010635   Narlikar J V (NO, Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune-411 007) : Quasi-steady state cosmology : theory and observations. Pramana J Phys 1999, 53(6), 1093-1104.

This is a review of an alternative cosmology, recently proposed by Fred Hoyle, Geoffrey Burbidge. It begins with a brief discussion of why one needs an alternative cosmology, when the standard hot big bang cosmology is claimed to be doing well. It is argued that the observational and theoretical constraints on the standard big bang cosmology, from various directions, leave a very narrow window, if any, in the parameter space of plausible models. There is thus a strong case for alternative cosmologies. The rest of the review concentrates on one alternative, the quasi steady state cosmology (QSSC) and summarises the recent work on this model. This includes, the theoretical formulation and simple exact solutions of the basic equations, their relationship to various observations, the stability of solutions and the toy model for understanding the growth of structures in the Universe.

24 ref

010634   Narasimha D (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Gravitational lensing by spiral galaxies. Pramana J Phys 1999, 53(6), 1039-42.

Spiral galaxies at moderate redshifts and oriented optimally could form characteristic multiple images of extended background sources from which the mass distribution in the galaxy can be estimated. The absorption profile due to the galaxy provides a reliable tool for the chemical and thermal diagnostic of the lends.

010633   Narasimha D (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Cosmological parameters. Pramana J Phys 1999, 53(6), 921-31.

There is some consensus emerging on the values of the basic parameters of classical cosmology. The baryon number density estimated from the light element abundance or X-ray gas in galaxy clusters tends towards 5% of closure density; the dark matter content based on a number of independent methods appears to be somewhat less than half the closure density; Hubble constant obtained from local measurement, gravitational lens or Sunyaev Zeldovich method are all probably centred around 60 km/sec/Mpc and the age of the Universe is generally agreed to be around 14 Gyr-all specified with bearable error bars. The supernova projects and CMBR anisotropy together favour a finite cosmological constant, and gravitational lens statistics support the same conclusion.

27 ref

010632   Mukhopadhyay B (Theoretical Astrophysics Group, S N Bose National Centre for Basic Sciences, J D Block, Salt Lake, Sector III, Calcutta-700 091) : Nucleosynthesis around black holes. Indian J Phys-Pt B 1999, 73(6), 917-24.

Nucleosynthesis in accretion disks around black holes was initiated. The state-of-the-art disk model, namely, Advective Accretion Disks. During the infall, matter temperature and density are generally increased which are first computed. These quantities are used to obtain local changes in composition, amount of nuclear energy released or absorbed, etc. under various inflow conditions. In the cases where the magnetic viscosity is dominant neutron torus may be formed. The outflowing winds from the disk could carry the new isotopes produced by nucleosynthesis and contaminate the surroundings. From the degree of contamination, one could pinpoint the inflow parameters.

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010631   Mukhopadhyay B (Theoretical Astrophysics Group, S N Bose National Centre for Basic Sciences, J D Block, Salt Lake, Sector-III, Calcutta-700 091) : Dirac equation in Kerr geometry. Indian J Phys-Pt B 1999, 73(6), 855-68.

Dirac equation in flat space, investigate the behaviour of half-integral spin particle. With the introduction of general relativistic effects of form of the Dirac equation will be modified. For the cases of different background geometry like Kerr, Schwarzschild etc. the corresponding form of the Dirac equation as well as the solution will be different. In 1972, Teukolsky wrote the Dirac equation in Kerr geometry. Chandrasekhar separated it into radial and angular parts in 1976. Later Chakrabarti solved the angular equation in 1984. In 1999 Mukhopadhyay and Chakrabarti have solved the radial Dirac equation in Kerr geometry in a spatially complete manner. Discuss these developments systematically and present some solutions.

14 ref

010630   Mukerjee K (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai-400 005) : Pulsar observations with the Indian X-ray astronomy experiment. Indian J Phys-Pt B 1999, 73(6), 957-66.

Observations of some X-ray pulsars were carried out by the pointed mode proportional counters of the Indian X-ray Astronomy Experiment (IXAE) on IRS-P3 satellite which has completed its three successful years of operation on 1999 March 21. The X-ray instrument details and results obtained from the observations of pulsars namely Crab pulsar, 4U 1626-67, Cep X-4 and 4U 1907+09 are discussed.

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010629   Mishra A P;DDubey S C (Department of Physics, A P S University, Rewa-486 003) : Geomagnetic disturbance and their variation with 11-year sunspot cycle during solar cycle 22. Asian J Phys 1997, 6(3), 413-6.

