2006 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/335

Browse

Now showing 1 - 20 of 55

On relativistic models of strange stars
(2006-01-01) Tikekar, R.; Jotania, Kanti
The superdense stars with mass to size ratio greater than 0.3 are expected to be made up of strange matter. The ansatz prescrbing ge- ometry of a 3-paraboloid immersed in 4-dimensional Euclidean space for the 3-space of the interior spacetime of superdense star is shown to provide two parameter family of physically viable relativistic models of strange stars. The compact stellar con gurations with mass to size ratio greater than 0.3 are likely to admit only strange star interpretation.
Plane symmetric inhomogeneous cosmological models with a perfect fluid in general relativity
(2006-01-01) Pradhan, Anirudh; Pandey, Purnima; Singh, Sunil Kumar
In this paper we investigate a class of solutions of Einstein equations for the plane- symmetric perfect ﬂuid case with shear and vanishing acceleration. If these solutions have shear, they must necessarily be non-static. We examine the integrable cases of the ﬁeld equations systematically. Among the cases with shear we ﬁnd three classes of solutions.
Cosmic mimicry: Is LCDM a braneworld in disguise?
(2006-01-01) Sahni, Varun; Shtanov, Yuri; Viznyuk, Alexander
Features in the primordial spectrum from WMAP: A wavelet analysis
(2006-01-01) Shafieloo, Arman; Souradeep, Tarun; Manimaran, P.
Precise measurements of the anisotropies in the cosmic microwave background enable us to do an accurate study on the form of the primordial power spectrum for a given set of cosmological parameters. In a previous paper [1], we implemented an improved (error sensitive) Richardson-Lucy deconvolution algorithm on the measured angular power spectrum from the ﬁrst year of WMAP data to determine the primordial power spectrum assuming a concordance cosmological model. This recovered spectrum has a likelihood far better than a scale invariant, or, ‘best ﬁt’ scale free spectra (∆ln L ≈ 25 w.r.t. Harrison Zeldovich, and, ∆ln L ≈ 11 w.r.t. power law with ns = 0.95). In this paper we use Discrete Wavelet Transform (DWT) to decompose the local features of the recovered spectrum individually to study their eﬀect and signiﬁcance on the recovered angular power spectrum and hence the likelihood. We show that besides the infra-red cut oﬀ at the horizon scale, the associated features of the primordial power spectrum around the horizon have a signiﬁcant eﬀect on improving the likelihood. The strong features are localized at the horizon scale.
Smoothing supernova data to reconstruct the expansion history of the universe
(2006-01-10) Shaﬁeloo, Arman; Alam, Ujjaini; Sahni, Varun
We propose a non-parametric method of smoothing supernova data over redshift using a Gaussian kernel in order to reconstruct important cosmological quantities including H(z) and w(z) in a model independent manner. This method is shown to be successful in discriminating between diﬀerent models of dark energy when the quality of data is commensurate with that expected from the future SuperNova Acceleration Probe (SNAP). We ﬁnd that the Hubble parameter is especially well-determined and useful for this purpose. The look back time of the universe may also be determined to a very high degree of accuracy ( < ∼ 0.2%) in this method. By reﬁning the method, it is also possible to obtain reasonable bounds on the equation of state of dark energy. We explore a new diagnostic of dark energy– the ‘w-probe’– which can be calculated from the ﬁrst derivative of the data. We ﬁnd that this diagnostic is reconstructed extremely accurately for diﬀerent reconstruction methods even if Ω0m is marginalized over. The w-probe can be used to successfully distinguish between ΛCDM and other models of dark energy to a high degree of accuracy.
