berman (Nuovo Cimento B74, 182, 1983) which yields constant deceleration parameter. We also discussed the physical behavior of the solutions by using some physical parameters.
Article ID 3 Research Article
K. K. Pandey K. M. Hiremath G. Yellaiah
Asymmetry, a well established fact, can be extracted from various solar atmospheric activity indices. Although asymmetry is being localized within short time scale, it also persists at different time scales. In the present study we examine the character and nature of asymmetry at various time scales by optimizing the data set, in units of Carrington Rotations (CRs), for Sunspot Area (SA) and soft X-ray flare index (FI_{SXR}). We find from three solar cycles (21-23) that at a small time scale (viz., daily, CRs and monthly) activity appears to be asymmetric with less significance. At larger time scales (≥01 CRs) strength of asymmetry enhances. Number of significant asymmetry points probably depends upon the solar heights. For different combination of data, asymmetry strength appears to be lowered at certain periods ∼06, ∼12, ∼18 CRs (164, 327 and 492 days i.e., harmonics of ∼1.3 years. Owing to similar behavior of emergence of magnetic flux, it is conjectured that emergence of flux on the surface probably contributes to the asymmetry of the solar activity.
Article ID 4 Research Article
Zahra Bagheri Pantea Davoudifar Gohar Rastegarzadeh Milad Shayan
In this paper, we used CORSIKA code to understand the characteristics of cosmic ray induced showers at extremely high energy as a function of energy, detector distance to shower axis, number, and density of secondary charged particles and the nature particle producing the shower. Based on the standard properties of the atmosphere, lateral and longitudinal development of the shower for photons and electrons has been investigated. Fluorescent light has been collected by the detector for protons, helium, oxygen, silicon, calcium and iron primary cosmic rays in different energies. So we have obtained a number of electrons per unit area, distance to the shower axis, shape function of particles density, percentage of fluorescent light, lateral distribution of energy dissipated in the atmosphere and visual field angle of detector as well as size of the shower image. We have also shown that location of highest percentage of fluorescence light is directly proportional to atomic number of elements. Also we have shown when the distance from shower axis increases and the shape function of particles density decreases severely. At the first stages of development, shower axis distance from detector is high and visual field angle is small; then with shower moving toward the Earth, angle increases. Overall, in higher energies, the fluorescent light method has more efficiency. The paper provides standard calibration lines for high energy showers which can be used to determine the nature of the particles.
Article ID 5 Research Article
Forbidden Zones for Circular Regular Orbits of the Moons in Solar System, R3BP
Previously, we have considered the equations of motion of the three-body problem in a Lagrange form (which means a consideration of relative motions of 3-bodies in regard to each other). Analysing such a system of equations, we considered the case of small-body motion of negligible mass m3 around the second of two giant-bodies $m_1$, $m_2$ (which are rotating around their common centre of masses on Kepler’s trajectories), the mass of which is assumed to be less than the mass of central body. In the current development, we have derived a key parameter $\eta$ that determines the character of quasi-circular motion of the small third body $m_3$ relative to the second body $m_2$ (planet). Namely, by making several approximations in the equations of motion of the three-body problem, such the system could be reduced to the key governing Riccati-type ordinary differential equations. Under assumptions of R3BP (restricted three-body problem), we additionally note that Riccati-type ODEs above should have the invariant form if the key governing (dimensionless) parameter $\eta$ remains in the range $10^{−2}$ ∻ $10^{−3}$. Such an amazing fact let us evaluate the forbidden zones for Moon’s orbits in the inner solar system or the zones of distances (between Moon and Planet) for which the motion of small body could be predicted to be unstable according to basic features of the solutions of Riccati-type.
Article ID 6 Research Article
Short-Term H$\alpha$ Line Variations in Classical Be Stars: 59 Cyg and OT Gem
K. T. Paul S. B. Shruthi Annapurni Subramaniam
We present the optical spectroscopic study of two classical Be stars, 59 Cyg and OT Gem obtained over a period of few months in 2009. We detected a rare triple-peak H$\alpha$ emission phase in 59 Cyg and a rapid decrease in the emission strength of H$\alpha$ in OT Gem, which are used to understand their circumstellar disks. We find that 59 Cyg is likely to be rapid rotator, rotating at a fractional critical rotation of $\sim$ 0.80. The radius of the H$\alpha$ emission region for 59 Cyg is estimated to be \( R_d/R_*\) $\sim$, assuming a Keplerian disk, suggesting that it has a large disk. We classify stars which have shown triple-peaks into two groups and find that the triple-peak emission in 59 Cyg is similar to $\zeta$ Tau. OT Gem is found to have a fractional critical rotation of $\sim$0.30, suggesting that it is either a slow rotator or viewed in low inclination. In OT Gem, we observed a large reduction in the radius of the H$\alpha$ emission region from $\sim$6.9 to $\sim$1.7 in a period of three months, along with the reduction in the emission strength. Our observations suggest that the disk is lost from outside to inside during this disk loss phase in OT Gem.
