2011 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/151

Browse

Search Results

Now showing 1 - 10 of 23
  • No Thumbnail Available
    Item
    X-Ray warm absorption and emission in the polar scattered seyfert 1 galaxy MRK 704
    (2011-04-10) Laha, Sibasish; Dewangan, Gulab Chand; Kembhavi, A.K.
  • No Thumbnail Available
    Item
    Statistics of bipolar representation of CMB maps
    (2011-09-04) Joshi, Nidhi; Rotti, Aditya; Souradeep, Tarun
    Gaussianity of temperature fluctuations in the Cosmic Microwave Background(CMB) implies that the statistical properties of the temperature field can be completely characterized by its two point correlation function. The two point correlation function can be expanded in full generality in the bipolar spherical harmonic(BipoSH) basis. Looking for significant deviations from zero for Bipolar Spherical Harmonic(BipoSH) Coefficients derived from observed CMB maps forms the basis of the strategy used to detect isotropy violation. In order to quantify ”significant deviation” we need to understand the distributions of these coefficients. We analytically evaluate the moments and the distribution of the coefficients of expansion(ALM l1l2 ), using characteristic function approach. We show that for BipoSH coefficients with M = 0 an analytical form for the moments up to any arbitrary order can be derived. For the remaining BipoSH coefficients with M = 0, the moments derived using the characteristic function approach need to be supplemented with a correction term. The correction term is found to be important particularly at low multipoles. We provide a general prescription for calculating these corrections, however we restrict the explicit calculations only up to kurtosis. We confirm our results with measurements of BipoSH coefficients on numerically simulated statistically isotropic CMB maps.
  • No Thumbnail Available
    Item
    Statefinder hierarchy: An extended null diagnostic for concordance cosmology
    (2011-01-04) Arabsalmani, Maryam; Sahni, Varun
    We show how higher derivatives of the expansion factor can be developed into a null diagnostic for concordance cosmology (ΛCDM). It is well known that the Statefinder – the third derivative of the expansion factor written in dimensionless form, a (ᶟ) /aH ᶟ , equals unity for ΛCDM. We generalize this result and demonstrate that the hierarchy, a (ᴺ) /aH ᴺ , can be converted to a form that stays pegged at unity in concordance cosmology. This remarkable property of the Statefinder hierarchy enables it to be used as an extended null diagnostic for the cosmological constant. The Statefinder hierarchy, combined with the growth rate of matter perturbations, can serve as a composite null diagnostic to differentiate between dark energy models.
  • No Thumbnail Available
    Item
    'Standard' Cosmological model & beyond with CMB
    (2011-04-16) Souradeep, Tarun
    Observational Cosmology has indeed made very rapid progress in the past decade. The ability to quantify the universe has largely improved due to observational constraints coming from structure formation Measurements of CMB anisotropy and, more recently, polarization have played a very important role. Besides precise determination of various parameters of the 'standard’ cosmological model, observations have also established some important basic tenets that underlie models of cosmology and structure formation in the universe – ‘acausally’ correlated initial perturbations in a flat, statistically isotropic universe, adiabatic nature of primordial density perturbations. These are consistent with the expectation of the paradigm of inflation and the generic prediction of the simplest realization of inflationary scenario in the early universe. Further, gravitational instability is the established mechanism for structure formation from these initial perturbations. The signature of primordial perturbations observed as the CMB anisotropy and polarization is the most compelling evidence for new, possibly fundamental, physics in the early universe. The community is now looking beyond the estimation of parameters of a working 'standard’ model of cosmology for subtle, characteristic signatures from early universe physics.
  • No Thumbnail Available
    Item
    Soft time lags in the X-ray emission of mrk 1040
    (2011-01-30) Tripathi, Shruti; Misra, Ranjeev; Dewangan, Gulab Chand; et al.
