2007 (IPP)

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3D Automated Classification Scheme for the TAUVEX data pipeline
(2007-11-28) Bora, Archana; Gupta, Ranjan; Singh, Harinder P.; et al.
In order to develop a pipeline for automated classi cation of stars to be observed by the TAUVEX ultraviolet space Telescope, we employ an arti cial neural network (ANN) technique for classifying stars by using synthetic spectra in the UV region from 1250 A to 3220 A as the training set and International Ultraviolet Explorer (IUE) low resolution spectra as the test set. Both the data sets have been pre-processed to mimic the observations of the TAUVEX ultraviolet imager. We have successfully classi ed 229 stars from the IUE low resolution catalog to within 3-4 spectral sub-class using two di erent simulated training spectra, the TAUVEX spectra of 286 spectral types and UVBLUE spectra of 277 spectral types. Further, we have also been able to obtain the colour excess (i.e. E(B-V) in magnitude units) or the interstellar reddening for those IUE spectra which have known reddening to an accuracy of better than 0.1 magnitudes. It has been shown that even with the limitation of data from just photometric bands, ANNs have not only classi ed the stars, but also provided satisfactory estimates for interstellar extinction. The ANN based classi cation scheme has been successfully tested on the simulated TAUVEX data pipeline. It is expected that the same technique can be employed for data validation in the ultraviolet from the virtual observatories. Finally, the interstellar extinction estimated by applying the ANNs on the TAUVEX data base would provide an extensive extinction map for our galaxy and which could in turn be modeled for the dust distribution in the galaxy.
Acoustic perturbations on steady spherical accretion in Schwarzschild geometry
(2007-06-10) Naskar, Tapan; Ray, Arnab K.
The stationary background ﬂow in the spherically symmetric infall of a compressible ﬂuid, coupled to the space-time deﬁned by the static chwarzschild metric, has been subjected to linearized acoustic perturbations. The perturbative procedure is based on the continuity condition and it shows that the coupling of the ﬂow with the geometry of space-time brings about greater stability for the ﬂow, to the extent that the amplitude of the perturbation, treated as a standing wave, decays in time, as opposed to the amplitude remaining constant in the Newtonian limit. In ualitative terms this situation simulates the effect of a dissipative mechanism in the classical Bondi accretion ﬂow, deﬁned in the Newtonian construct of space and time. As a result of this approach it becomes impossible to deﬁne an acoustic metric for a conserved spherically symmetric ﬂow, described within the framework of Schwarzschild geometry. In keeping with this view, the perturbation, considered eparately as a high-frequency travelling wave, also has its amplitude reduced.
Astronomical Data Management
(2007-01-01) Norris, Ray; Andernach, Heinz; Eichhorn, Guenther; et al.
We present a summary of the major contributions to the Special Session on Data Management held at the IAU General Assembly in Prague in 2006. While recent years have seen enormous improvements in access to astronomical data, and the Virtual Observatory aims to provide astronomers with seamless access to on-line resources, more attention needs to be paid to ensuring the quality and completeness of those resources. For example, data produced by telescopes are not always made available to the astronomical community, and new instruments are sometimes designed and built with insuﬃcient planning for data management, while older but valuable legacy data often remain undigitised. Data and results published in journals do not always appear in the data centres, and astronomers in developing countries sometimes have inadequate access to on-line resources. To address these issues, an “Astronomers Data Manifesto” has been formulated with the aim of initiating a discussion that will lead to the development of a “code of best practice” in astronomical data management.
Axisymmetric black hole accretion in the Kerr metric as an autonomous dynamical system
(2007-02-07) Goswami, Sanghamitra; Khan, Saba Nashreen; Ray, Arnab K.; et al.
In a stationary, general relativistic, axisymmetric, inviscid and rotational accretion ﬂow, described within the Kerr geometric framework, transonicity has been examined by setting up the governing equations of the ﬂow as a ﬁrst-order autonomous dynamical system. The consequent linearised analysis of the critical points of the ﬂow leads to a comprehensive mathematical prescription for classifying these points, showing that the only possibilities are saddle points and centre-type points for all ranges of values of the ﬁxed ﬂow parameters. The spin parameter of the black hole inﬂuences the multitransonic character of the ﬂow, as well as some of its speciﬁc critical properties. The special case of a ﬂow in the space-time of a non-rotating black hole, characterised by the Schwarzschild metric, has also been studied for comparison and the conclusions are compatible with what has been seen for the Kerr geometric case.