Strong mean field dynamos require supercritical helicity fluxes
(2006-01-10) Brandenburg, Axel; Subramanian, Kandaswamy
Several one and two dimensional mean ﬁeld models are analyzed where the effects of current helicity ﬂuxes and boundaries are included within the framework of the dynamical quenching model. In contrast to the case with periodic boundary conditions, the ﬁnal saturation energy of the mean ﬁeld decreases inversely proportional to the magnetic Reynolds number. If a nondimensional scaling factor in the current helicity ﬂux exceeds a certain critical value, the dynamo can operate even without kinetic helicity, i.e. it is based only on shear and current helicity ﬂuxes, as ﬁrst suggested by Vishniac & Cho (2001, ApJ 550, 752). Only above this threshold is the current helicity ﬂux also able to alleviate catastrophic quenching. The fact that certain turbulence simulations have now shown apparently non-resistively limited mean ﬁeld saturation amplitudes may be suggestive of the current helicity ﬂux having exceeded this critical value. Even below this critical value the ﬁeld still reaches appreciable strength at the end of the kinematic phase, which is in qualitative agreement with dynamos in periodic domains. However, for large magnetic Reynolds numbers the ﬁeld undergoes subsequent variations on a resistive time scale when, for long periods, the ﬁeld can be extremely weak.
Astrophysical magnetic fields and nonlinear dynamo theory
(2006-01-10) Brandenburg, Axel; Subramanian, Kandaswamy
The current understanding of astrophysical magnetic ﬁelds is reviewed, focusing on their generation and maintenance by turbulence. In the astrophysical context this generation is usually explained by a self-excited dynamo, which involves ﬂows that can amplify a weak ‘seed’ magnetic ﬁeld exponentially fast. Particular emphasis is placed on the nonlinear saturation of the dynamo. Analytic and numerical results are discussed both for small scale dynamos, which are completely isotropic, and for large scale dynamos, where some form of parity breaking is crucial. Central to the discussion of large scale dynamos is the so-called alpha effect which explains the generation of a mean ﬁeld if the turbulence lacks mirror symmetry, i.e. if the ﬂow has kinetic helicity. Large scale dynamos produce small scale helical ﬁelds as a waste product that quench the large scale dynamo and hence the alpha effect. With this in mind, the microscopic theory of the alpha effect is revisited in full detail and recent results for the loss of helical magnetic ﬁelds are reviewed.
Non subjective approach to the GP algorithm for analysing noisy time series
(2006-01-10) Harikrishnan, K. P.; Misra, Ranjeev; Ambika, G.; et al.
We present an adaptation of the standard Grassberger-Proccacia (GP) algorithm for estimating the Correlation Dimension of a time series in a non subjective manner. The validity and accuracy of this approach is tested using di erent types of time series, such as, those from standard chaotic systems, pure white and colored noise and chaotic systems added with noise. The e ectiveness of the scheme in analysing noisy time series, particularly those involving colored noise, is investigated. An interesting result we have obtained is that, for the same percentage of noise addition, data with colored noise is more distinguishable from the corresponding surrogates, than data with white noise. As examples for real life applications, analysis of data from an astrophysical X-ray object and human brain EEG, are presented.
On the variation of the fine-structure constant : Very high resolution spectrum of QSO HE 0515-4414
(2006-01-10) Chand, Hum; Srianand, R.; Petitjean, Patrick; et al.
We present a detailed analysis of a very high resolution (R ≈ 112, 000) spectrum of the quasar HE 0515−4414 obtained using the High Accuracy Radial velocity Planet Searcher (HARPS) mounted on the ESO 3.6 m telescope at the La Silla observatory. The main aim is to use HARPS spectrum of very high wavelength calibration accuracy (better than 1 mÅ), to constrain the variation of α = e² /~c and investigate any possible systematic inaccuracies in the wavelength calibration of the UV Echelle Spectrograph (UVES) mounted on the ESO Very Large Telescope (VLT). Methods. A cross-correlation analysis between the Th-Ar lamp spectra obtained with HARPS and UVES is carried out to detect any possible shift between the two spectra. Absolute wavelength calibration accuracies, and how that translate to the uncertainties in ∆ α/α are computed using Gaussian ﬁts for both lamp spectra. The value of ∆α/α at Zabs = 1.1508 is obtained using Many Multiplet method, and simultaneous Voigt proﬁle ﬁts of HARPS and UVES spectra. Results. We ﬁnd the shift between the HARPS and UVES spectra has mean around zero with a dispersion of σ ≃ 1 mÅ. This is shown to be well within the wavelength calibration accuracy of UVES (i.e σ ≃ 4 mÅ). We show that the uncertainties in the wavelength calibration induce an error of about, ∆α/α ≤ 10−⁶ , in the determination of the variation of the ﬁne-structure constant. Thus, the results of non-evolving ∆α/α reported in the literature based on UVES/VLT data should not be heavily inﬂuenced by problems related to wavelength calibration uncertainties. Our higher resolution spectrum of the zabs = 1.1508 Damped Lyman-α system toward HE 0515−4414 reveals more components compared to the UVES spectrum. Using only Fe II lines of zabs = 1.1508 system, we obtain ∆α/α = (0.05 ± 0.24) × 10−⁵ . This result is consistent with the earlier measurement for this system using the UVES spectrum alone.
