013831   Singh T P (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Gravitational collapse, black holes and naked singularities. J Astrophys Astr 1999, 20(3-4), 221-32.

Elementary review of gravitational collapse and the cosmic censorship hypothesis has been discussed. Known models of collapse resulting in the formation of black holes and naked singularities are summarized. These models, when taken together, suggest that the censorship hypothesis may not hold in classical general relatively. The nature of the quantum processes that take place near a naked singularity, and their possible implication for observations, is briefly discussed.

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013830   Shahbaz T (Department of Astrophysics, University of Oxford, Nuclear Physics Building, Keble Road, Oxfor, OXI 3RH, England) : Observational evidence for stellar mass black holes. J Astrophys Astr 1999, 20(3-4), 197-210.

Evidence for stellar mass black holes in the Galaxy has been studied. The unique properties of the soft X-ray transient (SXTs) have provided the first opportunity for detailed studies of the mass-losing star in low-mass X-ray binaries. The large mass functions of these systems imply that the compact object has a mass greater than the maximum mass of a neutron star, strengthening the case that they contain black holes. The results and techniques used are discussed. Study of a comparison of the luminosities of black hole and neutron star systems which has yielded compelling evidence for the existence of event horizons is made.

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013829   Sathyaprakash B S (NO, Cardiff University, 5, The Parade, Cardiff, CF2 3YB, U K) : Gravitational waves: the future of black hole physics. J Astrophys Astr 1999, 20(3-4), 211-20.

New millennium will witness the operation of several long-baseline ground-based interferometric detectors, possibly a space-based detector too, which will make it possible to directly observe black holes by catching gravitational waves emitted by them during their formation or when they are perturbed or when a binary consisting of black holes in-spirals due to radiation reaction. Such observations will help us not only to test some of the fundamental predictions of Einstein's general relatively but will also give us the unique opportunity to map black hole spacetimes, to measure and spins of black holes and their population.

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013828   Sasaki M (Department of Earth and Space Science, Osaka University, Toyonaka 560 0043, Japan) : Analytic black hole perturbation approach. J Astrophys Astr 1999, 20(3-4), 281-9.

An analytic method for calculating gravitational radiation from a small mass particle orbiting a massive black hole are studied. This method allows a systematic evaluation of the gravitational radiation to a very high order in post-Newtonian expansion, hence given useful information on the evolution of coalescing compact binary stars.

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013827   Rao M;Sengupta S (NO, Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore-560 080) : Arrested states of solids. Curr Sci 1999, 77(3), 382-7.

Solids produced as a result of fast quench across a freezing or a structural transition get stuck in long-lived metastable cofigurations of distinct morphology, sensitively dependent on the processing history. Martensites are particularly well-studied examples of nonequilibrium solid-solid transformations.

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013826   Penrose R (Department of Mathematics, University of Oxford, 24-29, St. Giles, Oxford OXI 3BD, UK) : Question of cosmic censorship. J Astrophys Astr 1999, 20(3-4), 233-48.

Cosmic censorship is discussed in its various facets. It is concluded that rather little clear-cut progress has been made to date and that the question is still very much open.

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013825   Nicolai H (NO, Max-Plack-Institut, fur Gravitationsphysik, Albert-Einstein-Institut, Am Muhlenberg 1, D-14476 Golm, Germany) : On M-theory. J Astrophys Astr 1999, 20(3-4), 149-64.

Contributions gives a personal view on recent attempts to find a unified framework for non-perturbative string theories, with special emphasis on the hidden symmetries of supergravity and their possible role in this endeavor. A reformulation of d = 11 supergravity with enlarged tangent space symmetry SO(1,2) x SO<16) is discussed from this perspective, as well as ansatz to construct yet further version with SO(1,4) x SO(16) and possibly even SO(1,1)+ x ISO(16)θ tangent space symmetry. It is suggested that upon "third quantization". dimensionally reduced maximal supergravity may have an equally important role to play in this unification as the dimensionally reduced maximally super-symmetric SU (θ) Yang Mills theory.

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013824   Moran J M;Greenhill L J (NO, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge Mass 02138, USA) : Observational evidence for massive black holes in the centers of active galaxies. J Astrophys Astr 1999, 20(3-4), 165-85.

