• Volume 3, Issue 4

      December 1982,   pages  363-497

    • Posterior probability of the deceleration parameter q0 from quasars provided with a luminosity indicator

      T. Kiang F. -H. Cheng

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      We have worked out a ’statistical algorithm’ for obtaining the posterior probability density of the deceleration parameter q0 from quasars where there is a luminosity indicator available. We point out that the role of the luminosity indicator is to provide asecond estimate of individual luminosities after a first estimate has been obtained from measured brightness and redshift together with an assumed q0. Discrimination of q0 is to be sought in the statistical properties of the set of differences between the two estimates (the residuals). We show that the variance of the residuals and their correlation with redshifts (further refined to luminosity distances) are two independent test-statistics for q0, whose known distributions then lead to the probability density sought.

      We have applied the above algorithm to a sample of flat-spectrum radio quasars with measured CIV, MgII and Ly α lines. A combined Baldwin’s relation was used for all 3 lines. Our result is that log q0 is normally distributed with a mean value of + 0.270± 0.135 (s.d.), or, q0= + 1.86 ± 0.135 dex. This result, we believe, is the sharpest result so far published on q0.

    • Morphology of H-alpha filaments and filament channel systems

      V. I. Makarov M. N. Stoyanova K. R. Sivaraman

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      Ring-like filaments have been detected on the spectroheliograms in the H-alpha line. Inside these filaments the magnetic field flux has a predominant polarity. Some of the dark filaments are connected by filament channels which can be seen at the limb either as (a) weak prominences or (b) dense low chromospheric features or (c) multi-channel system of matter flow between two prominences or (d) common quiescent prominences. The filament and the filament channel together form a continuous closed contour and outline the region of thef polarity particularly at the beginning of the solar cycle. The change in sign of the polar field of the Sun is associated with the drift of the filament band to high latitudes.

    • Spectroscopic binaries near the North Galactic Pole paper 6: BD 33° 2206

      R. F. Griffin

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      Photoelectric radial-velocity measurements have confirmed O. C. Wilson’s finding that BD 33° 2206, the secondary star in the wide visual binary ADS 8470, is a spectroscopic binary. It has an eccentric orbit with a period of 100 days. Its γ-velocity is close to the constant radial velocity of the visual primary, confirming the physical association of the stars.

    • On a nonlinear and Lorentz-invariant verson of Newtonian gravitation

      J. J. Rawal J. V. Narlikar

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      The Newtonian theory of gravitation is modified to include the gravitational energy as a source of gravitational potential, thus making the theory self-coupled and nonlinear. The modified theory can be derived from a Lorentz-invariant action principle. The Kepler problem is discussed in this theory and it is shown that the perihelion of the orbit steadily precesses. The rate of precession is, however, insufficient to account for the observed precession of the perihelion of Mercury. The differences from the Newtonian theory for the bending of light and the gravitational redshift of spectral lines are shown to be marginal.

    • Interstellar electron density

      M. Vivekanand R. Narayan

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      We impose the requirement that the spatial distribution of pulsars deduced from their dispersion measures using a model of the galactic electron density (ne) should be consistent with cylindrical symmetry around the galactic centre (assumed to be 10 kpc from the Sun). Using a carefully selected subsample of the pulsars detected by the II Molonglo Survey (II MS), we test a number of simple models and conclude that (i) the effective mean 〈ne〉) for the whole galaxy is 0.037-0.012+0.020 cm-3, (ii) the scale height of electrons is greater than 300 pc and probably about 1 kpc or more, and (iii) there is little evidence for variation of ne with galactic radius RGC for RGc ≳ 5 kpc. Further, we make a detailed analysis of the contribution to ne from H II regions. Combining the results of a number of relatively independent calculations, we propose a model for the galactic electron density of the formne (z) = 0.030 + 0.020 exp (- |z|/70) cm-3 where z(pc) is the height above the galactic plane and the second term describes the contribution from H II regions. We believe the statistical uncertainties in the parameters of this model are quite small.