    Temporal analysis of X-ray binaries and Active Galactic Nuclei have shown that hard X-rays react to variation of soft ones after a time delay. The opposite trend, or soft lag, has only been seen in a few rare Quasi-periodic Oscillations in X-ray binaries and recently for the AGN, 1H 0707-495, on short timescales of ∼ 103 secs. Here, we report analysis of a XMM-Newton observation of Mrk 1040, which reveals that on the dominant variability timescale of ∼ 104 secs, the source seems to exhibit soft lags. If the lags are frequency independent, they could be due to reverberation effects of a relativistically blurred reflection component responding to a varying continuum. Alternatively, they could be due to Comptonization delays in the case when high energy photons impinge back on the soft photon source. Both models can be verified and their parameters tightly constrained, because they will need to predict the photon spectrum, the r.m.s variability and time lag as a function of energy. A successful application of either model will provide unprecedented information on the radiative process, geometry and more importantly the size of the system, which in turn may provide stringent test of strong general relativistic effects.
  • No Thumbnail Available
    Item
    Perturbations in dark energy models with evolving speed of sound
    (2011-04-26) Ansari, Rizwan Ul Haq; Unnikrishnan, Sanil
    The behavior of perturbation in scalar field dark energy and its consequent effect on the cold dark matter (CDM) power spectrum is well understood to be governed by the equation of state (EOS)parameter and the effective speed of sound (ESS) of dark energy. In this paper, we investigate whether dark energy models whose ESS are epoch dependent leaves any distinct imprints on the large scale CDM power spectrum. In particular, we compare the cases where the ESS is decreasing with time with those where it increases. The CDM power spectrum is found to be generically suppressed in these cases as compared to the ΛCDM model. The degree of suppression at different length scales can, in principle, reflect the evolving nature of the ESS of dark energy. However, we find that the effect on the CDM power spectrum in cases where the ESS of dark energy is evolving with constant EOS parameter is significantly smaller as compared to the situation where ESS is constant whereas EOS parameter is evolving. Further, it is also shown that the effect of different evolution of ESS for a given evolution of EOS parameter of dark energy on the CDM power spectrum is significant only at the intermediate scales (around k ∼ 0.01h/Mpc). At scales much smaller and larger than the Hubble radius, it is the evolution of EOS parameter of dark energy which governs the degree of suppression of CDM power spectrum with respect to the ΛCDM model.
  • No Thumbnail Available
    Item
    Oscillation in the inflaton potential: Exact numerical analysis and comparison with the recent and forthcoming CMB datasets
    (2011-06-15) Aich, Moumita; Hazra, Dhiraj Kumar; Sriramkumar, L.; et al.
    Amongst the multitude of inflationary models currently available, models that lead to features in the primordial scalar spectrum are drawing increasing attention, since certain features have been found to provide a better fit to the CMB data than the conventional, nearly scale invariant, primordial spectrum. In this work, we carry out an exact numerical analysis of two models that lead to oscillations over all scales in the scalar power spectrum. We consider the model described by a quadratic potential which is superposed by a sinusoidal modulation and the recently popular axion monodromy model. Since the oscillations continue even onto smaller scales, in addition to the WMAP data, we also compare the models with the small scale data from ACT. Though, both the models, broadly, result in oscillations in the spectrum, interestingly, we find that, while the monodromy model leads to a considerably better fit to the data in comparison to the standard power law spectrum, the quadratic potential superposed with a sinusoidal modulation does not improve the fit to a similar extent. We also carry out forecasting of the parameters using simulated Planck data for both the models. We show that the Planck mock data performs better in constraining the model parameters as compared to the presently available CMB datasets.
  • No Thumbnail Available
    Item
    Odd-parity bipolar spherical harmonics
    (2011-09-13) Book, Laura G.; Kamionkowski, Marc; Souradeep, Tarun
    Bipolar spherical harmonics (BiPoSHs) provide a general formalism for quantifying departures in the cosmic microwave background (CMB) from statistical isotropy (SI) and from Gaussianity. However, prior work has focused only on BiPoSHs with even parity. Here we show that there is another set of BiPoSHs with odd parity, and we explore their cosmological applications. We describe systematic artifacts in a CMB map that could be sought by measurement of these odd-parity BiPoSH modes. These BiPoSH modes may also be produced cosmologically through lensing by gravitational waves (GWs), among other sources. We derive expressions for the BiPoSH modes induced by the weak lensing of both scalar and tensor perturbations. Our estimate of the expected signal-to-noise with which we could measure the weak lensing of GWs from the correlation of two BiPoSH modes is respectable, indicating that such a measurement may be merited. We also investigate the possibility to detect parity-breaking physics, such as chiral GWs, by cross-correlating opposite parity BiPoSH modes with multipole moments of the CMB polarization. We nd that the expected signal-to-noise of such a detection is modest, with errors dominated by cosmic variance.