Naturally occurring water vapor maser emission at 1.35 cm wavelength provides an accurate probe for the study of accretion disks around highly compact objects, thought to be black holes, in the centers of active galaxies. Because of the exceptionally fine angular resolution, 200 microarcseconds, obtainable with very long baseline interferometry, accompanied by high spectral resolution, <60>0.1 kms^-1, the dynamics and structures of these disks can be probed with exceptional clarity. The data on the galaxy NGC 4258 are discussed. The mass of the black hole binding the accretion disk is 3.9 x 10⁷M. Although the accretion disk has a rotational period of about 800 years, the physical motions of the masers have been directly measured with VLBI over a period of a few years. These measurements also allow the distance from the earth to the black hole to be estimated to an accuracy of 4 per cent. The status of the search for other maser/black hole candidates is also discussed.

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013823   Eckart A;Genzel R (NO, Max-Planck Institut fur extraterrestrische Physik, D 85740 Garching, Postfach 1603, Germany) : Massive black hole at the galactic center. J Astrophys Astr 1999, 20(3-4), 187-96.

Dynamic center of the Milky Way high spatial resolution, near-infrared imaging and spectroscopy have made it possible in the last few years to measure stellar velocities down to separations of less than five light days from the compact radio source SgrA^* (in the constellation Sagittarius). These measurements make a compelling case for the presence of a compact, central dark mass of 2.6 x 10⁶ solar masses. Simple physical considerations show that this dark mass cannot consist of a stable cluster of stars, stellar remnants substellar condensations or a degenerate gas of elementary particles. Energy equipartition requires that at least 10⁵ solar masses must be associated with SgrA^* itself and is enclosed within less than 8 light minutes (equivalent to 15 Schwarzschild radii of a million solar mass black hole). If one accepts these arguments it is hard to escape the conclusions that there must be a massive black hole at the core of the Milky Way.

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013822   Clarke C J S (Faculty of Mathematics Studies, University of Southampton, Southampton, SO17, 1BJ, UK) : New mathematical approaches is classical censorship problems. J Astrophys Astr 1999, 20(3-4), 249-57.

Recent advances in mathematical techniques for extending space-times through weak singularities have been discussed. This gives one more hope of proving a cosmic censorship theorem, and hence of understanding whether the final state of gravitational collapse will indeed be a black hole.

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013821   Andersson N (Department of Mathematics, University of Southampton, Southampton SO17 1BJ, UK) : Black hole dynamics : a survey of black hole physics from the point of view of perturbation theory. J Astrophys Astr 1999, 20(3-4), 269-80.

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013820   Alexeyev S O;Sazhin M V (NO, 20 Sternberg Astronomical Institute, Moscow State University, Universitetskii Prospect, 13, Moscow 119899) : Some aspects of four-dimensional black hole solutions in Gauss-Bonnet extended string gravity. J Astrophys Astr 1999, 20(3-4), 259-68.

Internal singularity of a string four-dimensional black hole with second order curvature corrections is investigated. A restriction to a minimal size of a neutral black hole is obtained in the frame of the model considered. Vacuum polarization of the surrounding space-time caused by this minimal-size black hole is also discussed.

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012743   Suleymanova S A;Izvekova V A;Rankin J M; Rathnasree N (Pushchino Radio Astronomy Observatory, P.N. Lebedev Physical Institute, Russian Academy of Science, Leninski Prospect, 53 Moscow 117 924) : Individual and integrated pulse properties of PSR B0943+10 involved in the mode-changing phenomenon. J Astrophys Astr 1998, 19(1-2), 1-18.

Characteristics of the "burst" (B) mode and "quiescent" (Q) mode pulse sequences - long known from studies at or below 103 MHz - are identified at 430 MHz for the first time. An 18-minute, polarimetric observation begins with a long B-mode sequence, which has a higher average intensity, regular drifting subpulses, and a preponderance of primary polarisation-mode radiation. An abrupt transition to a Q-mode sequence is then marked by a) weaker average intensity, but occasional very bright individual subpulses, b) a complete cessation of dirfting subpulses, with disorganized subpulses now being emitted over a much wider longitude interval, and c) near parity between the primary and secondary polarisation modes, resulting in pronounced depolarisation, both of individual pulses and the average profile. Careful study, however, of profile changes before and after this mode change reveals slower variations which both anticipate the abrupt transition and respond to it. A slow attenuation of the intensity level of the dominant component is observed throughout the duration of the B-mode sequence, which then accelerates with the onset of the Q-mode sequence. This slow variation appears to represent a "pre-switching transition" process; and the combination of effects on slow and abrupt time scales are finally responsible for the formation of the characteristic B- and Q-mode average profiles.

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012742   Sivaram C (NO, Indian Institute of Astrophysics, Bangalore-560 034) : Constraints on the photon mass and charge and test of equivalence principle from GRB 990123. Bull Astr Soc India 1999, 27(4), 627-30.