    • The role of proton cyclotron emission near acereting magnetic neutron stars

      K. M. V. Apparao S. M. Chitre M. J. Rees

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      The process of coherent and incoherent protonn cyyclotron emission occurring near the polar regions of a strongly magnetic accreting neutron star is considered. The soft X-ray flux resulting from the incoherent emission process is calculated for application to Her X-1 and also to gamma ray bursts, a consequence of which is that gamma ray bursts will be accompanied by soft X-ray bursts.

    • Maximum entropy image reconstruction-a practical non-information-theoretic approach

      Rajaram Nityananda Ramesh Narayan

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      The maximum entropy method (MEM) of image reconstructtion is discussed in the context of incomplete Fourier information (as in aperture synthesis). Several current viewpoints on the conceptual foundation of the method are analysed and found to be unsatisfactory. It is concluded that the MEM is a form of model-fitting, the model being a non-linear transform of a band-limited function. A whole family of ’entropies’ can be constructed to give reconstructions which (a) are individually unique, (b) have sharpened peaks and (c) have flattened baselines. The widely discussed 1nB and - B1nB forms of the entropy are particular cases and lead to Lorentzian and Gaussian shaped peaks respectively. However, they hardly exhaust the possibilities-for example, B1/2 is equally good.

      The two essential features of peak sharpening and baseline flattening are shown to depend on a parameter which can be controlled by adding a suitable constant to the zero spacing correlation ρ00. This process, called FLOATing, effectively tames much of the unphysical behaviour noted in earlier studies of the MEM. A numerical scheme for obtaining the MEM reconstruction is described. This incorporates the FLOAT feature and uses the fast Fourier transform (FFT), requiring about a hundred FFTs for convergence. Using a model brightness distribution, the MEM reconstructions obtained for different entropies and different values of the resolution parameter are compared. The results substantiate the theoretically deduced properties of the MEM.

      To allow for noise in the data, the least-squares approach has been widely used. It is shown that this method is biased since it leads to deterministic residuals which do not have a Gaussian distribution. It is suggested that fitting the noisy data exactly has the advantage of being unbiased even though the noise appears in the final map. A comparison of the strengths and weaknesses of the MEM and CLEAN suggests that the MEM already has a useful role to play in image reconstruction.

    • Near infrared observations of some of the IRC sources

      K. V. K. Iyengar F. Strafella D. Lorenzetti C. B. Cosmovici

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      Using an Infrared photometer with InSb photovoltaic detector at the 182cm Copernicus telescope of the Asiago Observatory, Italy, we have measured theJHKL’ M magnitudes of 12 IRC sources 7 of which are very late type stars with [ICIT -K] greater than 5 magnitudes. These data have been fitted to blackbody distributions to obtain their effective temperatures. The present data, in combination with other available photometric data at longer wavelengths seems to indicate excess emission at 11 μm from sources 10066, 10510 and 10234, and at 19.8 μm from source 20052. The source 60098 shows extreme infrared colours.

    • Relativistic beaming in the central components of double radio quasars

      V. K. Kapahi D. J. Saikia

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      Using a large sample of 78 well-observed double quasars, we have investigated several consequences of the relativistic beaming model. In this model the ratio of the strengths of the central component and outer lobes of a double source depends on whether the jet axis lies close to or away from the line of sight, If this is the actual situation, the fraction of emission from the core,fc, may be used as a statistical measure of the orientation of the source and should be correlated with other source parameters which also depend on the inclination of the jet axis to the line of sight.

      We findfc to be anticorrelated with the overall projected linear size of the extended emission but to exhibit a positive correlation with both the observed degree of misalignment from a collinear double structure, and the ratio of separations of the outer hotspots from the central component. As might be expected from these relationships, we also find sources of smaller projected linear sizes to appear more misaligned and the degree of misalignment to be correlated with the ratio of separations of the outer hotspots. All these correlations are consistent with the predictions of the relativistic beaming model.

    • Reflection effect in close binaries. I. Distribution of radiation from a point source

      A. Peraiah

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      The radiation field along an irradiated surface of a component in a binary system is calculated. The source of irradiation is assumed to be a point source. This is done primarily to understand easily how the incident radiation will get changed after it is being scattered by the atmosphere. It is noticed that the maximum radiation comes from intermediate points of the atmosphere, the reason being that here we have the combined radiation due to the star and incident radiation from the point source outside the star although both are diluted.

  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

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