Circular Polarization from Gamma-ray Burst Afterglows

We investigate the circular polarization (CP) from Gamma-Ray Burst (GRB) afterglows. We show that a tangled magnetic field cannot generate CP without an ordered magnetic field because there is always an oppositely directed field, so that no handedness exists. This implies the observation of CP could be a useful probe of an ordered field, which carries valuable information on the GRB central engine. By solving the transfer equation of polarized radiation, we find that the CP reaches 1% at radio frequencies and 0.01% at optical for the forward shock, and 10-1% at radio and 0.1-0.01% at optical for the reverse shock.Comment: 12 pages, 3 figure

Tables of Hyperonic Matter Equation of State for Core-Collapse Supernovae

We present sets of equation of state (EOS) of nuclear matter including hyperons using an SU_f(3) extended relativistic mean field (RMF) model with a wide coverage of density, temperature, and charge fraction for numerical simulations of core collapse supernovae. Coupling constants of Sigma and Xi hyperons with the sigma meson are determined to fit the hyperon potential depths in nuclear matter, U_Sigma(rho_0) ~ +30 MeV and U_Xi(rho_0) ~ -15 MeV, which are suggested from recent analyses of hyperon production reactions. At low densities, the EOS of uniform matter is connected with the EOS by Shen et al., in which formation of finite nuclei is included in the Thomas-Fermi approximation. In the present EOS, the maximum mass of neutron stars decreases from 2.17 M_sun (Ne mu) to 1.63 M_sun (NYe mu) when hyperons are included. In a spherical, adiabatic collapse of a 15$M_\odot$ star by the hydrodynamics without neutrino transfer, hyperon effects are found to be small, since the temperature and density do not reach the region of hyperon mixture, where the hyperon fraction is above 1 % (T > 40 MeV or rho_B > 0.4 fm^{-3}).Comment: 23 pages, 6 figures (Fig.3 and related comments on pion potential are corrected in v3.

Reconstructing the Primordial Spectrum with CMB Temperature and Polarization

We develop a new method to reconstruct the power spectrum of primordial curvature perturbations, $P(k)$, by using both the temperature and polarization spectra of the cosmic microwave background (CMB). We test this method using several mock primordial spectra having non-trivial features including the one with an oscillatory component, and find that the spectrum can be reconstructed with a few percent accuracy by an iterative procedure in an ideal situation in which there is no observational error in the CMB data. In particular, although the previous ``cosmic inversion'' method, which used only the temperature fluctuations, suffered from large numerical errors around some specific values of $k$ that correspond to nodes in a transfer function, these errors are found to disappear almost completely in the new method.Comment: 18 pages, 17 figures, submitted to PR

Reconstructing the primordial power spectrum - a new algorithm

We propose an efficient and model independent method for reconstructing the primordial power spectrum from Cosmic Microwave Background (CMB) and large scale structure observations. The algorithm is based on a Monte Carlo principle and therefore very simple to incorporate into existing codes such as Markov Chain Monte Carlo. The algorithm has been used on present cosmological data to test for features in the primordial power spectrum. No significant evidence for features is found, although there is a slight preference for an overall bending of the spectrum, as well as a decrease in power at very large scales. We have also tested the algorithm on mock high precision CMB data, calculated from models with non-scale invariant primordial spectra. The algorithm efficiently extracts the underlying spectrum, as well as the other cosmological parameters in each case. Finally we have used the algorithm on a model where an artificial glitch in the CMB spectrum has been imposed, like the ones seen in the WMAP data. In this case it is found that, although the underlying cosmological parameters can be extracted, the recovered power spectrum can show significant spurious features, such as bending, even if the true spectrum is scale invariant.Comment: 22 pages, 12 figures, matches JCAP published versio

Primordial Power Spectrum Reconstruction

In order to reconstruct the initial conditions of the universe it is important to devise a method that can efficiently constrain the shape of the power spectrum of primordial matter density fluctuations in a model-independent way from data. In an earlier paper we proposed a method based on the wavelet expansion of the primordial power spectrum. The advantage of this method is that the orthogonality and multiresolution properties of wavelet basis functions enable information regarding the shape of $P_{\rm in}(k)$ to be encoded in a small number of non-zero coefficients. Any deviation from scale-invariance can then be easily picked out. Here we apply this method to simulated data to demonstrate that it can accurately reconstruct an input $P_{\rm in}(k)$, and present a prescription for how this method should be used on future data.Comment: 4 pages, 2 figures. JCAP accepted versio

Scale dependence of the primordial spectrum from combining the three-year WMAP, Galaxy Clustering, Supernovae, and Lyman-alpha forests