Constraints on the mass and charge of a photon as well as on the parameters relevant to quantum gravity therories are put based on arrival times of radiation at different wavelengths from GRB 990123.

012741   Sivaram C (NO, Indian Institute of Astrophysics, Bangalore-560 034) : Non-anthropic origin or a small cosmological constant. Bull Astr Soc India 1999, 27(3), 377-82.

Impressive variety of recent observations which include luminosity evolutions of high red shift supernovae strongly suggest that the cosmological constant (Λ) is not zero. Even though the Λ-term may dominate cosmic dynamics at the present epoch, such a value for the vacuum energy is actually unnaturally small. The difficulties in finding a suitable explanation (based on fundamental physics) for such a small residual value for the cosmological term has led several authors to resort to an anthropic explanation for its existence. A few examples some based on phase transitions in the early universe involving strong or electro weak interactions and other on gravitational spin interactions to show how the cosmical term of the correct observed magnitude can arise from fundamental physics involving gravity has been presented.

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012740   Saha S K (NO, Indian Institute of Astrophysics, Bangalore-560 034) : Emerging trends of optical interferometry in astronomy. Bull Astr Soc India 1999, 27(4), 443-548.

An elaborate account of the random refractive index fluctuations of the atmosphere producing random aberrations in the telescope pupil, elucidating the trade offs between log-exposure and short-exposure imaging is given. The formation of speckles and of ways to detect them in the case of astronomical objects are discussed. Further, the other methods viz., (i) speckle spectroscopy, (ii) speckle polarimetry, (iii) phase closure, (iv) aperture synthesis, (v) pupil plane interferometry, (vi) differential speckle interferometry etc., using single moderate or large telescopes are described as well. The salient features of various detectors that are used for recording short-exposure images are summarized. The mathematical intricacies of the data processing techniques for both Fourier modulus and Fourier phase are analysed; the various schemes of image restoration techniques are examined as well with emphasis set on their comparisons. The recent technological innovation to compensate the deleterious effects of the atmosphere on the telescope image in real-time is enumerated. The experimental descriptions of several working long baseline interferometers in the visible band using two or more telescopes are summarized. The astrophysical results obtained till date using both single aperture interferometry, as well as long baseline interferometry with diluted apertures are highlighted.

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012739   Raman K S;Aleem P S M;Thiagarajan R (NO, Indian Institute of Astrophysics, Kodaikanal-624 103) : Evershed velocities in bipolar sunspots. Bull Astr Soc India 1999, 27(3), 383-9.

Spectra taken at the solar telescope-spectrograph, Kodaikanal, are analysed to find the sight-line velocity values in the regions between the opposite polarity sunspots. Velocities in opposite directions in the range 1 - 2.5 km per second are obtained in the cases of δ- type sunspots (spots having common penumbra), whereas opposite polarity sunspots having separate structures do not show any appreciable velocity in the regions between them. The velocities in the sunspots are restricted to the penumbral boundaries.

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012738   Prasad C D;Ambastha A;Mathew S K (NO, Udaipur Solar Observatory, Physical Research Laboratory, Udaipur-313 001) : Dynamical properties of quiescent prominence in He D3 5876 angstrom line emission. Bull Astr Soc India 1999, 27(3), 411-4.

Quiescent prominence with the Coude spectrograph of Udaipur Solar Observatory during May 24-25, 1995 have been observed. Analysis of two dimensinal spectroscopic data was carried out in order to obtain the line shifts and line widths of He D3 5876 Angstrom emission line. These quantities are used to study the dynamical properties of the observed prominence. The line-of-sight velocity distribution over the prominence follows a Gaussian distribution with a half width of ± 4.2 km s^-1. However the line width over the prominence does not show a well defined distribution as does the line shift. Most of the line profiles have the halfwidth at half-maximum (HWHM) ranging from 180 m Angstrom to 280 mAngstrom. Assuming a typical prominence temperature of 8000K, authors find turbulence velocity of around 4 km s^-1 within the prominence. In this paper authors present detailed results of the observations and discuss their implications for the structure and dynamics of the prominence.

012737   Paul B;Agrawal P C;Rao A R (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Low frequency quasi-periodic oscillations in the hard X-ray emission from cygnus X-1. Bull Indian Vacuum Soc 1999, 2(1), 55-61.

The observations of the black hole binary Cygnus X-1 were made in the energy band of 20-100 ke V with a balloon-borne Xenon-filled multiwire proportional counter telescope on 5th April 1992. Timing analysis of the data revealed the presence of Quasi-Periodic Oscillations (POP) in the hard X-ray emission from the source. The QPO feature in the power density spectrum is broad with a peak at a frequency of 0.06 Hz. This result is compared with similar reports of QPOs in Cyg X-1 in soft and hard X-rays. Short time scale random intensity variations in the X-ray light curve are described with a shot noise model.