On searches for gravitational waves from mini creation events by laser interferometric detectors
(2006-01-10) Sarmah, B. P.; Banerjee, S.K.; Dhurandhar, Sanjeev
As an alternative view to the standard big bang cosmology the quasi-steady state cosmol- ogy(QSSC) argues that the universe was not created in a single great explosion; it neither had a beginning nor will it ever come to an end. The creation of new matter in the universe is a regular feature occurring through nite explosive events. Each creation event is called a mini-bang or, a mini creation event(MCE). Gravitational waves are expected to be generated due to any anisotropy present in this process of creation. Mini creation event ejecting matter in two oppositely directed jets is thus a source of gravitational waves which can in principle be detected by laser interferometric detectors. In the present work we consider the gravitational waveforms propagated by linear jets and then estimate the response of laser interferometric detectors like LIGO and LISA.
Evolving turbulence and magnetic fields in galaxy clusters
(2006-01-10) Subramanian, Kandaswamy; Shukurov, A.; Haugen, N. E. L.
We discuss, using simple analytical models and MHD simulations, the origin and parameters of turbulence and magnetic ﬁelds in galaxy clusters. Any pre-existing tangled magnetic ﬁeld must decay in a few hundred million years by generating gas motions even if the electric conductivity of the intracluster gas is high. We argue that tur- bulent motions can be maintained in the intracluster gas and its dynamo action can prevent such a decay and amplify a random seed magnetic ﬁeld by a net factor typically 10⁴ in 5Gyr. Three physically distinct regimes can be identiﬁed in the evolution of turbulence and magnetic ﬁeld in galaxy clusters. Firstly, the ﬂuctuation dynamo will produce microgauss-strong, random magnetic ﬁelds during the epoch of cluster formation and major mergers. At this stage pervasive turbulent ﬂows with r.m.s. velocity of about 300 kms−ᶥ can be maintained at scales 100–200 kpc. The magnetic ﬁeld is intermittent, has a smaller scale of 20–30 kpc and average strength of 2 G. Secondly, turbulence will decay after the end of the major merger epoch; we discuss the dynamics of the decaying turbulence and the behavior of magnetic ﬁeld in it. Magnetic ﬁeld and turbulent speed undergo a power-law decay, decreasing by a factor of two during this stage, whereas their scales increase by about the same factor. Thirdly, smaller-mass subclusters and cluster galaxies will produce turbulent wakes where magnetic ﬁelds will be generated as well. Although the wakes plausibly occupy only a small fraction of the cluster volume, we show that their area covering factor can be close to unity, and thus they can produce some of the signatures of turbulence along virtually all lines of sight. The latter could potentially allow one to reconcile the possibility of turbulence with ordered ﬁlamentary gas structures, as in the Perseus cluster. The turbulent speeds and magnetic ﬁelds in the wakes are estimated to be of order 300 kms−ᶥ and 2 G, respectively, whereas the turbulent scales are of order 200 kpc for wakes behind subclusters of a mass 3 × 10ᶥᶟM⊙ and about 10 kpc in the galactic wakes. Magnetic ﬁeld in the wakes is intermittent and has the scale of about 30 kpc and 1 kpc in the subcluster and galactic wakes, respectively. Random Faraday rotation measure is estimated to be typically 100–200 radm−², in agreement with observations. We predict detectable polarization of synchrotron emission from cluster radio halos at wavelengths 3–6 cm, if observed at suﬃciently high resolution.
Dark energy
(2006-01-10) Sahni, Varun
The cosmological constant problem as well as the case for dark energy are briefly reviewed and some theoretical models of dark energy are discussed in detail. These include: the cosmological constant, quintessence, the Chaplygin gas and Braneworld models. I also discuss model independent measures of dark energy and conclude by mentioning some properties of the Statefinder diagnostic which can successfully differentiate between different families of dark energy models.
