IUCAA Preprints
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Item Cosmic Microwave Background Trispectrum and Primordial Magnetic Field Limits(2012-06-08) Trivedi, Pranjal; Seshadri, T. R.; Subramanian, KandaswamyPrimordial magnetic fields will generate non-Gaussian signals in the cosmic microwave background (CMB) as magnetic stresses and the temperature anisotropy they induce depend quadratically on the magnetic field. We compute a new measure of magnetic non-Gaussianity, the CMB trispectrum, on large angular scales, sourced via the Sachs-Wolfe effect. The trispectra induced by magnetic energy density and by magnetic scalar anisotropic stress are found to have typical magnitudes of approximately a few times 10 29 and 10 19, respectively. Observational limits on CMB non-Gaussianity from WMAP data allow us to conservatively set upper limits of a nG, and plausibly sub-nG, on the present value of the primordial cosmic magnetic field. This represents the tightest limit so far on the strength of primordial magnetic fields, on Mpc scales, and is better than limits from the CMB bispectrum and all modes in the CMB power spectrum. Thus, the CMB trispectrum is a new and more sensitive probe of primordial magnetic fields on large scales.Item Effects of curvature and interactions on the dynamics of the deconfinement phase transition(2011-07-06) Subramanian, Kandaswamy; Seshadri, T. R.; Barrow, John D.Item CMB Anisotropy Due to Tangled magnetic fields in re-ionized models(2005-04-01) Seshadri, T. R.; Subramanian, KandaswamyPrimordial tangled cosmological Magnetic Fields source rotational velocity perturbations of the baryon fluid, even in the post-recombination universe. These vortical modes inturn leave a characteristic imprint on the temperature anisotropy of the Cosmic Microwave Background (CMB), if the CMB photons can be re-scatterred after recombination. Observations from WMAP in- dicate that the Universe underwent a relatively early re-ionization (zri ∼ 15), which does indeed lead to a significant optical depth for re-scattering of CMB photons after the re-ionization epoch. We compute the resulting additional temperature anisotropies, induced by primordial magnetic fields in the postrecombination universe. We show that in models with early re-ionization, a nearly scale-invariant spectrum of tangled magnetic fields which redshift to a present value of B0 ∼ 3 × 10−9 Gauss, produce vector mode perturbations which in turn induce additional temperature anisotropy of about 0.3 to 0.4 µK over very small angular scales, with l upto ∼ 10000 or so.98.62.En, 98.70.Vc, 98.80.Cq, 95.30.QdItem Cosmic Microwave Background Bispectrum from Primordial Magnetic Fields on Large Angular Scales(2009-09-01) Seshadri, T. R.; Subramanian, KandaswamyPrimordial magnetic fields lead to non-Gaussian signals in the Cosmic Microwave Background (CMB) even at the lowest order, as magnetic stresses, and the temperature anisotropy they induce, depend quadratically on the magnetic field. In contrast, CMB non-Gaussianity due to inflationary scalar perturbations arise only as a higher order effect. We propose here a novel probe of stochastic primordial magnetic fields that exploits the characteristic CMB non-Gaussianity that they induce. In particular, we compute the CMB bispectrum (bl 1 l2 l3)induced by stochastic primordial fields on large angular scales. We find a typical value of l1(l1 + 1)l3(l3 + 1)bl1 l2 l3∼ 10−22, for magnetic fields of strength B0 ∼ 3 nano Gauss and with a nearly scale invariantmagnetic spectrum. Current observational limits on the bispectrum allow us to set upper limits on B0 ∼ 35 nano Gauss, which can be improved by including other magnetically induced contributions to the bispectrumItem CCD photometric study of the late type contact binary EK comae berenices(2010-04-26) Deb, Sukanta; Singh, Harinder P.; Seshadri, T. R.; et al.We present CCD photometric observations of the W UMa type contact binary EK Comae Berenices using the 2 metre telescope of IUCAA Girawali Observatory, India. The star was classified as a W UMa type binary of subtype-W by Samec et al. [20]. The new V band photometric observations of the star reveal that shape of the light curve has changed significantly from the one observed by Samec et al. [20]. A detailed analysis of the light curve obtained from the high-precision CCD photometric observations of the star indicates that EK Comae Berenices is not a W-type but an A-type totally eclipsing W UMa contact binary. The photometric mass ratio is determined to be 0.349 ± 0.005. A temperature difference of ∆T = 141 ± 10 K between the components and an orbital inclination of i[°] = 89.800 ± 0.075 were obtained for the binary system. Absolute values of masses, radii and luminosities are estimated by means of the standard mass-luminosity relation for zero age main-sequence stars. The star shows O’Connell effect, asymmetries in the light curve shape around the primary and secondary maximum. The observed O’Connell effect is explained by the presence of a hot spot on the primary component.