Sunspots are most obvious feature of disturbed surface of the photosphere on solar atmosphere and appear to play a key role in major solar and geomagnetic disturbances. The geomagnetic disturbances are directly linked with solar source activity. The solar source activity varies with 11-year sunspot cycle. During solar activity maximum years, the huge amount of solar energetic particles (SEPs) are released form the Sun and enters in the Earth's magnetosphere which are found to be responsible for producing large number of geomagnetic storms, whereas at solar activity minimum years a very few number of geomagnetic storms are observed. In the analysis, examined the variation of geomagnetic storms and number of disturbed days (Ap≥ 30) with various phases of 11-year sunspot cycle during the solar cycle 22. It is found that the yearly occurrence of geomagnetic storms show the close correspondence with the various phases of 11-year sunspot cycle, but the yearly occurrence of number of disturbed days did not vary with 11-year sunspot cycle. The monthly distribution of disturbed days have been also discussed.

010628   Mazumdar A (Theoretical Astrophysics Group, Tata Institute of Fundamental Research, Mumbai-400 005) : Cepheid distance to the vigro cluster. Pramana J Phys 1999, 53(6), 933-6.

HST data on Cepheid variables in one Virgo Cluster spiral galaxy is re-analyzed, taking into account flux-limited incompleteness in the sample and calibration of the period-luminosity relation in the relevant period range. Distance to the Virgo Cluster is estimated to be 19.6±1.7 (random)±2.6 (systematic) Mpc.

010627   Manickam S G;Chakrabarti S K (NO, S N Bose National Centre for Basic Sciences, JD Block, Sector III, Salt Lake, Calcutta-700 091) : On the nature of quasi periodic oscillations in the black hole candidate GRS1915+105. Indian J Phys-Pt B 1999, 73(6), 967-76.

Analyze RXTE data of the transient black hole candidate GRS1915+105 and find that the Quasi Periodic Oscillations (QPO) are of variable frequency. Power density spectra reveal that there is more power in high energy X-rays, indicating that the QPO might be originated due to shock oscillations. A few timescales have been identified and generally it is found that the longest time-scale (<126> 100s) is related to the time it takes to fill the volume upto sonic sphere, and the shorter time scale (<126>0.1s) is related to the free-fall time in the post-shock region.

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010626   Manickam S G (NO, S N Bose National Centre for Basic Sciences, J D Block, Sector III, Salt Lake, Calcutta-700 091) : Introduction to quasi periodic oscillations (QPO) in the X-ray emission from neutron stars and black hole candidates. Indian J Phys-Pt B 1999, 73(6), 925-30.

The status of the nature of Quasi Periodic Oscillations (QPO) observed in X-rays from black holes and neutron star candidates.

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010625   Majumdar S;Subrahmanyan R (Joint Astrophysics Programme, Department of Physics, Indian Institute of Science, Bangalore-560 012) : Constraining cosmological parameters through Sunyaev-Zel'dovich surveys. Pramana J Phys 1999, 53(6), 971-5.

For cold dark matter models, images of temperature fluctuations in the cosmic microwave background (CMB), due to Sunyaev Zel'dovich (SZ) effect have been simulated taking a cosmological distribution of clusters into account. All the models are normalised to the 4-year COBE data. The image statistics are compared with the ATCA limits on arcmin scale anisotropy. The comparison appears to favour low-Ω0 open universe models.

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010624   Majumdar S (NO, Indian Institute of Astrophysics, Bangalore) : Cosmology with Sunyaev-Zeldovich effect. Indian J Phys-Pt B 1999, 73(6), 835-42.

Use of Sunyaev Zeldovich effect (SZE) as a cosmological probe is briefly reviewed, with special emphasis on using SZE of clusters to constrain Ω0. Particular attention is paid to recent and upcoming black sky surveys. Lastly, it is shown how the combined analysis of SZE and X-ray emission from clusters can be used to derive the Hubble Constant.

14 ref

010623   Majumdar A S (NO, S N Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Calcutta-700 091) : Inflationary universe in Kaluza-Klein theories. Indian J Phys-Pt B 1999, 73(6), 843-54.

Describes extended inflation and its typical problems and then briefly review essential features of Kaluza-Klein theory, and show that it leads to a scenario of inflationary cosmology in four dimensions. The problem of stable compactification of extra spatial dimensions is discussed. The requirements for successful extended inflation lead to constraints on the parameters of higher dimensional models.

45 ref

010622   Maharana J;Mukherji S (NO, Institute of Physics, Bhubaneswar-751 005) : String cosmology. Pramana J Phys 1999, 53(6), 1105-14.

