2004 (IPP)

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Angular momentum transport in quasi-Keplerian accretion disks
(2011-07-06) Subramanian, Prasad; Pujari, B. S.; Becker, Peter A.
We reexamine arguments advanced by Hayashi &Matsuda (2001), who claim that several simple, physically motivated deriva- tions based on mean free path theory for calculating the viscous torque in a quasi-Keplerian accretion disk yield results that are inconsistent with the generally accepted model. If correct, the ideas proposed by Hayashi & Matsuda would radically alter our understanding of the na- ture of the angular momentum transport in the disk, which is a central feature of accretion disk theory. However, in this paper we point out several fallacies in their arguments and show that there indeed exists a simple derivation based on mean free path theory that yields an expres- sion for the viscous torque that is proportional to the radial derivative of the angular velocity in the accretion disk, as expected. The deriva- tion is based on the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disk.
Aspects of scalar field dynamics in Gauss-Bonnet Brane Worlds
(2011-07-06) Sami, M.; Savchenko, N.; Toporensky, A.
The Einstein-Gauss-Bonnet equations projected from the bulk to brane lead to a complicated Friedmann equation which simpliﬁes to H2 ∼ ρq in the asymptotic regimes. The Randall-Sundrum (RS) scenario corresponds to q = 2 whereas q = 2/3 & q = 1 give rise to high energy Gauss-Bonnet (GB) regime and the standard GR respectively. Amazingly, while evolving from RS regime to high energy GB limit, one passes through a GR like region which has important implications for brane world inﬂation. For tachyon GB inﬂation with potentials V (φ) ∼ φp investigated in this paper, the scalar to tensor ratio of perturbations R is maximum around the RS region and is generally suppressed in the high energy regime for the positive values of p. The ratio is very low for p > 0 at all energy scales relative to GB inﬂation with ordinary scalar ﬁeld. The models based upon tachyon inﬂation with polynomial type of potentials with generic positive values of p turn out to be in the 1σ observational contour bound at all energy scales varying from GR to high energy GB limit. The spectral index nS improves for the lower values of p and approaches its scale invariant limit for p = −2 in the high energy GB regime. The ratio R also remains small for large negative values of p, however, diﬀerence arises for models close to scale invariance limit. In this case, the tensor to scale ratio is large in the GB regime whereas it is suppressed in the intermediate region between RS and GB. Within the frame work of patch cosmologies governed by H2 ∼ ρq , the behavior of ordinary scalar ﬁeld near cosmological singularity and the nature of scaling solutions are distinguished for the values of q < 1 and q > 1. The tachyon dynamics, on the other hand, exhibits stable scaling solutions ∀q if the adiabatic
Automated Classification of 2000 Bright IRAS Sources
(2011-07-06) Gupta, Ranjan; Singh, Harinder P.; Volk, K.; et al.
An Artiﬁcial Neural Network (ANN) scheme has been employed that uses a supervised back-propagation algorithm to classify 2000 bright sources from the Calgary database of IRAS (Infrared Astronomical Satellite) spectra in the region 8µm to 23µm. The database has been classiﬁed into 17 predeﬁned classes based on the spectral morphology. We have been able to classify over 80 percent of the sources correctly in the ﬁrst instance. The speed and robustness of the scheme will allow us to classify the whole of the LRS database, containing more that 50,000 sources, in the near future.
BVR photometry of a newly identified RS CVn binary star HD 61396
(2011-07-06) Barway, Sudhanshu; Pandey, S. K.
BVR photometry of a recently identiﬁed RS CVn binary star HD 61396, carried out during 2001, is presented. The new photometry reveal signiﬁcant evolution in the shape and amplitude of light curve when compared with those reported earlier by Padmakar et al.(2000). The traditional two-starspot model has been used to obtain the spot parameters from the observed light curve. Changes in the spot area and their location on the stellar surface are discernible from the extracted parameters from the new photometry.
Case for dynamical dark energy revisited
(2011-07-06) Alam, Ujjaini; Sahni, Varun; Starobinsky, A. A.