  • No Thumbnail Available
    Item
    Numerical simulation of time delay interferometry for LISA with one arm dysfunctional
    (2011-02-25) Dhurandhar, Sanjeev; Ni, W.-T.; Wang, G.
    In order to attain the requisite sensitivity for LISA, laser frequency noise must be suppressed below the secondary noises such as the optical path noise, acceleration noise etc. In a previous paper (Dhurandhar et al., Class. Quantum Grav., 27, 135013, 2010), we have found a large family of second generation analytic solutions of time delay interferometry with one arm dysfunctional and also estimated the laser noise due to residual time-delay semi-analytically from orbit perturbations due to Earth. Since other planets and solar-system bodies also perturb the orbits of LISA spacecraft and affect the time delay interferometry (TDI), we simulate the time delay numerically in this paper for all solutions with n ≤ 3. To conform to the actual LISA planning, we have worked out a set of 3-year optimized mission orbits of LISA spacecraft starting at June 21, 2021 using CGC2.7 ephemeris framework. We then use this numerical solution to calculate the residual optical path differences in the second generation solutions of our previous paper, and compare with the semi-analytic error estimate. The accuracy of this calculation is better than 1 cm (or 30 ps). The maximum path length difference, for all configuration calculated, is below 1 m (3 ns). This is well below the limit under which the laser frequency noise is required to be suppressed.
  • No Thumbnail Available
    Item
    Infrared emission from the composite grains: Effects of inclusions and porosities on the 10 and 18 µm features
    (2011-01-11) Vaidya, D.B.; Gupta, Ranjan
    In this paper we study the effects of inclusions and porosities on the emission properties of silicate grains and compare the model curves with the observed infrared emission from circumstellar dust. Methods. We calculate the absorption efficiency of the composite grain, made up of a host silicate oblate spheroid and inclusions of ice/graphite/or voids, in the spectral region 5.0-25.0 m. The absorption efficiencies of the composite spheroidal oblate grains for three axial ratios are computed using the discrete dipole approximation (DDA). We study the absorption as a function of the volume fraction of the inclusions and porosity. In particular, we study the variation in the 10 m and 18 m emission features with the volume fraction of the inclusions and porosities. We then calculate the infrared fluxes for these composite grains at several dust temperatures (T=200-350K) and compare the model curves with the average observed IRAS-LRS curve, obtained for circumstellar dust shells around oxygen rich M-type stars. The model curves are also compared with two other individual stars. Results. The results on the composite grains show variation in the absorption efficiencies with the variation in the inclusions and porosities. In particular, it is found that the wavelength of peak absorption at 10 m, shifts towards longer wavelengths with variation in the volume fraction of the inclusions of graphite. The spheroidal composite grains with axial ratio ∼ 1.33; volume fraction of f=0.1 and dust temperature between 210-340K, fit the observed infra-red emission from circumstellar dust reasonably well in the wavelength range 5-25 m. The model flux ratio, R=Flux(18 )/Flux(10 ), compares well with the observed ratio for the circumstellar dust.Conclusions. The results on the composite grains clearly indicate that the silicate feature at 10 µm shifts with the volume fraction of graphite inclusions. The feature does not shift with the porosity. Both the features do not show any broadening with the inclusions or porosity. The absorption efficiencies of the composite grains calculated using DDA and Effective Medium Approximation (EMA)do not agree. The composite grain models presented in this study need to be compared with the observed IR emission from the circumstellar dust around a few more stars.