Discusses various cosmological issues related to Universe from a string theoretic perspective, analyse the pre-big bang cosmological scenario which appears naturally in this context due to the existence of scale factor duality symmetry in string theory. Discusses some of the attractive and problematic features of this scenario. Finally, introduce a method which is powerful enough to search for cosmological solutions in various low energy limits of string theories.

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010621   Kanekar N;Chengalur J N (National Centre for Radio Astrophysics, Tata Intitute of Fundamental Research, University of Pune Campus, P O Bag 3, Ganeshkhind, Pune-411 007) : Nature of low redshift damped Ly-α systems. Pramana J Phys 1999, 53(6), 1013-9.

Damped Ly-α systems are the major repository of the observed neutral HI at high redshift. These systems are most efficiently detected via absorption spectra taken against distant QSOs. The observational constraints on the nature of these objects, and also discuss the implications of recent observations of two low redshift damped absorbers, made with the Giant Metrewave Radio Telescope. Find that, for the lowest redshift (z=0.0912) damped Ly-α system, if the system is a rotating gas disk, then the total associated HI mass has to be less than 2 x 10⁹ M, if the disk is at low inclination angles, and less than 10¹⁰M if the system is edge on. All limits are 3σ.

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010620   Deb R N (NO, Mehta Research Institute, Allhabad) : Determination of the pressure and energy density of the universe, implying the presence of missing mass. Indian J Phys-Pt B 1999, 73(6), 869-75.

Pressure and energy density of the universe are computed and they are used to show that the universe must have missing mass.

010619   Das T K;De B K;Bhattachryya J (Eastern Centre for Research in Astrophysics, Institute of Radio Physics & Electronics, 92, A P C Road, Calcutta-700 009) : Some aspects of solar X-ray flares. Indian J Phys-Pt B 1999, 73(5), 695-9.

The Solar X-ray flares observed by GOES during the period July, 1993 - June, 1994, which encompasses the post-maximum phase of 22nd solar cycle. It has been examined that N-S asymmetry remains positive in respect of both latitudinal position and intensity of X-ray flares and increases with the increase of intensity. The E-W asymmetry is found to be zero, when longitudinal distribution is considered; but it remains positive upto a certain intensity value above which it becomes negative. The duration and rise time lie respectively in the ranges of values 5-15 min. and 0-5 min. The distribution of impulsiveness of X-ray flares follows almost Binomial distribution pattern peaking around 0.45.

010618   Das T K (NO, S N Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Calcutta-700 091) : Modeling the origin of astrophysical jets from galactic and extra-galactic sources. Indian J Phys-Pt B 1999, 73(6), 899-915.

Astrophysical jets are physical conduits along which mass, momentum, energy and magnetic flux are channeled from the stellar, galactic and extra-galactic objects to the outer medium. Geometrically these jets are narrow (small opening angle) conical or cylindrical/semi-cylindrical protrusions covering an astonishing range in size. While the jets associated with young stars are typically 10¹⁷ cm in length, jets from some giant extra-galactic sources have an overall extent in excess of 10²⁴cm. Thus the jet phenomenon is seen on scales that cover more than seven orders of magnitude and some of the extra-galactic radio jets are considered to be the largest single coherent structure found in the universe. Though it is now an well-known fact that a variety of celestial objects, spanning from stars (having all masses) during their formation (Young Stellar Objects) to Active Galactic Nuclei (and possibly sources of γ ray bursts also suffer mass loss through jets, the detailed nature of the origin of the extragalactic jets is not quite clear due to the lack of proper understanding of the underlying physical mechanism responsible for jet production. Efforts have been made to justify the validity of a model very recently proposed, which, for the first time believe, is able to explain the formation of extragalactic cosmic jets using a self-consistent inflow-outflow system. Primarily intended to make intelligible to readership not working exclusively ....

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010617   Chattopadhyay I (NO, S N Bose National Centre for Basic Sciences, J D Block, Salt Lake, Sector-III, Calcutta-700 091) : Theoretical investigation and numerical simulation of radiative outflows around compact objects. Indian J Phys-Pt B 1999, 73(6), 881-8.