012736   Nilakshi;Yadav R K S;Mohan V;Pandey A K;Sagar R (NO, U P State Observatory, Manora Peak, Nainital-263 129) : BVRI CCD photometric standards in the field of GRB 990123. Bull Astr Soc India 1999, 27(3), 405-10.

The CCD magnitudes in Johnson BV and Cousins RI photometric passbands are determined for 18 stars in the field of GRB 990123. These measurements can be used in carrying out precise CCD photometry of the optical transient of GRB 990123 using differential photometric techniques during non-photometric sky conditions. A comparison with previous photometry indicates that the present photometry is more precise.

012735   Mookerjea B;Ghosh S K (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Infrared diagnostics for compact H II regions. II. Bull Astr Soc India 1999, 27(4), 583-600.

Study has been divided into two categories : either all the model clouds are constrained to have (i) a fixed total mass; or (ii) a fixed physical size. Presents the second category of models, where (a) the study has been extended by including high resolution spectroscpic diagnostics; and (b) a wider range of radial density distribution laws have been considered. Three types of embedded stars, viz., ZAMS 04, 07 and B0.5 have been considered and a wide range of optical depth has been covered. All relevant model predictions have been quantified in terms of observables with different instruments onboard the Infrared Space Observatory (ISO) for direct comparability. Study has quantified the importance of high resolution mid and far infrared spectroscopy in determining the elemental abundances, embedded stellar type, temperature and density distribution and geometrical details of compact H II regions.

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012734   Mookerjea B;Ghosh S K (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Infrared diagnostics for compact H II regions. I. Bull Astr Soc India 1999, 27(4), 567-82.

Radiation transfer calculations through the gas and the dust components have been carried out to predict the emergent spectral energy distribution (SED) and directly observables like : infrared photometric colours, angular sizes and radio continuum emission. Many of these predictions have been quantified in such a way that, they can be directly compared with the data from the instruments onboard the Infrared Space Observatory (ISO). In general mid-IR photometric data are found to be diagnostically much more powerful compared to the far-IR measurements. The capability of different far and mid infrared colours to extract information about the embedded stellar type, radial density distribution, and the total optical depth have been demonstrated. Work has been followed up with another study, in which, the constraints on the interstellar cloud are more relaxed. Additionally, the gas component has been considered in much greater details so as to include predictions about fine structure line emission of several heavy elements (Mookerjea and Ghosh, 1999, Paper II), observable using ISO.

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012733   Mohan V;Uddin W;Ram Sagar; (U.P. State Observatory, , Manora Peak, Naini Tal-263 129) : Atmospheric extinction at Devasthal, Naini Tal. Bull Astr Soc India 1999, 27(4), 601-8.

Atmospheric extinction coefficients in Johnson U, B, V, R and I photometric passbands are determined for the first time at Devasthal, Naini Tal during 1998 and 1999. A solid state stellar potometer mounted on a 52-cm reflector was used for the measurements. The minimum atmospheric extinction was observed on Jan 21, 1998 with the values of coefficients 0.38, 0.22, 0.12 and 0.06 mag in U, B, V and R respectively. However, mean values of the coefficients during the observations are 0.49±0.09, 0.32±0.06, 0.21±0.05, 0.13±0.04 and 0.08±0.04 mag in U, B, V, R and I respectively. A comparison of the atmospheric extinction observed at Devasthal with those observed at other optical astronomical sites in India as well as abroad indicates that Earth's atmosphere at Devasthal site is fairly transparent and is suitable for precise photometric observations. The observed extinction values also agree fairly well with those expected due to molecular absorption, Rayleigh and aerosol scatterings by molecules and dust particles present in the Earth's atmosphere at an altitude of ~ 2.45 km. Variation in aerosol content of the Earth's atmosphere seems to produce the observed night to night variation in extinction at Devasthal.

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012732   Krishan V;Chitre S M (NO, Indian Institute of Astrophysics, Bangalore-560 034) : Generation of kinetic helicity from irrotational motions. Bull Astr Soc India 1999, 27(3), 391-403.