Article ID 7 Research Article
Benford’s Distribution in Extrasolar World: Do the Exoplanets Follow Benford’s Distribution?
Abhishek Shukla Ankit Kumar Pandey Anirban Pathak
In many real life situations, it is observed that the first digits (i.e., 1,2,…,9) of a numerical data-set, which is expressed using decimal system, do not follow a uniform distribution. In fact, the probability of occurrence of these digits decreases in an almost exponential fashion starting from 30.1 % for 1 to 4.6 % for 9. Specifically, smaller numbers are favoured by nature in accordance with a logarithmic distribution law, which is referred to as Benford’s law. The existence and applicability of this empirical law have been extensively studied by physicists, accountants, computer scientists, mathematicians, statisticians, etc., and it has been observed that a large number of data-sets related to diverse problems follow this distribution. However, except two recent works related to astronomy, applicability of Benford’s law has not been tested for extrasolar objects. Motivated by this fact, this paper investigates the existence of Benford’s distribution in the extrasolar world using Kepler data for exoplanets. The quantitative investigations have revealed the presence of Benford’s distribution in various physical properties of these exoplanets. Further, some specific comments have been made on the possible generalizations of the obtained results, its potential applications in analysing the data-set of candidate exoplanets.
Article ID 8 Research Article
N. Lahkar S. Kalita H. L. Duorah K. Duorah
Yields of nature’s rarest isotopes La^{138} and Ta^{180} are calculated by neutrino processes in the Ne-shell of density $\rho ≈ 10^4 g/cc$ in a type II supernova (SN II) progenitor of mass 20 $M_\odot$. Two extended sets of neutrino temperature $- T_{\nue}$ = 3, 4, 5, 6 MeV and $T_{\nu(\mu/\tau)}$= 4, 6, 8, 10, 12 MeV respectively for charged and neutral current processes are taken. Solar mass fractions of the seeds La^{139}, Ta^{181}, Ba^{138} and Hf^{180} are taken for calculation. They are assumed to be produced in some s-processing events of earlier generation massive ‘seed stars’ with average interior density range $\langleρ\rangle \approx 10^3−10^6 g/cc$. The abundances of these two elements are calculated relative to O^{16} and are found to be sensitive to the neutrino temperature. For neutral current processes with the neutron emission branching ratio, $b_n = 3.81 \times 10^{-4}$ and $b_n = 9.61 \times 10^{−1}$, the relative abundances of La^{138} lie in the ranges $4.48 \times 10^{−14}−2.94 \times 10^{−13}$ and $1.13 \times 10^{−10} − 7.43 \times 10^{−10}$ respectively. Similarly, the relative abundances of Ta^{180} lie in the ranges $1.80 \times 10^{−15} − 1.17 \times 10^{−14}$ and 4.53 \times 10^{−12}−2.96 × 10^{−11} respectively for the lower and higher values of the neutron emission branching ratio. For charged current processes, the relative abundances of La^{138} and Ta^{180} are found to be in the ranges $1.38 \times 10^{−9} − 7.62 \times 10^{−9}$ and $2.09 \times 10^{-11} − 1.10 \times 10^{−10}$ respectively. Parametrized by density of the ‘seed stars’, the yields are found to be consistent with recent supernova simulation results throughout the range of neutrino temperatures. La^{138} and Ta^{180} are found to be efficiently produced in charged current interaction.
Article ID 9 Special Section on "The Ooty Wide Field Array"
Jayaram N. Chengalur Somnath Bharadwaj
Article ID 10 Review Article
C. R. Subrahmanya P. K. Manoharan Jayaram N. Chengalur
We describe here an ongoing upgrade to the legacy Ooty Radio Telescope (ORT). The ORT is a cylindrical parabolic cylinder 530 m × 30 m in size operating at a frequency of 326.5 (or $z \sim 3.35$ for the HI 21-cm line). The telescope has been constructed on a North–South hill slope whose gradient is equal to the latitude of the hill, making it effectively equatorially mounted. The feed consists of an array of 1056 dipoles. The key feature of this upgrade is the digitization and cross-correlation of the signals of every set of 4-dipoles. This converts the ORT into a 264 element interferometer with a field-of-view of $ 2^{\circ} \times 27.4^{\circ} \cos(\delta)$. This upgraded instrument is called the Ooty Wide Field Array (OWFA). This paper briefly describes the salient features of the upgrade, as well as its main science drivers. There are three main science drivers viz. (1) observations of the large scale distribution of HI in the post-reionization era, (2) studies of the propagation of plasma irregularities through the inner heliosphere and (3) blind surveys for transient sources. More details on the upgrade, as well as on the expected science uses can be found in other papers in this special issue.