Braneworld dynamics in Einstein Gauss Bonnet gravity
(2007-10-10) Maeda, Hideki; Sahni, Varun; Shtanov, Yuri
We discuss the cosmological evolution of a braneworld in ﬁve dimensional Gauss– Bonnet gravity. Our discussion allows the ﬁfth (bulk) dimension to be space-like as well as time-like. The resulting equations of motion have the form of a cubic equation in the H2, (ρ + σ)2 plane, where σ is the brane tension and ρ is the matter density. This allows us to conduct a comprehensive pictorial analysis of cosmological evolution for the Gauss–Bonnet brane. The many interesting properties of this braneworld include the possibility of accelerated expansion at late times. For a ﬁnite region in parameter space the accelerated expansion can be phantom-like so that w < −1. At late times, this branch approaches de Sitter space (w = −1) and avoids the big-rip singularities usually present in phantom models. For a time-like extra dimension the Gauss–Bonnet brane can bounce and avoid the initial singularity.
C iv absorption in damped and sub-damped Lyman-alpha systems: Correlations with metallicity and implications for galactic winds at z~2-3
(2007-07-27) Fox, Andrew J.; Ledoux, C.; Petitjean, Patrick; et al.
We present a study of Civ absorption in a sample of 63 damped Lyman-α (DLA) systems and 11 sub-DLAs in the redshift range 1.75
CMB anisotropy power spectrum using linear combinations of WMAP maps
(2007-06-25) Saha, Rajib; Souradeep, Tarun; Jain, Pankaj; et al.
In recent years the goal of estimating diﬀerent cosmological parameters precisely has set new challenges in the eﬀort to accurately measure the angular power spectrum of CMB. This has required removal of foreground contamination as well as detector noise bias with reliability and precision. Recently, a novel model-independent method for the estimation of CMB angular power spectrum solely from multi-frequency observations has been proposed and implemented on the ﬁrst yearWMAP data by Saha et al. 2006. All previous estimates of power spectrum of CMB are based upon foreground templates using data sets from diﬀerent experiments. However our methodology demonstrates that CMB angular spectrum can be reliably estimated with precision from a self contained analysis of the WMAP data. In this work we provide a detailed description of this method. We also study and identify the biases present in our power spectrum estimate. We apply our methodoly to extract the power spectrum from the WMAP 1 year and 3 year data.
CMB power spectrum estimation with non-circular beam and incomplete sky coverage
(2007-02-05) Mitra, Sanjit; Sengupta, Anand; Souradeep, Tarun; et al.
Over the last decade, measurements of the CMB anisotropy has spearheaded the remarkable transition of cosmology into a precision science. However, addressing the systematic eﬀects in the increasingly sensitive, high resolution, ‘full’ sky measurements from diﬀerent CMB experiments pose a stiﬀ challenge. The analysis techniques must not only be computationally fast to contend with the huge size of the data, but, the higher sensitivity also limits the simplifying assumptions which can then be invoked to achieve the desired speed without compromising the ﬁnal precision goals. While maximum likelihood is desirable, the enormous computational cost makes the suboptimal method of power spectrum estimation using Pseudo-Cl unavoidable for high resolution data. The debiasing of the Pseudo-Cl needs account for non-circular beams, together with non-uniform sky coverage. We provide an analytic framework for correcting the power spectrum for the eﬀect of beam noncircularity and non-uniform sky coverage (including incomplete/masked sky maps). The approach is perturbative in the distortion of the beam from non-circularity allowing for rapid computations when the beam is mildly non-circular. When non-circular beam eﬀect is important, we advocate that it is computationally advantageous to employ ‘soft’ azimuthally apodized masks whose spherical harmonic transform die down fast with m.
Composite Interstellar Grains
(2007-01-20) Vaidya, D.B.; Gupta, Ranjan; Snow, T. P.
A composite dust grain model which is consistent with the observed interstellar extinction and linear polarization is presented. The composite grain is made up of a host silicate spheroid and graphite inclusions. The extinction e ciencies of the composite spheroidal grains for three axial ratios are computed using the discrete dipole approximation (DDA). The interstellar extinction curve is evaluated in the spectral region 3.40{0.10 musing the extinction e ciencies of the composite spheroidal grains. The model extinction curves are then compared with the average observed interstellar extinction curve.We also calculate the linear polarization for the spheroidal composite grains at three orientation angles and nd the wavelength of maximum polarization. Further, we estimate the volume extinction factor, an important parameter from the point of view of cosmic abundance, for the composite grain models that reproduce the average observed interstellar extinction. The estimated abundances derived from the composite grain models for both carbon and silicon are found to be lower than that are predicted by the bare silicate/graphite grain models but these values are still higher than that are implied from the recent ISM values
Critical properties of spherically symmetric accretion in a fractal medium
(2007-07-18) Roy, Nirupam; Ray, Arnab K.