We investigate the behaviour of dark energy using the recently released supernova data of Riess et al., 2004 and a model independent parameterization for dark energy (DE). We ﬁnd that, if no priors are imposed on Ω0m and h, DE which evolves with time provides a better ﬁt to the SNe data than ΛCDM. This is also true if we include results from the WMAP CMB data. From a joint analysis of SNe+CMB, the best-ﬁt DE model has w0 < ∼ − 1 at the present epoch and the transition from deceleration to acceleration occurs at zT = 0.39±0.03. However, DE evolution becomes weaker if the ΛCDM based CMB results Ω0m = 0.27 ± 0.04, h = 0.71 ± 0.06 are incorporated in the analysis. In this case, zT = 0.57±0.07. Our results also show that the extent of DE evolution is sensitive to the manner in which the supernova data is sampled.
CMB power spectrum estimation using non-circular beam
(2011-07-06) Mitra, Sanjit; Sengupta, Anand; Souradeep, Tarun
The measurements of the angular power spectrum of the Cosmic Microwave Background (CMB) anisotropy has proved crucial to the emergence of cosmology as a precision science in recent years. In this remarkable data rich period, the limitations to precision now arise from the the inability to account for ﬁner systematic eﬀects in data analysis. The non-circularity of the experimental beam has become progressively important as CMB experiments strive to attain higher angular resolution and sensitivity. We present an analytic framework for studying the leading order eﬀects of a non- circular beam on the CMB power spectrum estimation. We consider a non-circular beam of ﬁxed shape but variable orientation. We compute the bias in the pseudo-Cl power spectrum estimator and then construct an unbiased estimator using the bias matrix. The covariance matrix of the unbiased estimator is computed for smooth, non-circular beams. Quantitative results are shown for CMB maps made by a hypothetical experiment with a non-circular beam comparable to our ﬁts to the WMAP beam maps described in the appendix and uses a toy scan strategy. We ﬁnd that signiﬁcant eﬀects on CMB power spectrum can arise due to non-circular beam on multipoles comparable to, and beyond, the inverse average beam-width where the pseudo-Cl approach may be the method of choice due to computational limitations of analyzing the large datasets from current and near future CMB experiments.
Comprehensive CCd photometric study of the open clusterNGC 2421
(2011-07-06) Yadav, R. K. S.; Sagar, Ram
We present the UBV RI CCD photometry in the region of the open cluster NGC 2421. Radius of the cluster is determined as ∼ 3′ .0 using stellar density proﬁle. Our Study indicates that metallicity of the cluster is Z ∼ 0.004. The reddening E(B − V ) = 0.42±0.05 mag is determined using two colour (U − B) versus (B − V ) diagram. By combining the 2MASS JHK data with the optical data we determined E(J −K) = 0.20±0.20 mag and E(V −K) = 1.15±0.20 mag for this cluster. Colour- excess diagrams show normal interstellar extinction law in the direction of the cluster. We determined the distance of the cluster as 2.2±0.2 Kpc by comparing the ZAMS with the intrinsic CM diagram of the cluster. The age of the cluster has been esti- mated as 80±20 Myr using the stellar isochrones of metallicity Z = 0.004. The mass function slope x = 1.2±0.3 has been derived by applying the corrections of ﬁeld stars contamination and data incompleteness. Our analysis indicate that the cluster NGC 2421 is dynamically relaxed.
Constraints on Dirac-Born-Infeld type dark energy models from varying alpha
(2011-07-06) Garousi, M. R.; Sami, M.; Tsujikawa, Shinji
We study the variation of the eﬀective ﬁne structure constant alpha for Dirac-Born-Infeld (DBI) type dark energy models. The DBI action based on string theory naturally gives rise to a coupling between gauge ﬁelds and a scalar ﬁeld responsible for accelerated expansion of the universe. This leads to the change of alpha due to a dynamical evolution of the scalar ﬁeld, which can be compatible with the recently observed cosmological data around the redshift ˜ z < ∼ 3. We place constraints on several diﬀerent DBI models including exponential, inverse power-law and rolling massive scalar potentials. We ﬁnd that these models can satisfy the varying alpha constraint provided that mass scales of the potentials are ﬁne-tuned. When we adopt the mass scales which are motivated by string theory, both exponential and inverse power-law potentials give unacceptably large change of alpha, thus ruled out from observations. On the other hand the rolling massive scalar potential is compatible with the observationally allowed variation of alpha. Therefore the information of varying alpha provides a powerful way to distinguish between a number of string-inspired DBI dark energy models.