Winds and Jets form in active galaxies and in binary systems which are known to harbour compact objects at the centre. Winds are likely to originate from centrifugal barriers in advective accretion flows, but the acceleration of the flow is a puzzle. Matter likely to start subsonically from a disk must be accelerated very close to the black hole in order to rach a velocity comparable to the velocity of light, which is actually observed. But the terminal velocity achieved by jets are still an enigma. Author try to answer some of these questions by studying critical point behaviour of the outflow in presence of radiative acceleration. Show that the momentum deposition term changes the character of the solution drastically depending on the magnitude and the exact location of the deposition. Accelerate the matter from very close to the compact object to infinity, particularly interesting is the case, where radiative momentum deposition force (hereafter RAMOD) actually drive away bound matter as winds. Also study the time evolution of the outflows by numerical simulation, and find these new solutions to be stable.

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010616   Chakrabarti S K (NO, S N Bose National Centre for Basic Sciences, J D Block, Salt Lake, Sector III, Calcutta-700 091) : Latest trends in the study of accretion and outflows around compact objects. Indian J Phys-Pt B 1999, 73(6), 931-44.

Astrophysics of black holes and neutron stars has taken a new turn in the present decade with the realization that sub-Keplerian flows and the associated centrifugal barrier near the horizon or the surface of a neutron star play a major role in deciding the nature of the emitted spectra and the formation of outflows from the accreting matter. This region may remain steady or oscillate depending on the accretion rate, specific angular momentum and specific energy of the flow. Intricacies of oscillation may depend on the degree of feedback the inflow receives from the outflow. This region may emit hard or soft X-rays depending on relative numbers of hot elections and soft photons intercepted by this region. Discuss how these properties come about and how they explain the observational results of black hole candidates.

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010615   Chakrabarti S (NO, S N Bose National Centre for Basic Sciences, JD Block, Sector III, Salt Lake, Calcutta-700 091) : Search for prebiotic molecules in space. Indian J Phys-Pt B 1999, 73(6), 991-6.

The search for prebiotic molecules in space is a part of an endeavour to solve the problem of how life originated on earth. The search includes multiwave-length observations and analysis of the spectrum from cold dark clouds, dense molecular clouds, pre-protostellar, protostellar and star forming regions, complemented with laboratory and mathematical simulations of the corresponding dynamical and chemical conditions. Studying the dynamics of the collapse of a typical molecular cloud core prior to a protostar formation, including chemistry involved with the evolution of the inorganic and organic molecules observed to be present in the dense molecular clouds.

15 ref

010614   Bhattacharyya S (NO, Indian Institute of Astrophysics, Bangalore) : Neutron stars and accretion disks in low mass X-ray binaries. Indian J Phys-Pt B 1999, 73(6), 889-98.

Evolution of Low Mass X-ray Binaries will be described briefly. Accretion disks around neutron stars in LMXBs will also be discussed and the importance of general relativistic effect of rapid rotation on the disk will be pointed out. Millisecond pulsars and some of the observed aspects of LMXBs will be mentioned.

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010613   Bhattacharyya A;Basu A M (Department of Physics, Jadavpur University, Calcutta-700 032) : Non-linear vortices in planetary ionosphere. Indian J Phys-Pt B 1999, 73(6), 985-90.

Effect of dynamics of dust grains in the planetary magnetospheric plasma is studied. The simulation suggests to study further to find the possibility of formation of vortices.

010612   Bharadwaj S (Department of Physics and Meteorology and Centre for Theoretical Studi, Indian Institute of Technology, Kharagpur-721 302) : Large scale structure. Pramana J Phys 1999, 53(6), 977-87.

Discusses some aspects of the problem of forming large scale structures in the Universe. The base picture that initially small perturbations generated by inflation grow by the process of gravitational instability to give the observed structure is largely consistent with the observations. The growth of the perturbations depends crucially on the contents of the Universe, and discuss a few variants of the cold dark matter model. Many of these models are consistent with observations. Future observations hold the possibility of deciding amongst these models.

23 ref

006419   Svestka Z (CASS UCSD, La Jolla, California, CA 92093-0424, USA and SROn Laboratory for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands) : Solar activity: an overview. Curr Sci 1999, 77(11), 1503-10.

006418   Rai Choudhuri A (Department of Physics, Indian Institute of Science, Bangalore-560 012) : Solar dynamo. Curr Sci 1999, 77(11), 1475-86.

It is believed that the magnetic field of the sun is produced by the dynamo process, which involves nonlinear interactions between the solar plasma and the magnetic field. After summerizing the main characteristics of solar magnetic fields, the basic ideas of dynamo theory are presented. Then an appraisal is made of the current status of solar dynamlo theory.

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006417   Phillips K J H;Dwivedi B N (Space Science Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK) : Probing the sun's hot atmosphere. Curr Sci 1999, 77(11), 1511-20.