Spherically symmetric transonic accretion of a fractal medium has been studied in both the stationary and the dynamic regimes. The stationary transonic solution is greatly sensitive to inﬁnitesimal deviations in the outer boundary condition, but the ﬂow becomes transonic and stable, when its evolution is followed through time. The evolution towards transonicity is more pronounced for a fractal medium than what is it for a continuum. The dynamic approach also shows that there is a remarkable closeness between an equation of motion for a perturbation in the ﬂow, and the metric of an analogue acoustic black hole. The stationary inﬂow solutions of a fractal medium are as much stable under the inﬂuence of linearised perturbations, as they are for the ﬂuid continuum.
Critical properties of spherically symmetric black hole accretion in Schwarzschild geometry
(2007-02-28) Mandal, Ipsita; Ray, Arnab K.; Das, Tapas K.
The stationary spherically symmetric accretion ﬂow in the Schwarzschild metric has been set up as an autonomous ﬁrst-order dynamical system, and it has been studied completely analytically. Of the three possible critical points in the ﬂow, the one that is physically realistic behaves like the saddle point of the standard Bondi accretion problem. One of the two remaining critical points exhibits the strange mathematical behaviour of being either a saddle point or a centre-type point, depending on the values of the ﬂow parameters. The third critical point is always unphysical and behaves like a centre-type point. The treatment has been extended to pseudo-Schwarzschild ﬂows for comparison with the general relativistic analysis.
Cross-correlation search for periodic gravitational waves
(2007-12-10) Dhurandhar, Sanjeev; Mukhopadhyay, Himan; Badri, Krishnan; et al.
In this paper we study the use of cross-correlations between multiple gravitational wave (GW) data streams for detecting long-lived periodic signals. Cross orrelation searches between data from multiple detectors have traditionally been used to search for stochastic GW signals, but recently they have also been used in directed searches for periodic GWs. Here we further adapt the cross-correlation statistic for periodic GW searches by taking into account both the non-stationarity and the long term-phase coherence of the signal. We study the statistical properties and sensitivity of this search, its relation to existing periodic wave searches, and describe the precise way in which the cross- correlation statistic interpolates between semi-coherent and fully-coherent methods. Depending on the maximum duration over we wish to preserve phase coherence, the cross-correlation statistic can be tuned to go from a standard cross-correlation statistic using data from distinct detectors, to the semi-coherent time-frequency methods with increasing coherent time baselines, and all the way to a full coherent search. This leads to a uni ed framework for studying periodic wave searches and can be used to make informed trade-o s between computational cost, sensitivity, and robustness against signal uncertainties.
Detecting gravitional waves from inspiraling binaries with a network of detectors: coherent strategies by correlated detectors
(2007-02-03) Tagoshi, H.; Mukhopadhyay, Himan; Dhurandhar, Sanjeev; et al.
We discuss the coherent search strategy to detect gravitational waves from inspiraling compact binaries by a network of correlated laser interferometric detectors. From the maximum likelihood ratio statistic, we obtain a coherent statistic which is slightly diﬀerent from and generally better than what we obtained in our previous work. In the special case when the cross spectrum of two detectors normalized by the power spectrum density is constant, the new statistic agrees with the old one. The quantitative diﬀerence of the detection probability for a given false alarm rate is also evaluated in a simple case.
Estimation of Primordial Spectrum with post-WMAP 3 year data
(2007-07-14) Shafieloo, Arman; Souradeep, Tarun
In this paper we implement an improved (error sensitive) Richardson-Lucy deconvolution algorithm on the measured angular power spectrum from the WMAP 3 year data to determine the primordial power spectrum assuming diﬀerent points in the cosmological parameter space for a ﬂat ΛCDM cosmological model. We also present the preliminary results of the cosmological parameter estimation by assuming a free form of the primordial spectrum, for a reasonably large volume of the parameter space. The recovered spectrum for a considerably large number of the points in the cosmological parameter space has a likelihood far better than a ‘best ﬁt’ power law spectrum up to ∆χ2 eﬀ ≈ −30. We use Discrete Wavelet Transform (DWT) for smoothing the raw recovered spectrum from the binned data. The results obtained here reconﬁrm and sharpen the conclusion drawn from our previous analysis of the WMAP 1st year data. A sharp cut oﬀ around the horizon scale and a bump after the horizon scale seem to be a common feature for all of these reconstructed primordial spectra. We have shown that although the WMAP 3 year data prefers a lower value of matter density for a power law form of the primordial spectrum, for a free form of the spectrum, we can get a very good likelihood to the data for higher values of matter density. We have also shown that even a ﬂat CDM model, allowing a free form of the primordial spectrum, can give a very high likelihood ﬁt to the data. Theoretical interpretation of the results is open to the cosmology community. However, this work provides strong evidence that the data retains discriminatory power in the cosmological parameter space even when there is full freedom in choosing the primordial spectrum.