Cosmological perturbations from brane inflation with a Gauss-Bonnet term
(2011-07-06) Dufaux, Jean-Francois; Lidsey, James E.; Roy, Maartens
Braneworld inﬂation is a phenomenology related to string theory that describes high-energy mod- iﬁcations to general relativistic inﬂation. The observable universe is a braneworld embedded in 5-dimensional anti de Sitter spacetime. When the 5-dimensional action is Einstein-Hilbert, we have a Randall-Sundrum type braneworld. The amplitude of tensor and scalar perturbations from in- ﬂation is strongly increased relative to the standard results, although the ratio of tensor to scalar amplitudes still obeys the standard consistency relation. If a Gauss-Bonnet term is included in the action, as a high-energy correction motivated by string theory, we show that there are important changes to the Randall-Sundrum case. We give an exact analysis of the tensor perturbations. They satisfy the same wave equation and have the same spectrum as in the Randall-Sundrum case, but the Gauss-Bonnet change to the junction conditions leads to a modiﬁed amplitude of gravitational waves. The amplitude is no longer monotonically increasing with energy scale, but decreases asymp- totically after an initial rise above the standard level. Using an approximation that neglects bulk eﬀects, we show that the amplitude of scalar perturbations has a qualitatively similar behaviour to the tensor amplitude. In addition, the tensor to scalar ratio breaks the standard consistency relation.
Cosmology from rolling massive scalar field on the anti-D3 Brane of de Sitter Vacua
(2012-07-06) Garousi, M. R.; Sami, M.; Tsujikawa, Shinji
We investigate a string-inspired scenario associated with a rolling massive scalar ﬁeld on D-branes and discuss its cosmological implications. In particular, we discuss cosmological evolution of the massive scalar ﬁeld on the ant-D3 brane of KKLT vacua. Unlike the case of tachyon ﬁeld, because of the warp factor of the anti-D3 brane, it is possible to obtain the required level of the amplitude of density perturbations. We study the spectra of scalar and tensor perturbations generated during the rolling scalar inﬂation and show that our scenario satisﬁes the observational constraint coming from the Cosmic Microwave Background anisotropies and other observational data. We also implement the negative cosmological constant arising from the stabilization of the modulus ﬁelds in the KKLT vacua and ﬁnd that this leads to a successful reheating in which the energy density of the scalar ﬁeld eﬀectively scales as a pressureless dust. The present dark energy can be also explained in our scenario provided that the potential energy of the massive rolling scalar does not exactly cancel with the amplitude of the negative cosmological constant at the potential minimum.
Current status of observational cosmology
(2011-07-06) Ostriker, Jeremiah P.; Souradeep, Tarun
Observational cosmology has indeed made very rapid progress in recent years. The ability to quantify the universe has largely improved due to observational constraints coming from structure formation. The transition to precision cosmology has been spear- headed by measurements of the anisotropy in the cosmic microwave background (CMB) over the past decade. Observations of the large scale structure in the distribution of galax- ies, high red-shift supernova, have provided the required complementary information. We review the current status of cosmological parameter estimates from joint analysis of CMB anisotropy and large scale structure (LSS) data. We also sound a note of caution on overstating the successes achieved thus far.
Cylindrically symmetric inhomogeneous cosmological models with viscous fluid & varying /\>
(2012-07-06) Pradhan, A.; Singh, P. K.; Jotania, Kanti
Cylindrically symmetric non-static cosmological models representing a bulk viscous fluid distribution have been obtained which are inhomogeneous and anisotropic. Without assuming any adhoc law, we obtain a cosmological constant as a decreasing function of time. Various physical and geometrical features of the models are also discussed.