Article ID 11 Review Article
The Receiver System for the Ooty Wide Field Array
C. R. Subrahmanya P. Prasad B. S. Girish R. Somashekar P. K. Manoharan A. K. Mittal
The legacy Ooty Radio Telescope (ORT) is being reconfigured as a 264-element synthesis telescope, called the Ooty Wide Field Array (OWFA). Its antenna elements are the contiguous 1.92 m sections of the parabolic cylinder. It will operate in a 38-MHz frequency band centred at 326.5 MHz and will be equipped with a digital receiver including a 264-element spectral correlator with a spectral resolution of 48 kHz. OWFA is designed to retain the benefits of equatorial mount, continuous 9-hour tracking ability and large collecting area of the legacy telescope and use of modern digital techniques to enhance the instantaneous field-of-view by more than an order of magnitude. OWFA has unique advantages for contemporary investigations related to large scale structure, transient events and space weather watch. In this paper, we describe the RF subsystems, digitizers and fibre optic communication of OWFA and highlight some specific aspects of the system relevant for the observations planned during the initial operation.
Article ID 12 Review Article
Prowess – A Software Model for the Ooty Wide Field Array
One of the scientific objectives of the Ooty Wide Field Array (OWFA) is to observe the redshifted Hi emission from $z \sim 3.35$. Although predictions spell out optimistic outcomes in reasonable integration times, these studies were based purely on analytical assumptions, without accounting for limiting systematics. A software model for OWFA has been developed with a view to understanding the instrument-induced systematics, by describing a complete software model for the instrument. This model has been implemented through a suite of programs, together called Prowess, which has been conceived with the dual role of an emulator as well as observatory data analysis software. The programming philosophy followed in building Prowess enables a general user to define an own set of functions and add new functionality. This paper describes a co-ordinate system suitable for OWFA in which the baselines are defined. The foregrounds are simulated from their angular power spectra. The visibilities are then computed from the foregrounds. These visibilities are then used for further processing, such as calibration and power spectrum estimation. The package allows for rich visualization features in multiple output formats in an interactive fashion, giving the user an intuitive feel for the data. Prowess has been extensively used for numerical predictions of the foregrounds for the OWFA \HI~ experiment.
Article ID 13 Review Article
Prospects of Detecting HI using Redshifted 21-cm Radiation at $z \sim 3$
Bharat Kumar Gehlot J. S. Bagla
Distribution of cold gas in the post-reionization era provides an important link between distribution of galaxies and the process of star formation. Redshifted $21$-cm radiation from the hyperfine transition of neutral hydrogen allows us to probe the neutral component of cold gas, most of which is to be found in the interstellar medium of galaxies. Existing and upcoming radio telescopes can probe the large scale distribution of neutral hydrogen via HI intensity mapping. In this paper, we use an estimate of the HI power spectrum derived using an ansatz to compute the expected signal from the large scale HI distribution at $z \sim 3$. We find that the scale dependence of bias at small scales makes a significant difference to the expected signal even at large angular scales. We compare the predicted signal strength with the sensitivity of radio telescopes that can observe such radiation and calculate the observation time required for detecting neutral hydrogen at these redshifts. We find that OWFA (Ooty Wide Field Array) offers the best possibility to detect neutral hydrogen at $z \sim 3$ before the SKA (Square Kilometer Array) becomes operational. We find that the OWFA should be able to make a $3 \sigma$ or a more significant detection in $2000$ hours of observations at several angular scales. Calculations done using the Fisher matrix approach indicate that a $5\sigma$ detection of the binned HI power spectrum via measurement of the amplitude of the HI power spectrum is possible in $1000$ h (Sarkar et al. 2017).
Article ID 14 Review Article
Anjan Kumar Sarkar Somnath Bharadwaj Sk. Saiyad Ali
We use the Fisher matrix formalism to predict the prospects of measuring the redshifted 21-cm power spectrum in different $k$-bins using observations with the upcoming Ooty Wide Field Array (OWFA) which will operate at $326.5 {\rm MHZ}$. This corresponds to neutral hydrogen (HI) at $z=3.35$, and a measurement of the 21-cm power spectrum provides a unique method to probe the large-scale structures at this redshift. Our analysis indicates that a $5 \sigma$ detection of the binned power spectrum is possible in the $k$ range $0.05 \leq k \leq 0.3 \, {\rm Mpc}^{-1}$ with $1000$ hours of observation. We find that the signal- to-noise ratio (${\rm SNR}$) peaks in the $k$ range $0.1- 0.2\, {\rm Mpc}^{-1}$ where a $10 \sigma$ detection is possible with $2000$ hours of observations. Our analysis also indicates that it is not very advantageous to observe beyond $1000$ h in a single field-of-view as the (${\rm SNR}$) increases rather slowly beyond this in many of the small $k$-bins. The entire analysis reported here assumes that the foregrounds have been completely removed.