We probe the scale dependence of the primordial spectrum in the light of the three-year WMAP (WMAP3) alone and WMAP3 in combination with the other cosmological observations such as galaxy clustering and Type Ia Supernova (SNIa). We pay particular attention to the combination with the Lyman $\alpha$ (Ly$\alpha$) forest. Different from the first-year WMAP (WMAP1), WMAP3's preference on the running of the scalar spectral index on the large scales is now fairly independent of the low CMB multipoles $\ell$. A combination with the galaxy power spectrum from the Sloan Digital Sky Survey (SDSS) prefers a negative running to larger than 2$\sigma$, regardless the presence of low $\ell$ CMB ($2\le \ell \le 23$) or not. On the other hand if we focus on the Power Law $\Lambda$CDM cosmology with only six parameters (matter density $\Omega_m h^2$, baryon density $\Omega_b h^2$, Hubble Constant $H_0$, optical depth $\tau$, the spectral index, $n_s$, and the amplitude, $A_s$, of the scalar perturbation spectrum) when we drop the low $\ell$ CMB contributions WMAP3 is consistent with the Harrison-Zel'dovich-Peebles scale-invariant spectrum ($n_s=1$ and no tensor contributions) at $\sim 1\sigma$. When assuming a simple power law primordial spectral index or a constant running, in case one drops the low $\ell$ contributions ($2\le \ell \le 23$) WMAP3 is consistent with the other observations better, such as the inferred value of $\sigma_8$. We also find, using a spectral shape with a minimal extension of the running spectral index model, LUQAS$+$ CROFT Ly$\alpha$ and SDSS Ly$\alpha$ exhibit somewhat different preference on the spectral shape.Comment: 16 pages, 13 figures Revtex

Cosmology with CMB anisotropy

Measurements of CMB anisotropy and, more recently, polarization have played a very important role allowing precise determination of various parameters of the `standard' cosmological model. The expectation of the paradigm of inflation and the generic prediction of the simplest realization of inflationary scenario in the early universe have also been established -- `acausally' correlated initial perturbations in a flat, statistically isotropic universe, adiabatic nature of primordial density perturbations. Direct evidence for gravitational instability mechanism for structure formation from primordial perturbations has been established. In the next decade, future experiments promise to strengthen these deductions and uncover the remaining crucial signature of inflation -- the primordial gravitational wave background.Comment: Plenary talk at the IXth. International Workshop on High Energy Physics Phenomenology (WHEPP-9), Institute of Physics, Bhubaneshwar, India. Jan 3-14, 2006; To appear in the Proceedings to be published in Pramana; 12 pages, 2 figure

`Standard' Cosmological model & beyond with CMB

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.Comment: 16 pages, 6 figures, Plenary talk, Proc. of GR-19, Mexico City, Mexico (Jul 5-9, 2010). To appear in a special issue in Class. Q. Gra

Detailed optical and near-infrared polarimetry, spectroscopy and broadband photometry of the afterglow of GRB 091018: Polarisation evolution

[Abridged] A number of phenomena have been observed in GRB afterglows that defy explanation by simple versions of the standard fireball model, leading to a variety of new models. Polarimetry can be a major independent diagnostic of afterglow physics, probing the magnetic field properties and internal structure of the GRB jets. In this paper we present the first high quality multi-night polarimetric light curve of a Swift GRB afterglow, aimed at providing a well calibrated dataset of a typical afterglow to serve as a benchmark system for modelling afterglow polarisation behaviour. In particular, our dataset of the afterglow of GRB 091018 (at redshift z=0.971) comprises optical linear polarimetry (R band, 0.13 - 2.3 days after burst); circular polarimetry (R band) and near-infrared linear polarimetry (Ks band). We add to that high quality optical and near-infrared broadband light curves and spectral energy distributions as well as afterglow spectroscopy. The linear polarisation varies between 0 and 3%, with both long and short time scale variability visible. We find an achromatic break in the afterglow light curve, which corresponds to features in the polarimetric curve. We find that the data can be reproduced by jet break models only if an additional polarised component of unknown nature is present in the polarimetric curve. We probe the ordered magnetic field component in the afterglow through our deep circular polarimetry, finding P_circ < 0.15% (2 sigma), the deepest limit yet for a GRB afterglow, suggesting ordered fields are weak, if at all present. Our simultaneous R and Ks band polarimetry shows that dust induced polarisation in the host galaxy is likely negligible.Comment: 20 pages, 14 figures, 3 tables. Accepted for publication in MNRAS. Some figures are reduced in quality to comply with arXiv size requirement