Dark matter and dark energy
(2011-07-06) Sahni, Varun
Abstract. I brieﬂy review our current understanding of dark matter and dark en- ergy. The ﬁrst part of this paper focusses on issues pertaining to dark matter includ- ing observational evidence for its existence, current constraints and the ‘abundance of substructure’ and ‘cuspy core’ issues which arise in CDM. I also brieﬂy describe MOND. The second part of this review focusses on dark energy. In this part I dis- cuss the signiﬁcance of the cosmological constant problem which leads to a predicted value of the cosmological constant which is almost 10123 times larger than the ob- served value Λ/8πG ≃ 10−47 GeV4 . Setting Λ to this small value ensures that the acceleration of the universe is a fairly recent phenomenon giving rise to the ‘cosmic coincidence’ conundrum according to which we live during a special epoch when the density in matter and Λ are almost equal. Anthropic arguments are brieﬂy dis- cussed but more emphasis is placed upon dynamical dark energy models in which the equation of state is time dependent. These include Quintessence, Braneworld models, Chaplygin gas and Phantom energy. Model independent methods to deter- mine the cosmic equation of state and the Stateﬁnder diagnostic are also discussed. The Stateﬁnder has the attractive property ... a /aH3 = 1 for LCDM, which is helpful for diﬀerentiating between LCDM and rival dark energy models. The review ends with a brief discussion of the fate of the universe in dark energy models.
Data analysis techniques for gravitational wave observations
(2011-07-06) Dhurandhar, Sanjeev
Astrophysical sources of gravitational waves fall broadly into three categories: (i) transient and bursts, (ii) periodic or continuous wave and (iii) stochastic. Each type of source requires a di®erent type of data analysis strategy. In this talk various data analysis strategies will be reviewed. Optimal ¯ltering is used for extracting binary inspirals; Fourier transforms over Doppler shifted time intervals are computed for long duration periodic sources; optimally weighted cross-correlations for stochastic background. Some recent schemes which e±ciently search for inspirals will be described. The performance of some of these techniques on real data obtained will be discussed. Finally, some results on cancellation of systematic noises in laser interferometric space antenna (LISA) will be presented and future directions indicated.
Evalution of a CMDS imager as readout for imaging photon counters
(2011-07-06) Tandon, S. N.
This work is motivated by a desire to check if CMOS detectors, which are somewhat inferior to CCDs in terms of noise and dark current, but are much simpler to use, could be used as readout in imaging photon counters. Tests have been made on the performance of a CMOS detector used as readout of an image intensifier. The main conclusions are that: a) the detector has adequate dynamic range as well as sensitivity for photon counting, b) with an optics for reduction by a factor about three between the intensifier and the readout, it is found that 10-20 micron wide slits are recorded with an rms width of - 0.14 pixels, i.e. - 11 microns on the slit-plane - this includes effects due to finite resolution of the intensifier, and c) the detector is capable of giving a rms precision of - 0.03 pixels ( - 0.7 microns) on centroids of images similar to those of intensified photon events.
From Gravitons to Gravity: Myths and Reality
(2011-07-06) Padmanabhan, T.