Solar corona is an extremely hot (10⁶ K), almost fully ionized plasma which extends from a few thousand km above the photosphere to where it freely expands into the solar system as the solar wind. The exact reasons for its high temperature is still unknown, despite more than 50 years of research, but magnetic fields are certainly involved. This reviews some recent progress in understanding using data from spacecraft (SOHO, Yohkoh, and TRACE) as well as ground-based eclipse experiments.

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006416   Parker E N (Enrico Fermi Institute, University of Chicago, 1323 Evergreen Road, Homewood, Illinois 60430 USA) : Physics laboratory in the sky. Curr Sci 1999, 77(11), 1445-53.

Sun, by virtue of its large mass, size and temperature, exhibits a variety of effects that are unknown in the terrestrial laboratory, thereby challenging the physicist to relate them to the basic principles of physics derived from the terrestrial laboratory. A number of solar phenomena are reviewed, with comments on the degree to which they are presently understood.

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006415   Kosovichev A G;Duvall Jr T L (W.W. Hansen Experimental Physics Laboratory, Stanford University, CA 95305-4085, USA) : Solar tomography. Curr Sci 1999, 77(11), 1467-74.

Solar tomography (or time-distance helioseismology) is a new promising method for probing 3-D structures and flows beneath the solar surface, which is potentially important for studying the birth of active regions in the Sun's interior and for understanding the relation between the internal dynamics of the active regions, and the chromospheric and coronal activity. In this method, the time for waves to travel along sub-surface ray paths is determined from the temporal cross correlation of signals at two separated surface points. By measuring the times for many pairs of points from Dopplergrams, covering the visible hemisphere, a tremendous quantity of information about the state of the solar interior is derived. As an example, results on the internal structures of supergranulation, meridional circulation, active regions and sunspots. An active region which emerged on the solar disk in January 1998, was studied from SOHO/MDI for nine days, both before and after its emergence at the surface. The results show a complicated structure of the emerging region in the interior, and suggest that the emerging flux ropes travel very quickly through the depth range of our observations.

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006414   Kanekar Nissim (National Centre for Radio Astrophysics (TIFR), University of Pune Campus, Pune-411 007) : Fire in the sky. Phys Educ 1999, 16(2), 177-83.

006413   Kalkofen W;Ulmschneider P (NO, Harvard-Smithsonian Center for Astrophysics, St. Cambridge, MA02138, USA) : Dynamics and heating of the quiet solar chromosphere. Curr Sci 1999, 77(11), 1496-1502.

Solar chromosphere can be characterized by two signatures: the spectroscopic signature is an emission spectrum for all radiation originating in the chromosphere; only NLTE effects in the cores of strong lines producing absorption features. And the dynamical signature is in the form of oscillations, with a period of 3 min in the nonmagnetic chromosphere and a period of 7 min in magnetic regions. Explains these signatures in terms of waves: The dynamics of the chromosphere is due to acoustic waves in the magnetic-field-free atmosphere, which produce K2v bright points, and to kink and longitudinal waves in magnetic flux tubes, which produce network bright points. The heating of the chromosphere is caused by acoustic waves whose dissipation makes the kinetic temperature rise in the outward direction, producing the emission spectrum. As far as energy fluxes are concerned, the energy dissipated in chromospheric heating outweighs the energy visible in bright points by two orders of magnitude. Interprets the observed oscillation periods in the chromosphere as cutoff periods: for the 3 min period, as the cutoff period of acoustic waves in a non-magnetic, stratified atmosphere; and for the 7-min period, as the cutoff period of kink waves in magnetic flux tubes for field strengths typical of the magnetic network.

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006412   Dwivedi B N;Anita Mohan;Wilhlem K (Department of Applied Physics, Institute of Technology, Banaras Hindu University, Varanasi-221 005) : New sun: probing solar plasmas in the extreme-ultraviolet light from SUMER on SOHO. Curr Sci 1999, 77(11), 1521-9.

Authors briefly outline the extreme-ultraviolet spectro-scopy of solar plasmas in the light of a wealth of high-resolution observations, both in spectral and spatial regimes, from the SUMER spectrograph (Solar Ultra-violet Measurements of Emitted Radiation) on the spacecraft SOHO (Solar and Heliospheric Observatory). Some of the new results on a new sun seen by the SUMER spectrograph. In particular, discuss coronal holes and the solar wind, the `ref'/`blue' sun, abundance anomalies, explosive events and sunspot transition region oscillations. Reiterating clues, obtained from SUMER, that provide information essential for solving solar riddles of coronal heating and the wind acceleration.