Vortical and helical fluctuations are an integral part of any turbulent fluid. A turbulent magnetohydrodynamic system with net kinetic helicity is known to support the growth of a large scale magnetic field through the dynamo mechanism. An analogous effect, called the kinetic α-effect has been shown to give rise to large scale structures in a turbulent hydrodynamic system. Thus, it appears that the mechanism for formation of large scale structures, magnetic or hydrodynamic, hinges on the presence of kinetic helicity, in an essential manner. It has been demonstrated that systems possessing nonzero net kinetic helicity and those with zero net kinetic helicity, but nonzero mean square helicity, can support the formation of large scale structures via the mechanism of inverse cascade of energy. The question, then arises, "what are the mechanisms for the production of small scale vortical and helical fluctuations, especially in astrophysical situations"? Authors examine two mechanisms for the generation of vorticity and helicity from longitudinal velocity fields, like those associated with, for example, acoustic waves, which by themselves are irrotational. Authors assume the existence of finite amplitude acoustic waves: these could be generated in a highly compressible medium such as obtains in astrophysical situations. Authors discuss growth of vorticity field through a mechanism, known as the Langmuir circulation, in which the ambient shear in the fluid couples with the longitudinal waves to produce vorticity and helicity and the conversion of part of the energy contained in a longitudinal velocity field into vortical and helical velocity fields when it propagates through a turbulent medium; this latter mechanism may be seen as the inverse of the Lighthill mechanism where part of the turbulent energy is converted into sound waves.

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012731   Jagadish Singh;Bhola Ishwar (Department of Mathematics, B. R. A. Bihar University, Muzaffarpur-842 001) : Stability of triangular points in the generalised photogravitational restricted three body problem. Bull Astr Soc India 1999, 27(3), 415-24.

Reports the effect of oblateness and radiation pressure forces of the primaries on the location and the stability of the triangular points in the restricted three body problem. Authors observe that the equations of motion and locations of the triangular points are affected by the radiation pressure forces and oblateness of the primaries. It is further seen that these points are stable for 0 ≤ μ <60> μ_C, and unstable for μ_C ≤ μ ≤ 1/2. It is also seen that for these points the range of stability increases or decreases according to p <62> or <60> 0 where p depends upon the radiating and oblateness coefficients.

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012730   Golap K;Udaya Shankar N U;Sachdev S;Dodson R; Sastry C V (NO, University of Mauritius, Reduit, Mauritius) : Low frequency radio telescope at Mauritius for a southern sky survey. Bull Indian Vacuum Soc 1999, 2(1), 35-53.

New, meter-wave radio telescope has been built in the north-east of Mauritius, an island in the Indian Ocean, at a latitude of -20.14°. The Mauritius Radio Telescope (MRT) is a Fourier Synthesis T-Shaped array, consisting of a 2048 m long East-West arm and an 880 m long South arm. In the East-West arm 1024 fixed helices are arranged in 32 groups and in the South arm 16 trolleys, with four helices on each, which move on a rail are used. A 512-channel digital complex correlation receiver is used to measure the visibility function. At least 60 days of observing are required for obtaining the visibilities up to 880 m spacing. The Fourier transform of the calibrated visibilities produces a map of the area of the sky under observation with a synthesized beam width 4' x 4.6' sec (δ + 20.14°) at 151.5 MHz. The Primary objective of the telescope is to produce a sky survey in the declination range - 70° to -10° with a point source sensitivity of about 200 mJy (3σ level). This will be the southern sky equivalent of the Cambridge 6C survey. Describe the telescope, discuss the array design and the calibration techniques used, and present a map made using the telescope.

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012729   Das H K;Menon S M;Paranjpye A;Tandon S N (NO, IUCAA, Post Bag 4, Ganeshkhind, Pune-411 007) : Site characterisation for the IUCAA telescope. Bull Astr Soc India 1999, 27(4), 609-26.

Reports on characterisation of the site for IUCAA telescope which was conducted during October `96 to April `97. The main points highlighted are the results on : seeing, contribution of atmospheric turbulence near the surface to seeing, extinction and sky brightness, cloud coverage and surface weather darta. In addition the key design features of the instruments developed for these observations are presentecd.

012728   Blaauw A;Ramachandran R (Department of Astronomy, Kapteyn Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands) : Local pulsars: a note on the birth-velocity distribution. J Astrophys Astr 1998, 19(1-2), 19-33.

Simple model for the representation of the observed distributions of the motions, and the characteristic ages of the local population of pulsars. The principal difference from earlier models is the introduction of a unique value, S, for the kick velocity with which pulsars are born. It has been consider separately the proper motion components in galactic longitude and latitude, and find that the distributions of the velocity components parallel and perpendicular to the galactic plane are represented satisfactorily by S = 200 km/sec, and leave no room for a significant fraction of much higher velocities. The successful proposition of a unique value for the kick velocity may provide an interesting tool in attempts to understand the physical process leading to the expulsion of the neutron star.