Search for 1₁₁ - 1₁ₒ and 2₁₁ - 2₁₂ transitions of H₂CCO, H₂CCC, and H₂CCCC, in cosmic objects
(2006-01-10) Chandra, S.; Musrif, P. G.; Dharmkare, Ram M.; et al.
Galactic dynamo and helicity losses through fountain flow
(2006-01-19) Shukurov, A.; Sokoloff, Dmitry; Subramanian, Kandaswamy
Nonlinear behaviour of galactic dynamos is studied, allowing for magnetic helicity removal by the galactic fountain ﬂow. Methods. A suitable advection speed is estimated, and a one-dimensional mean-ﬁeld dynamo model with dynamic α-effect is explored. Results. It is shown that the galactic fountain ﬂow is eﬃcient in removing magnetic helicity from galactic discs. This alleviates the constraint on the galactic mean-ﬁeld dynamo resulting from magnetic helicity conservation and thereby allows the mean magnetic ﬁeld to saturate at a strength comparable to equipartition with the turbulent kinetic energy.
Primordial magnetic fields and CMB anisotropies
(2006-01-25) Subramanian, Kandaswamy
Possible signatures of primordial magnetic ﬁelds on the Cosmic Microwave Background (CMB) temperature and polarization anisotropies are reviewed. The signals that could be searched for include excess temperature anisotropies particularly at small angular scales below the Silk damping scale, B-mode polarization, and non-Gaussian statistics. A ﬁeld at a few nG level produces temperature anisotropies at the 5µK level, and B-mode polarization anisotropies 10 times smaller, and is therefore potentially detectable via the CMB anisotropies. An even smaller ﬁeld, with B0 < 0.1 nG, could lead to structure formation at high redshift z > 15, and hence naturally explain an early re-ionization of the Universe.
Non-linear behaviour of the black hole system GRS 1915+ 105
(2006-02-05) Misra, Ranjeev; Harikrishnan, K. P.; Ambika, G.; et al.
Using non-lin~ar ti?Ie seri.e~ analysis, along with surrogate data analysis, it is shown that the various types of lon~ t~rI? varla~lbty exhibited by the black hole system GRS 1915+105, can be explained in terms of a ~et~rmlmstlc.n?n-bnear system with some inherent stochastic noise. Evidence is provided for a non-linear limit cycle ongm of one of.the low f~eque~cy QPO detected in the source, while some other types of variability could. be due ~oan underlying low dimensional chaotic system. These results imply that the partial differential equanons which govern the magneto- ~drodynamic flow of the inner accretion disk, can be approximated by a by a small number ( ~ 3 - 5) of non-linear but ordinary differential equations. While this analysis does not reveal the exact nature of these approximate equations, they may be obtained in the future, after results of magneto-hydrodynamic simulation of realistic accretion disks become available.
Molecular hydrogen in a damped lyman-alpha system at Zabs = 4.224
(2006-02-09) Ledoux, C.; Petitjean, Patrick; Srianand, R.
We present the direct detection of molecular hydrogen at the highest redshift known today (zabs = 4.224) in a Damped Lyman-α (DLA) system toward the quasar PSS J 1443+2724. This absorber is remarkable for having one of the highest metallicities amongst DLA systems at zabs > 3, with a measured iron abundance relative to Solar of −1.12 ± 0.10. We provide for the ﬁrst time in this system accurate measurements of Ni, Mg ii, S ii and Ar i column densities. The sulfur and nitrogen abundances relative to Solar, −0.63 ± 0.10 and −1.38 ± 0.10 respectively, correspond exactly to the primary nitrogen production plateau. H2 absorption lines are detected in four diﬀerent rotational levels (J = 0, 1, 2 and 3) of the vibrational ground-state in three velocity components with total column densities of logN(H2) = 17.67, 17.97, 17.48 and 17.26 respectively. The J = 4 level is tentatively detected in the strongest component with logN(H2) ∼ 14. The mean molecular fraction is log f = −2.38 ± 0.13, with f = 2N(H2)/(2N(H2) + N(Hi)). We also measure logN(HD)/N(H2) < −4.2. The excitation temperatures T01 for the two main components of the system are 96 and 136 K respectively. We argue that the absorbing galaxy, whose star formation activity must have started at least 2− 5× 108 yrs before z = 4.224, is in a quiescent state at the time of observation. The density of the gas is small, nH ≤ 50 cm−3 , and the temperature is of the order of T ∼ 90 − 180 K. The high excitation of neutral carbon in one of the components can be explained if the temperature of the Cosmic Microwave Background Radiation has the valueexpected at the absorber redshift, T = 14.2 K. These observations demonstrate the feasibility to study H2 at the highest redshifts provided high enough spectral resolution and good S/N ratio are achieved.