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006411   Christensen-Dalsgaard J;Thompson M J (Teoretisk Astrofysik Center, Danmarks Grundforskningsfond, and Institut for Fysik og Astronomi, Aar, DK-8000, Aarhus C,Denmark) : Rotation of the solar interior. Curr Sci 1999, 77(11), 1460-6.

Helioseismology has allowed to infer the rotation in the greater part of the solar interior with high precision and resolution. The results show interesting conflicts with earlier theoretical expectations, indicating that the sun is host to complex dynamical phenomena, so far hardly understood. This has important consequences for ideas about the evolution of stellar rotation, as well as for models for the generation of the solar magnetic field. An overview of current knowledge about solar rotation, much of it obtained from observations from the SOHO spacecraft, and discuss the broader implications.

006410   Chitre S M;Antia H M (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Seismic sun. Curr Sci 1999, 77(11), 1454-9.

Even though the interior of the sun is not directly accessible to observations, it is nonetheless possible to infer the physical conditions inside the sun using the theory of stellar structure and the accurately measured frequencies of solar oscillations. The helioseismic data has provided a powerful tool to probe the sun and also to test physical theories describing its internal constitution.

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004261   Tickoo A K;Koul R;Kaul S K;Kaul I K;Bhat C L; Bhatt N (Bhabha Atomic Research Centre, Nuclear Research Laboratory, Mumbai-400 085) : Drive control system for the Tactic γ-ray telescope. Bull Astr Soc India 1999, 27(2), 301-4.

Some salient features of the hardware and software aspects of the 2-axes drive system developed for the TACTIC γ-ray telescope array has been presented.

004260   Thakur P;Sahu D K;Chakraborty D K (School of Stdudies in Physics, Pt. Ravishankar Shukla University, Raipur-492 010) : Dynamical modeling of elliptical galaxy NGC 2513. Bull Astr Soc India 1999, 27(2), 277-80.

Analysed the photometric and kinematical data of the elliptical galaxy NGC 2513 to determine its intrinsic structure. The galaxy shows isophote twist and further, the line of sight velocity along the apparent minor axis is comparable to that along the major axis. These indicate that the galaxy is triaxial. A model which is a triaxial generalisation of the γ model of Jaffe and Hernquist. The projected surface density has central cusp (for γ<62>1), ellipticity variations and isophote twists. Compare the projected surface density by taking a suitable line of sight and choosing the parameters in the model, so as to obtain a best fit with the photometric data. The results of analysis show that models with cusp fits the data better.

004259   Sathyabama N;Sapru M L;Bhat C L;Rannot R C; Razdan A K;Tickoo A K;Koul M K;Gandhi V N (NO, Bhabha Atomic Research Centre, Nuclear Research Laboratory, Mumbai-400 085) : Threshold energy estimates of TACTIC gamma-ray telescope array - inputs from simulation studies (I). Bull Astr Soc India 1999, 27(2), 309-14.

Comprehensive simulation studies, using the CORSIKA air-shower code are in progress, to predict and optimize the performance of the TACTIC γ-ray telescope array. Preliminary results are presented on the <103>-ray threshold energies likely to be achieved by its Central Imaging Element and 3 Vertex Elements, using 2 independent, innovative trigger-generation criteria.

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004258   Sapre A K;Nashine R;Mishra V DD (School of Studies in Physics, Pt. Ravishankar Shukla University, Raipur-492 010) : θ-z test for QSO-galaxy physical association. Bull Astr Soc India 1999, 27(2), 285-7.

The θ-z test devised by Burbidge et al, (1972, 1990) may serve as a powerful test to decide the reality or otherwise of the physical association hypothesis for the QSO-bright galaxy close pairs with discrepant redshifts. Recently, Burbidge (1996) has published a sample of 33 QSO-bright galaxy close pairs with discrepant redshifts as the best condidates for the physical association. Authors have subjected this sample of 33 pairs to the θ-z test with a view to test the reality of the physical association of these pairs.

004257   Sahu D K;Pandey S K;Kembhavi A K (School of Studies in Physics, Pt. Ravishankar University, Raipur-492 001) : Properties of dust extinction in NGC 2076. Bull Astr Soc India 1999, 27(2), 271-6.