012727   Banerjee D P K;Rawat S D;Pathan F M;Anandarao B G (Astronomy and Astrophysics Division, Physical Research Laboratory, , Navrangpura, Almedabad-380 009) : FLAGS - a fibre linked astronomical grating spectrograph. Bull Astr Soc India 1999, 27(3), 425-31.

Design and construction of a fibre - linked grating spectrograph, developed for astronomical studies, is described here. The spectrograph has a resolving power of about 10,000 over the visible region (4000-7000 Angstrom). Authors have used optical fibres to guide light from the telescope to the spectrograph. Some preliminary results, demonstrating the performance of the instrument, are presented.

010656   Widyan H (Department of Physics and Astrophysics, University of Delhi, Delhi-110 007) : Topological defects in the left-right symmetric model and their relevance to cosmology. Pramana J Phys 1999, 53(6), 1077-80.

It is shown that the minimal left-right symmetric model admits cosmic string and domain wall solutions.

010655   Vadawale S V (NO, Tata Institute of Fundamental Research, Mumbai-400 005) : X-ray imaging with proportional counters. Indian J Phys-Pt B 1999, 73(6), 997-1004.

Proportional counters are entensively used in X-ray astronomy. Position sensitive proportional counters have been used as imaging detector in the focal plane of the X-ray telescopes in the Einstein Observatory and ROSAT satellite. Here various methods of position sensing by proportional counters are briefly discussed. Design and construction of a Position Sensitive Proportional Counter (PSPC) being developed at TIFR is described. The PSPC has sensitive energy range from 2 keV to 20 keV. With the sensitive circular area of 30 mm diameter, it can be used as an imaging detector at the focal plane of a soft X-ray optics. For two dimensional position determination of an X-ray event, two multi wire cathode grids of 50μm wire spaced at 0.5 mm are used. The final position of the event is obtained by calculating the center of gravity of the charge spread. Position resolution of less than 100μm can be achieved by this method. The main anode grid made up o 25 μm wire spaced at 1 mm gives the standard energy resolution of the proportional counter. Another anode grid is used an an anti-coincidence counter to achieve high degree of background rejection.

010654   Uttam K N;Mahipal Singh;Gopal R;Gaur N K S (Saha's Spectroscopy Laboratory, Physcis Department, Allahabad University, Allahabad-211 002) : Design of astrospectrograph. Asian J Phys 1997, 6(3), 447-50.

Astrospectrograph has been designed and fabricated for the observation of total solar eclipse of October 24, 1995. The dispersion of the spectrograph is 35 A°/mm in the wavelength region λ λ 4000-3500 A°. It has been designed for the use with 150 mm Meniscus Cassegrain Telescope of Carl Ziess available at Physics Department, Allahabad University, Allahabad. During the solar eclipse, absorption lines of the sun have been recorded. The spectrograph can be mounted at the Cassegrain focus of the telescope and replaced by eyepiece whenever required. The set-up can be conveniently utilized to carry out stellar spectroscopic work.

010653   Uttam K N;Joshi M M (Saha's Spectroscopy Laboratory,Physics Department, Allahabad University, Allahabad-211 002) : Variation in temperature and photo-voltage during total solar eclipse. Asian J Phys 1997, 6(3), 363-4.

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010652   Uttam K N;Gopal R;Joshi M M (Saha's Spectroscopy Laboratory, Physics Department, Allahabad University, Allahabad-211 001) : Asymmetry in solar corona. Asian J Phys 1997, 6(3), 365-8.

Corona of the sun has been photographed using Amateur Astro-camera 56/250 at the time of totality of total solar eclipse on October 24, 1995. The photographed corona is found to asymmetrical having major and minor axes 1327840 km and 1317070 km respectively.

010651   Srivastava A M (NO, Institute of Physics, Sachivalaya Marg, Bhubaneswar-751 005) : Topological defects in cosmology. Pramana J Phys 1999, 53(6), 1069-76.

Present status of theories of topological defects in particle theory models of the early Universe is discussed. Various consequences of topological defects in cosmology, such as constraints on particle theory models, structure formation etc. are discussed.

010650   Singh K P (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : X-ray emission from clusters a galaxies and cosmological parameters. Pramana J Phys 1999, 53(6), 1043-51.