Article ID 15 Review Article
Simulating the z = 3.35 HI 21-cm Visibility Signal for the Ooty Wide Field Array (OWFA)
Suman Chatterjee Somnath Bharadwaj Visweshwar Ram Marthi
The upcoming Ooty Wide Field Array (OWFA) will operate at $326.5 \, {\rm MHz}$ which corresponds to the redshifted 21-cm signal from neutral hydrogen (HI) at z = 3.35. We present two different prescriptions to simulate this signal and calculate the visibilities expected in radio-interferometric observations with OWFA. In the first method we use an input model for the expected 21-cm power spectrum to directly simulate different random realizations of the brightness temperature fluctuations and calculate the visibilities. This method, which models the HI signal entirely as a diffuse radiation, is completely oblivious to the discrete nature of the astrophysical sources which host the HI. While each discrete source subtends an angle that is much smaller than the angular resolution of OWFA, the velocity structure of the HI inside the individual sources is well within the reach of OWFA’s frequency resolution and this is expected to have an impact on the observed HI signal. The second prescription is based on cosmological N-body simulations. Here we identify each simulation particle with a source that hosts the HI, and we have the freedom to implement any desired line profile for the HI emission from the individual sources. Implementing a simple model for the line profile, we have generated several random realizations of the complex visibilities. Correlations between the visibilities measured at different baselines and channels provides an unique method to quantify the statistical properties of the HI signal. We have used this to quantify the results of our simulations, and explore the relation between the expected visibility correlations and the underlying HI power spectrum.
Article ID 16 Review Article
Space Weather and Solar Wind Studies with OWFA
P. K. Manoharan C. R. Subrahmanya J. N. Chengalur
In this paper, we review the results of interplanetary scintillation (IPS) observations made with the legacy system of the Ooty Radio Telescope (ORT) and compare them with the possibilities opened by the upgraded ORT, the Ooty Wide Field Array (OWFA). The stability and the sensitivity of the legacy system of ORT allowed the regular monitoring of IPS on a grid of large number of radio sources and the results of these studies have been useful to understand the physical processes in the heliosphere and space weather events, such as coronal mass ejections, interaction regions and their propagation effects. In the case of OWFA, its wide bandwidth of 38 MHz, the large field-of-view of $\sim$27$^\circ$ and increased sensitivity provide a unique capability for the heliospheric science at 326.5 MHz. IPS observations with the OWFA would allow one to monitor more than 5000 sources per day. This, in turn, will lead to much improved studies of space weather events and solar wind plasma, overcoming the limitations faced with the legacy system. We also highlight some of the specific aspects of the OWFA, potentially relevant for the studies of coronal plasma and its turbulence characteristics.
Article ID 17 Review Article
FRB Event Rate Predictions for the Ooty Wide Field Array
Siddhartha Bhattacharyya Apurba Bera Somnath Bharadwaj N. D. Ramesh Bhat Jayaram N. Chengalur
We developed a generic formalism to estimate the event rate and the redshift distribution of Fast Radio Bursts (FRBs) in our previous publication (Bera et al. 2016), considering FRBs are of an extragalactic origin. In this paper, we present (a) the predicted pulse widths of FRBs by considering two different scattering models, (b) the minimum total energy required to detect events, (c) the redshift distribution and (d) the detection rates of FRBs for the Ooty Wide Field Array (OWFA). The energy spectrum of FRBs is modelled as a power law with an exponent $-\alpha$ and our analysis spans a range $-3\leq \alpha \leq 5$. We find that OWFA will be capable of detecting FRBs with $\alpha\geq 0$. The redshift distribution and the event rates of FRBs are estimated by assuming two different energy distribution functions; a Delta function and a Schechter luminosity function with an exponent $-2\le \gamma \le 2$. We consider an empirical scattering model based on pulsar observations (model I) as well as a theoretical model (model II) expected for the intergalactic medium. The redshift distributions peak at a particular redshift $z_p$ for a fixed value of α, which lie in the range $0.3\leq z_p \leq 1$ for the scattering model I and remain flat and extend up to high redshifts ($z\lesssim 5$) for the scattering model II.
Current Issue
Volume 38 | Issue 2
June 2017
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.