Early-type galaxies with large-scale dust are suitable objects to investigate the properties of dust extinction in extragalactic environments because the underlying galaxies have fairly smooth distribution of the starlight. Total dust content and the physical properties of dust grain may be a function of time and galaxy environment, and thus detailed studies of dust properties may provide important clues about the nature, origin and subsequent evolution of dust in galaxies. Multiband CCD surface photometry of NGC 2076, a lenticular, having a very prominent dust lane running parallel to the apparent major axis of the galaxy. The main objective is to investigate the wavelength dependence of the dust extinction and to estimate dust mass from optical as well as FIR data. The extinction curve is very similar to Galaxy, having a value of 2.7 for RV, defined as the ratio of total visual extinction AV to selection extinction between B and V, E (B-V). The smaller value of RV for NGC 2076 than the canonical value for our Galaxy RV = 3.1, implies that the size of dust grains responsible for extinction in NGC 2076 is less than that in Galaxy. The estimate of dust mass from optical colour excess, total extinction as well as from FIR flux are found to be of the same order and are therefore consistent with one another.

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004256   Saha S K;Chinnappan V (NO, Indian Institute of Astrophysics, Bangalore-560 034) : Night time variation of Fried's parameter at VBT, Kavalur. Bull Astr Soc India 1999, 27(2), 327-9.

The night time variation of the Fried's parameter, ro is measured on 28-29 March, 1991, at the Cassegrain focus of 2.34 m Vainu Bappu Telescope (VBT), at Vainu Bappu Observatory (VBO), Kavalur, India, using speckle interferometer; the results are discussed.

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004255   Rao A R (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Indian multi-wavelength astronomy satellite. Bull Astr Soc India 1999, 27(2), 289-93.

Indian X-ray Astronomy Experiment (IXAE) is currently in operation on-board the IRS-P3 satellite launched by the PSLV-D3 rocket in 1996 March 21. Based on IXAE observations, several interesting results have been obtained so far, including possible evidence for matter disappearing behind the event horizon of a black hole in the superluminal Galactic X-ray source GRS 1915+105. Encouraged by the success of IXAE, a full-fledged saztellite for astronomical observations is currently under consideration. A large area xenon filled proportional counter array (LAXPAC), a soft X-ray telescope (SXT), one X-ray sky monitor (XRM) and an UV/optical telescope (UVT) are being considered for inclusion in the Indian Multi-wavelength Astronomy Satellite (IMAS).

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004254   Pati A K (NO, Indian Institute of Astrophysics, Bangalore-560 034) : Proposed UV-optical payload for the Indian astronomy satellite. Bull Astr Soc India 1999, 27(2), 295-300.

Ultraviolet Imaging Telescope is a part of the proposed Indian Multiwavelength Astronomy Satellite mission. The primary objective of the UV Imaging Telescope is an all-sky survey in two bands in the wavelength region 1200 to 3000 A^o. The aperture of the telescope will be 50 cm and there will be two channels for simultaneous observations, one for the range 1200-1900 A^o and the other longwards of 1800 A^o. The goal for spatial resolution on the sky over a full field of 2° is <126> 2 seconds of arc. The detectors of choice are photon counting devices using MCP intensifiers and different photocathodes for the two channels. Each channel will have a filter changing mechanism, with a provision for upto 4 full size filters and 20 small (20 arc minutes) filters. It will also be possible to mount grisms for field spectroscopy. The all-sky survey is expected to reach a UV (1500 A^o) limiting magnitude of 20. After the all sky survey, deeper surveys over selected regions of the sky can also be done, interspersed with programs for the X-ray payload. A key area will be simultaneous UV / optical and X-ray observations of a variety of astronomical objects ranging from binaries to quasi-stellar objects and active galactic nuclei. The launch of the mission is expected between the years 2002 and 2004.

004253   Narasimhan K S V S;Alladin S M;Sastry K S (NO, Inter-University Centre for Astrophysics, Pune) : Escape velocities of interacting galaxies in Hernquist's model. Bull Astr Soc India 1999, 27(2), 281-3.

Hernquist's (1990) mass model for spherical galaxies and bulges described by de Vaucouleur's r^{1/4 profile represents the observed properties of elliptical galaxies quite well. Using this model, derive simple exact analytic formulae for the parabolic velocities of escape of two spherical galaxies and deduce analytically that for r = O, Vesc (O)/(Vrms)1 = 2√3 for α1 <62> α2 and 2√2 for α1}

004252   Mondal S;Chandrasekhar T;Ashok N M;Kikani P K (NO, Physical Research Laboratory, Ahmedabad-380 009) : Two channel high speed photometer for lunar occultation studies in the near-IR. Bull Astr Soc India 1999, 27(2), 335-8.