Clusters of galaxies are excellent probes of cosmic structure and evolution. X-ray studies of clusters provide some of their key parameters, viz., temperature of the hot intra-cluster gas, its metallicity, Z-ray luminosity and surface brightness giving mass distribution and mass-flow rate in the case of cooling flows. X-ray measurements for a large sample of clusters have lead to estimates of the total gravitating mass in them, which can be compared to the virial masses derived from dynamical considerations and gravitational lensing in some of them. X-ray derived total masses are consistent with masses obtained from the other methods after the effects due to the presence of cooling flows are taken into account in the analyses. Estimated virial masses, lack of evolution in X-ray properties, and detection of several very hot clusters at high redshifts indicate a Universe with a low value (<131><255>0.3) for the Ω parameter.

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010649   Seshadri T R (NO, Mehta Research Institute, Chhatnag Road, Jhusi, Allahabad-211 019) : Multi-fractal analysis of the galaxy distribution in the Las Campanas redshift survey. Pramana J Phys 1999, 53(6), 989-93.

Author have carried out a multi-fractal analysis of the distribution of galaxies in the three Northern slices of the Las Campanas redshift survey. Have studied the scaling properties of the distribution of galaxies on length scales from 20 h^-1Mpc to 200 h^-1Mpc. has been studied. Main results are: The distribution of galaxies exhibits a multi-fractal scaling behaviour over the scales 20 h^-1Mpc to 80 h^-1Mpc, and the distribution is consistent with homogeneity on the scales 80 h^-1Mpc to 200 h^-1 Mpc. Conclude that results are consistent with the Universe being homogeneous at large scales and the transition to homogeneity occurs somewhere in the range 80h^-1Mpc to 100 h^-1Mpc.

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010648   Seshadri T R (NO, Mehta Research Institute, Chhatnag Road, Jhusi, Allhabad-211 019) : Polarization of the cosmic microwave background radiation. Pramana J Phys 1999, 53(6), 963-9.

In re-ionized models, the measurement of polarization of CMBR can be a good criterion to narrow down the parameter space for cosmological models. A Vishniac-type effect in second order polarization over arc minute scales has been calculated. It has been shown that while the effect is very small (<126>10^-2μK) for CDM models, it can be significant (<126>0.3μK) for some isocurvature models.

010647   Sarma N C;Gogoi M;Saikia J;Tamuli R;Hazarika S;Saikia P K;Gohan Barua A;Rai S;Baruah G D (Laboratory of Nonlinear Optics and Spectroscopy, Department of Physics, Dibrugarh University, Dibrugarh-786 004) : Visible spectrum of coma of comet Hale-Bopp (C 1995 01). Asian J Phys 1997, 6(3), 361-2.

Diffuse spectrum in the visible region of 6000-4500 A° has been recorded for the come of the comet Hale-Bopp (C 1995 01). Two discrete bands are observed and the possibility of the presence of sodium and swan features is indicated.

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010646   Sahni V (NO, Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune-411 007) : Pancakes and filaments in cosmological gravitational clustering. Pramana J Phys 1999, 53(6), 995-1000.

Considers the geometrical properties of a distribution of matter evolving under gravitational clustering. Such a distribution can be studied using standard statistical indicators such as the correlation function as well as geometrical descriptors sensitive to `connectedness' such as percolation analysis and Minkowski functionals. Applying these methods to N-body simulations and galaxy catalogues find that the filling factor at the percolation threshold is usually very small reflecting the fact that the Universe consists of a network of filaments and pancakes, the latter being statistically more prominent.

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010645   Sahni V (NO, Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune-411 007) : Cosmological constant revisited. Pramana J Phys 1999, 53(6), 937-44.

Observational evidence for a small cosmological constant at the present epoch. This evidence mainly comes from high redshift observations of Type 1a supernovae, which, when combined with CMB observations strongly support a flat Universe with Ωm + Ω^ ≈ 1. Theoretically a cosmological constant can arise from zero point vacuum fluctuations. In addition ultra-light scalar fields could also give rise to a Universe which is accelerating driven by a time dependent term induced by the scalar field potential. Finally, a dominated Universe also finds support from observations of galaxy clustering and the age of the Universe.

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010644   Roukema B F (NO, Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune-411 007) : Topology of the universe : background and recent observational approaches. Pramana J Phys 1999, 53(6), 945-9.

Is the Universe (a spatial section therof) finite or inifinite? Knowing the global geometry of a Friedmann-Lemaitre (FL) universe requires knowing both its curvature and its topology. A flat or hyperbolic (`open') FL universe is not necessarily infinite in volume. Multiply connected flat and hyperbolic models are, in general, as consistent with present observations on scales of 1-20 h^-1 Gpc as are the corresponding simply connected flat and hyperbolic models. The methods of detecting multiply connected models (MCM's) are presently in their pioneering phase of development and the optimal observationally realistic strategy is probably yet to be calculated. Constraints against MCM's on <126>1-4 h^-1 Gpc scales have been claimed, but relate more to inconsistent assumptions in perturbation statistics rather than just to topology. Candidate 3-manifolds based on hypothesised multiply imaged objects are being offered for observational refutation. The theoretical and observational sides of this rapidly developing subject have yet to make any serious contact, but the prospects of a significant detection in the coming decade may well propel the two together.