Evidence for overdensity around z_em > 4 quasars from the proximity effect
(2007-02-14) Guimaraes, R.; Srianand, R.; Petitjean, Patrick; et al.
We study the density ﬁeld around zem > 4 quasars using high quality medium spectral resolution ESI-Keck spectra (R ∼ 4300, SNR > 25) of 45 high-redshift quasars selected from a total of 95 spectra. This large sample considerably increases the statistics compared to previous studies. The redshift evolution of the mean photo-ionization rate and the median optical depth of the intergalactic medium (IGM) are derived statistically from the observed transmitted ﬂux and the pixel optical depth probability distribution function respectively. This is used to study the so-called proximity effect, that is, the observed decrease of the median optical depth of the IGM in the vicinity of the quasar caused by enhanced photo-ionization rate due to photons emitted by the quasar. We show that the proximity effect is correlated with the luminosity of the quasars, as expected. By comparing the observed decrease of the median optical depth with the theoretical expectation we ﬁnd that the optical depth does not decrease as rapidly as expected when approaching the quasar if the gas in its vicinity is part of the standard IGM. We interpret this effect as revealing gaseous overdensities on scales as large as ∼15h−1 Mpc. The mean overdensity is of the order of two and ﬁve within, respectively, 10 and 3h−1 Mpc. If true, this would indicate that high redshift quasars are located in the center of overdense regions that could evolve with time into massive clusters of galaxies. The overdensity is correlated with luminosity: brighter quasars show higher overdensities.
Evolution of transonicity in an accretion disc
(2007-01-10) Ray, Arnab K.; Bhattacharjee, Jayanta K.
For inviscid, rotational accretion ows driven by a general pseudo Newtonian potential on to a Schwarzschild black hole, the only possible xed points are saddle points and centre-type points. For the speci c choice of the Newtonian potential, the ow has only two critical points, of which the outer one is a saddle point while the inner one is a centre-type point. A restrictive upper bound is imposed on the admissible range of values of the angular momentum of sub-Keplerian ows through a saddle point. These ows are very unstable to any deviation from a necessarily precise boundary condition. The di culties against the physical realisability of a solution passing through the saddle point have been addressed through a temporal evolution of the ow, which gives a non-perturbative mechanism for selecting a transonic solution passing through the saddle point. An equation of motion for a real-time perturbation about the stationary ows reveals a very close correspondence with the metric of an acoustic black hole, which is also an indication of the primacy of transonicity
Excitation mechanisms in newly discovered H2-bearing Damped Lyman-alpha clouds: systems with low molecular fractions
(2007-07-30) Noterdaeme, P.; Ledoux, C.; Petitjean, Patrick; et al.