Computational study for neutral and cationic pericondensed polycyclic aromatic hydrocarbons
(2006-02-10) Pathak, Amit; Rastogi, Shantanu
Quantum chemical calculations using density functional theory are presented for small to medium sized pericondensed PAHs includ- ing some being reported for the first time. Bond lengths and charge distribution have been computed for these PAHs in both neutral and cationic forms. Upon ionization, significant change in fractional charge on atoms is present particularly for the outer carbon atoms. The charge on the internal carbon atoms tends towards zero in cations. Vibrational frequencies and infrared intensities have been calculated for the optimized structures of PAH neutrals and cations. The drastic intensity alterations occurring upon ionization are discussed and related to specific changes occurring in the charge distribution. The C-H stretch intensity depends on the partial charge on peripheral hydrogen atoms and reduces in cations as hydrogen atoms become more positive. Pericondensed PAHs show better matching with the observed interstellar infrared bands. The co-added model spectra show profiles similar to the observed astrophysical bands.
Further constraints on electron acceleration in solar noise storms
(2006-02-28) Subramanian, Prasad; Becker, Peter A.
We reexamine the energetics of nonthermal electron acceleration in solar noise storms. A new result is obtained for the minimum nonthermal electron number density required to produce a Langmuir wave population of sufficient intensity to power the noise storm emission. We combine this constraint with the stochastic electron acceleration formalism developed by Subramanian & Becker (2005) to derive a rigorous estimate for the efficiency of the overall noise storm emission process, beginning with nonthermal electron acceleration and culminating in the observed radiation. We also calculate separate efficiencies for the electron acceleration - Lang-muir wave generation stage and the Langmuir wave - noise storm production stage. In addition, we obtain a new theoretical estimate for the energy density of the Langmuir waves in noise storm continuum sources.
Confronting braneworld cosmology with supernova data and baryon oscillations
(2006-03-18) Alam, Ujjaini; Sahni, Varun
Braneworld cosmology has several attractive and distinctive features. For instance the eﬀective equation of state in braneworld models can be both quintessence-like (w0 ≥ −1) as well as phantom like (w0 ≤ −1). Models with w0 ≥ −1 (w0 ≤ −1) are referred to as Brane 2 (Brane 1) and correspond to complementary embeddings of the brane in the bulk. (The equation of state in Brane 1 can successfully cross the ‘phantom divide’ at w = −1.) In this paper we compare the predictions of braneworld models to two recently released supernova data sets: the ‘Gold’ data (Riess et al., 2004) and the data from the Supernova Legacy Survey (SNLS) (Astier et al., 2005). We also incorporate the recent discovery of the baryon acoustic peak in the Sloan Digital Sky Survey (Eisenstein et al., 2005) into our analysis. Our main results are that braneworld models satisfy both sets of SNe data. Brane 1 (with w0 ≤ −1) shows very good agreement with data for values of the matter density bounded from below: Ω0m > ∼ 0.25 (Gold) and Ω0m > ∼ 0.2 (SNLS). On the other hand Brane 2 (with w0 ≥ −1) shows excellent agreement with data for values of the matter density which are bounded from above: Ω0m < ∼ 0.45 (Gold) and Ω0m < ∼ 0.35 (SNLS). The DGP model is excluded at 3σ by SNLS and at 1σ by the Gold dataset. Braneworld models with future ‘quiescent’ singularities (at which the Hubble parameter and the matter density remain ﬁnite but higher derivatives of the expansion factor diverge) are excluded by both datasets.