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010643   Roscoe D F (School of Mathematics, Sheffield University, Sheffield, S3 7RH, UK) : Analysis of 900 optical rotation curves : dark matter in a corner?. Pramana J Phys 1999, 53(6), 1033-7.

One of the largest Hα rotation curve data bases of spiral galaxies currently available is that provided by Persic and Salucci (PS) [1], which has been derived by them from unreduced rotation curve data of 965 southern sky spirals obtained by Mathewson, Ford and Buchhorn (MFB) [2]. Of the original sample of 965 galaxies, the observations on 900 were considered by PS 1995 to be good enough for rotation curve studies, and the present analysis concerns itself with these 900 rotation curves. Analysis shows that the rotation velocity, V, at any radial displacement, R, in the optical disc of any given spiral galaxy satisfies the law V<124>Rα = V0<124>Rα0, where R0 and V0 are given as approximate functions of the galaxy's absolute magnitude and surface brightness whilst α is an unidentified function of other galaxy parameters - of which the most significant ones will be the relative proportions of the disc, bulge and halo mass-components. It is this latter function which provides the opportunity for a dark-matter modelling process which is independent of any particular dynamical theory.

010642   Ray R (NO, Institute of Physics, Sachivalaya Marg, Bhubaneswar-751 005) : Defect correlation in liquid crystal : experimental verification of cosmological Kibble mechanism. Pramana J Phys 1999, 53(6), 1087-91.

Observations of correlated production off strength one defects and anti-defects formed in isotropic-nematic phase transition in NLC. Find the width σ of the distribution of net winding number, to be in good agreement with the value predicted by the Kibble mechanism for defect production.

010641   Rao A R (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : X-ray detectors in astronomy. Indian J Phys-Pt B 1999, 73(6), 945-56.

Developments in the field of X-ray detectors used in astronomy are described. A brief sketch of the historical developments is given, with particular emphasis on the efforts made in the Indian context. Results obtained from a balloon borne hard X-ray telescope and the Indian X-ray Astronomy Experiment are also highlighted.

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010640   Rangarajan R (Theoretical Physics Division, Physical Research Laboratory, Ahmedabad-380 009) : Baryogenesis in the early Universe. Pramana J Phys 1999, 53(6), 1061-7.

Reviews various attempts to create the observed baryon asymmetry of the Universe. In particular, consider models of GUT baryogenesis, baryogenesis via leptogenesis, the Affleck-Dine mechanism, electroweak baryogenesis and baryogenesis via topological defects and primordial black holes.

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010639   Paul B C (Physics Department, North Bengal University, Siliguri, District Darjeeling-734 430) : Exact scalar field cosmologies in a higher derivative theory. Pramana J Phys 1999, 53(5), 833-41.

Exact cosmological solutions of a higher derivative theory described by the Lagrangian L = R + 2αR² in the presence of interacting scalar field. The interacting scalar field potential required for a known evolution of the FRW universe in the framework of the theory is obtained using a technique different from the usual approach to solve the Einstein field equations. A technique to determine potential similar to that used by Ellis and Madsen in Einstein gravity. Some new and interesting potentials are noted in the presence of R² term in the Einstein action for the known behaviours of the universe. These potentials in general do not obey the slow rollover approximation.

010638   Pal P B (NO, Saha Institute of Nuclear Physics, 1/AF Bidhan Nagar, Calcutta-700 064) : Particle dark matter. Pramana J Phys 1999, 53(6), 1053-9.

Discusses some compelling suggestions about particles which could be dark matter in the Universe, with special attention to experimental searches for them.

010637   Pal P B (NO, Saha Institute of Nuclear Physics, 1/AF Bidhan-Nagar, Calcutta-700 064) : Cosmological parameters. Indian J Phys-Pt B 1999, 73(6), 877-9.

010636   Nath B B (NO, Raman Research Institute, Bangalore-560 012) : Cosmology with the intergalactic medium. Pramana J Phys 1999, 53(6), 1021-5.

Discusses a few new results which points out the importance of the intergalactic medium as a diagnostic for the formation and evolution of galaxies in the Universe. Discusses the recent studies to determine the power spectrum of fluctuation from QSO-absorption line studies, and then some feedback processes from early galaxies which influence the intergalactic medium.

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