Aims.We probe the physical conditions in high-redshift damped Lyman-α systems (DLAs) using the observed molecular fraction and the rotational excitation of molecular hydrogen. Methods. We search for Lyman- and Werner-band absorption lines of molecular hydrogen in the VLT/UVES spectra of background QSOs at the redshift of known DLAs. Results. We report two new detections of molecular hydrogen in the systems at zabs = 2.402 and 1.989 toward, respectively, HE 0027−1836 and HE 2318−1107, discovered in the course of the Hamburg-ESO DLA survey. We also present a detailed analysis of our recent H2 detection toward Q2343+125. All three systems have low molecular fractions, log f ≤ −4, with f = 2N(H2)/(2N(H2) + N(H)). Only one such H2 system was known previously. Two of them (toward Q2343+125 and HE 2318−1107) have high-metallicities, [X/H] > −1, whereas the DLA toward HE 0027−1836 is the system with the lowest metallicity ([Zn/H] = −1.63) among known H2-bearing DLAs. The depletion patterns for Si, S, Ti, Cr, Mn, Fe and Ni in the three systems are found to be very similar to what is observed in diﬀuse gas of the Galactic halo. Molecular hydrogen absorption from rotational levels up to J = 5 is observed in a single well-deﬁned component toward HE 0027−1836. We show that the width (Doppler parameter) of the H2 lines increases with increasing J and that the kinetic energy derived from the Doppler parameter is linearly dependent on the relative energy of the rotational levels. There is however no velocity shift between lines from diﬀerent rotational levels. The excitation temperature is found to be 90 K for J = 0 to J = 2 and ∼500 K for higher J levels. Single isothermal PDR models fail to reproduce the observed rotational excitations. A two-component model is needed: one component of low density (∼50 cm−3 ) with weak illumination (χ = 1) to explain the J ≤ 2 rotational levels and another of high density (∼500 cm−3 ) with strong illumination (χ = 30) for J ≥ 3 levels. However, the juxtaposition of these two PDR components may be ad-hoc and the multicomponent structure could result either from turbulent dissipation or C-shocks.
Galactic dynamos supported by magnetic helicity fluxes
(2007-03-08) Sur, Sharanya; Shukurov, A.; Subramanian, Kandaswamy
We present a simple semi-analytical model of nonlinear, mean-ﬁeld galactic dynamos and use it to study the effects of various magnetic helicity ﬂuxes. The dynamo equations are reduced using the ‘no-z’ approximation to a nonlinear system of ordinary differential equations in time; we demonstrate that the model reproduces accurately earlier results, including those where nonlinear behaviour is driven by a magnetic helicity ﬂux. We discuss the implications and interplay of two types of magnetic helicity ﬂux, one produced by advection (e.g., due to the galactic fountain or wind) and the other, arising fromanisotropy of turbulence as suggested by Vishniac & Cho (2001). We argue that the latter is signiﬁcant if the galactic differential rotation is strong enough: in ourmodel, forRω . −10 in terms of the corresponding turbulent magnetic Reynolds number. We conﬁrm that the intensity of gas outﬂow from the galactic disc optimal for the dynamo action is close to that expected for normal spiral galaxies. The steady-state strength of the large-scale magnetic ﬁeld supported by the helicity advection is still weaker than that corresponding to equipartition with the turbulent energy. However, the Vishniac-Cho helicity ﬂux can boost magnetic ﬁeld further to achieve energy equipartition with turbulence. For stronger outﬂows that may occur in starburst galaxies, the Vishniac-Cho ﬂux can be essential for the dynamo action. However, this mechanism requires a large-scale magnetic ﬁeld of at least≃ 1 Gto be launched, so that it has to be preceded by a conventional dynamo assisted by the advection of magnetic helicity by the fountain or wind.
Giant Ringlike Radio Structures Around Galaxy Cluster Abell 3376
(2007-11-09) Bagchi, Joydeep; Durret, Florence; Paul, Surajit; et al.
In the current paradigm of cold dark matter cosmology, large-scale structures are assembling through hierarchical clustering of matter. In this process, an important role is played by megaparsec (Mpc)-scale cosmic shock waves, arising in gravity-driven supersonic ﬂows of intergalactic matter onto dark matter-dominated collapsing structures such as pancakes, ﬁlaments and clusters of galaxies. Here we report Very Large Array telescope observations of giant (∼ 2 Mpc ×1.6 Mpc), ring-shaped non-thermal radio emitting structures, found at the outskirts of the rich cluster of galaxies Abell 3376. These structures may trace the elusive shock waves of cosmological large scale matter ﬂows, which are energetic enough to power them. These radio sources may also be the acceleration sites where magnetic shocks are possibly boosting 1cosmic-ray particles with energies of up to 1018 to 1019 electron volts.
Gravitational collapse of radiating Dyon solution and cosmic censorship hypothesis
(2007-11-20) Patil, K. D.; Zade, S. S.; Mohod, A. N.
We investigate the possibility of cosmic censorship violation in the gravitational collapse of radiating dyon solution. We show that the ﬁnal outcome of the collapse depends sensitively on the electric and magnetic charge parameters. The graphs of the outer apparent horizon, inner Cauchy horizon for diﬀerent values of parameters have been drawn. It is found that the the electric and magnetic components push the apparent horizon towards the advanced time coordinate axis, which in turn reduces the radius of the apparent horizon in Vaidya space- time. In the present paper we extend the earlier work of A. Chamorro and K. S. Virbhadra.