email: nabhan@iucaa.ernet.in There is a general belief, reinforced by statements in standard textbooks, that: (i) one can obtain the full non-linear Einstein’s theory of gravity by coupling a massless, spin-2 ﬁeld hab self-consistently to the total energy momentum tensor, including its own; (ii) this procedure is unique and leads to Einstein-Hilbert action and (iii) it only uses standard concepts in Lorentz invariant ﬁeld theory and does not involve any geometrical assumptions. After providing several reasons why such beliefs are suspect — and critically re-examining several previous attempts — we provide a detailed analysis aimed at clarifying the situation. First, we prove that it is impossible to obtain the Einstein-Hilbert (EH) action, starting from the standard action for gravitons in linear theory and iterating repeatedly. This result follows from the fact that EH action has a part (viz. the surface term arising from second derivatives of the metric tensor) which is non-analytic in the coupling constant, when expanded in terms of the graviton ﬁeld. Thus, at best, one can only hope to obtain the remaining, quadratic, part of the EH Lagrangian (viz. the Γ2 lagrangian) if no additional assumptions are made. Second, we use the Taylor series expansion of the action for Einstein’s theory, to identify the tensor Sab , to which the graviton ﬁeld hab couples to the lowest order (through a term of the form Sab hab in the lagrangian). We show that the second rank tensor Sab is not the conventional energy momentum tensor Tab of the graviton and provide an explanation for this feature. Third, we construct the full nonlinear Einstein’s theory with the source being spin-0 ﬁeld, spin-1 ﬁeld or relativistic particles by explicitly coupling the spin-2 ﬁeld to this second rank tensor Sab order by order and summing up the inﬁnite series. Finally, we construct the theory obtained by self consistently coupling hab to the conventional energy momentum tensor Tab order by order and show that this does not lead to Einstein’s theory. The implications are discussed.
Fundamentals of the LISA Stable Flight Formation
(2011-07-06) Dhurandhar, Sanjeev; Nayak, K. R.; Koshti, S.
General Relativistic Effects of Strong Magnetic Fields on the gravitational force: A driving engine for Gamma-$ SGRM?
(2012-07-06) Malheiro, Manuel; Ray, Subharthi; Mosquera Cuesta, Herman J.; et al.
Abstract. In general relativity all forms of energy contribute to gravity and not only just ordinary matter as in Newtonian Physics. This fact can be seen in the modiﬁed hydrostatic equilibrium equation for relativistic stars pervaded by magnetic (B) ﬁelds. It has an additional term coupled to the matter part as well as an anisotropic term which is purely of magnetic origin. That additional term coming from the pressure changed by the radial component of the diagonal electromagnetic ﬁeld tensor, weakens the gravitational force when B is strong enough and can even produce an unexpected change in the attractive nature of the force by reversing its sign. In an extreme case, this new general relativistic (GR) eﬀect can even trigger an instability in the star as a consequence of the sudden reversal of the hydrostatic pressure gradient. We suggest here that this GR eﬀect may be the possible central engine driving the transient giant outbursts observed in Soft Gamma-ray Repeaters (SGRs). In small regions of the neutron star (NS), strong magnetic condensation can take place. Beyond a critical limit, these highly magnetised bubbles may explode releasing the trapped energy as a burst of γ-rays of ∼ 1036−40 erg.
Generating dynamical black hole solutions
(2011-07-06) Dawood, A.K.; Ghosh, S. G.
We prove a theorem that characterizes a large family of non-static solutions to Einstein equations, representing, in general, spherically symmetric Type II ﬂuid. It is shown that the best known dynamical black hole solutions to Einstein equations are particular cases from this family. Thus we extend a recent work of Salgado [12] to non-static case. The spherically symmetric static black hole solutions, for Type I ﬂuid, are also retrieved.
Generation of electromagnetic fields in string cosmology with a massive scalar field on the anti D-brane
(2012-07-06) Garousi, M. R.; Sami, M.; Tsujikawa, Shinji
We study the generation of electromagnetic ﬁelds in a string-inspired scenario associated with a rolling massive scalar ﬁeld φ on the anti-D3 branes of KKLT de Sitter vacua. The 4-dimensional DBI type eﬀective action naturally gives rise to the coupling between the gauge ﬁelds and the inﬂaton φ, which leads to the production of cosmological magnetic ﬁelds during inﬂation due to the breaking of conformal invariance. We ﬁnd that the amplitude of magnetic ﬁelds at decoupling epoch can be larger than the limiting seed value required for the galactic dynamo. We also discuss the mechanism of reheating in our scenario and show that gauge ﬁelds are suﬃciently enhanced for the modes deep inside the Hubble radius with an energy density greater than that of the inﬂaton.