Lunar occultation in the near infrared has been demonstrated by the PRL group to be an effective method of determining the angular size (in milliarsecond range) and accurate effective temperature of red giants, supergiants and carbon stars. Many of these object (Teff <126> 3000 K) have cooler circumstellar material at temperatures less than <126> 1000 K. A lunar occultation observed simultaneously of such a star in the K(2.2 μm) and L(3.6μm) filter bands can provide a complete high angular resolution information on both the star and the circumstellar region. For this purpose a two-channel fast photometer operating in the K and L bands has been developed recently at PRL. It incorporates a special dichroic beam-splitter for separating the K and L beams and several improvements over existing single channel system.

004251   Mazumdar A;Narasimha D (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : Distance to the virgo cluster and estimation of the Hubble constant. Bull Astr Soc India 1999, 27(2), 267-9.

004250   Manian K S B;Shah V;Thomas M;Joshi U C;Ganesh S (NO, Physical Research Laboratory, Ahmedabad-380 009) : Near infrared polarimeter for extended sources. Bull Astr Soc India 1999, 27(2), 331-4.

State of art near IR polarimeter was designed and developed at Physical Research Laboratory, Ahmedabad and the polarimeter is operational for the last three years. During the last one year a new mode of operation has been implemented to measure polarization of extended sources. The modulation of incoming light signal is generated by fast rotating half wave plate (10 revolution/sec which generates 40 Hz modulation of polarized light). The fast modulation takes care of sky background variation.

004249   Kulinder Pal Singh (NO, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005) : X-ray spectroscopy in astronomy. Bull Astr Soc India 1999, 27(2), 243-52.

X-ray spectroscopy can probe conditions in (a) high temperature plasmas found in stellar coronae, supernova remnants, and clusters of galaxies, and (b) in the accretion powered sources around compact objects - white dwarfs, neutron stars, and black-holes. X-ray spectroscopic measurements of hot plasmas determine the temperatures, densities, abundances of elements, and equilibrium conditions of plasmas. In accretion powered sources, the strong X-ray continuum lights up the surrounding medium and generates a number of discrete X-ray emission and absorption features which tell us about the physical conditions in the surrounding matter. In thin hot astrophysical plasmas, X-ray line emission in the soft and medium energies contains information from the innermost K-shell transitions of nearly all the abundant elements (Carbon to Zinc), and the L-shell transitions of most elements. The study of line spectra, however, requires medium-to-high energy resolution and some progress is now being made using line spectra for detailed diagnoses of X-ray sources. Examples of new results based on spectra of X-ray sources and the diagnostic power of spectroscopy are briefly described. High resolution X-ray spectroscopy using X-ray gratings and calorimeters, will be available with the launch of new X-ray observations - AXAF, XMM, SPECTRUM - XG and ASTRO - E in the next few years.

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004248   Koul M K;Rannot R C;Sapru M L;Sathyabama N; Bhat C L;Tickoo A K;Razdan A K;Gandhi V N (NO, Bhabha Atomic Research Centre, Nuclear Research Laboratory, Mumbai-400 085) : Sensitivity and energy resolution estimates of the TACTIC gamma-ray telescope - inputs from simulation studies (II). Bull Astr Soc India 1999, 27(2), 315-8.

Response of the recently-commissioned TACTIC imaging Cerenkov telescope to atmospheric Cerenkov pulses produced by cosmic gamma-ray and hadron-initiated extensive air-showers has been studied through Monte Carlo simulations, using the CORSIKA code (Capdeville et al., 1992). Preliminary results are presented on the likely performance of 3 different imaging camera sizes : 2.8° x 2.8°, 4.7° x 4.7° and 5.9° x 5.9°, with regard to event calorimetry and characterization.

004247   Kantharia N G (Joint Astronomy Programme, Department of Physics, Indian Institute of Science & Raman Research Institute, Bangalore) : High Rydberg state recombination lines from interstellar carbon : an observational study. Bull Astr Soc India 1999, 27(2), 253-6.

004246   Kanda A;Koul R;Bhatt N;Kaul S R (NO, Bhabha Atomic Research Centre, Nuclear Research Laboratory, Trombay, Mumbai-400 085) : Computer network-based data acquisition and control system for the imaging element of the TACTIC array. Bull Astr Soc India 1999, 27(2), 319-21.

An interrupt-driven distributed Data AcQuisition and Control System (DAQCS) has been developed for the Imaging Element of the 4-element γ-ray telescope array, TACTIC. A network of PCs, running the QNX RTOS, have been coupled to the front-end CAMAC-based instrumentation modules. Single-point monitoring and control from a designated node with an elaborate GUI has greatly facilitated the operation of the telescope and its sophisticated, multi-pixel imaging camera.