arXiv subjects

Asantha Cooray

Publications and source records attributed to Asantha Cooray.

At least 289 records · Page 16Linked to original sources

arXiv2004

Gravitational Wave Background of Neutron Star-White Dwarf Binaries

We discuss the stochastic background of gravitational waves from ultra compact neutron star-white dwarf (NS-WD) binaries at cosmological distances. Under the assumption that accreting neutron stars and donor white dwarf stars form most of the low mass X-ray binaries (LMXBs), our calculation makes use of recent results related to the luminosity function determined from X-ray observations. Even after accounting for detached NS-WD binaries not captured in X-ray data, the NS-WD background is at least an order of magnitude below that due to extragalactic white dwarf-white dwarf binaries and below the detectability level of the Laser Interferometer Space Antenna (LISA) at frequencies between 10^-5 Hz and 10^-1 Hz. While the extragalactic background is unlikely to be detected, we suggest that around one to ten galactic NS-WD binaries may be resolved with LISA such that their positions are determined to an accuracy of several degrees on the sky.

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arXiv2004

Cross-Correlation Studies between CMB Temperature Anisotropies and 21 cm Fluctuations

During the transition from a neutral to a fully reionized universe, scattering of cosmic microwave background (CMB) photons via free-electrons leads to a new anisotropy contribution to the temperature distribution. If the reionization process is inhomogeneous and patchy, the era of reionization is also visible via brightness temperature fluctuations in the redshifted 21 cm line emission from neutral Hydrogen. Since regions containing electrons and neutral Hydrogen are expected to trace the same underlying density field, the two are (anti) correlated and this is expected to be reflected in the anisotropy maps via a correlation between arcminute-scale CMB temperature and the 21 cm background. In terms of the angular cross-power spectrum, unfortunately, this correlation is insignificant due to a geometric cancellation associated with second order CMB anisotropies. The same cross-correlation between ionized and neutral regions, however, can be studied using a bispectrum involving large scale velocity field of ionized regions from the Doppler effect, arcminute scale CMB anisotropies during reionization, and the 21 cm background. While the geometric cancellation is partly avoided, the signal-to-noise ratio related to this bispectrum is reduced due to the large cosmic variance related to velocity fluctuations traced by the Doppler effect. Unless the velocity field during reionization can be independently established, it is unlikely that the correlation information related to the relative distribution of ionized electrons and regions containing neutral Hydrogen can be obtained with a combined study involving CMB and 21 cm fluctuations.

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arXiv2004

Search for Small-Mass Black Hole Dark Matter with Space-Based Gravitational Wave Detectors

The high sensitivity of upcoming space-based gravitational wave detectors suggests the possibility that if halo dark matter were composed of primordial black holes (PBHs) with mass between $10^{16}$ g and 10$^{20}$ g, the gravitational interaction with detector test masses will lead to a detectable pulse-like signal during the fly-by. For an improved version of the Laser Interferometer Space Antenna with a reduced acceleration noise at the low-end of its frequency spectrum, we find an event rate, with signal-to-noise ratios greater than 5, of $\sim$ a few per decade involving black holes of mass $\sim$ 10$^{17}$ g. The detection rate improves significantly for second generation space based interferometers that are currently envisioned, though these events must be distinguished from those involving perturbations due to near-Earth asteroids. While the presence of primordial black holes below a mass of $\sim$ 10$^{16}$ g is now constrained based on the radiation released during their evaporation, the gravitational wave detectors will extend the study of PBHs to a several orders of magnitude higher masses.

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arXiv2004

Cross-Correlation Studies with CMB Polarization Maps

The free-electron population during the reionized epoch rescatters CMB temperature quadrupole and generates a now well-known polarization signal at large angular scales. While this contribution has been detected in the temperature-polarization cross power spectrum measured with WMAP data, due to the large cosmic variance associated with anisotropy measurements at tens of degree angular scales only limited information related to reionization, such as the optical depth to electron scattering, can be extracted. The inhomogeneities in the free-electron population lead to an additional secondary polarization anisotropy contribution at arcminute scales. While the fluctuation amplitude, relative to dominant primordial fluctuations, is small, we suggest that a cross-correlation between arcminute scale CMB polarization data and a tracer field of the high redshift universe, such as through fluctuations captured by the 21 cm neutral Hydrogen background or those in the infrared background related to first proto-galaxies, may allow one to study additional details related to reionization. For this purpose, we discuss an optimized higher order correlation measurement, in the form of a three-point function, including information from large angular scale CMB temperature anisotropies in addition to arcminute scale polarization signal related to inhomogeneous reionization. We suggest that the proposed bispectrum can be measured with a substantial signal-to-noise ratio and does not require all-sky maps of CMB polarization or that of the tracer field. A measurement such as the one proposed may allow one to establish the epoch when CMB polarization related to reionization is generated and to address if the universe was reionized once or twice.

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arXiv2004

First Sources in Infrared Light: Stars, Supernovae and Miniquasars

The cosmic infrared background (IRB) at wavelengths between 1 micron and 3 microns provides a useful probe of early star-formation prior to and during reionization. To explain the high optical depth to electron scattering, as measured by the Wilkinson Microwave Anisotropy Probe (WMAP), one requires significant star-formation activity at redshifts 10 and higher. In addition to massive stars, the IRB flux may be contributed by a population of early miniquasars. We study the relative contributions from first stars, supernovae and quasars to the IRB for reasonable star formation rates at high redshift. If miniquasars radiate efficiently at the Eddington-limit, current background measurements limit the fraction of mass in first stars that is converted to seed black holes to be roughly less than 10%. In the case of supernovae, though an individual supernova is much brighter than the progenitor star, due to the shorter lifetime of order few months, the fractional contribution to the IRB remains at a level of 10% and below when compared to the same contribution from stars. The bright supernovae may, however, be directly detectable by future large ground-based and space telescopes.

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arXiv2004

Uncorrelated Estimates of Dark Energy Evolution

Type Ia supernova data have recently become strong enough to enable, for the first time, constraints on the time variation of the dark energy density and its equation of state. Most analyses, however, are using simple two or three-parameter descriptions of the dark energy evolution, since it is well known that allowing more degrees of freedom introduces serious degeneracies. Here we present a method to produce uncorrelated and nearly model-independent band power estimates of the equation of state of dark energy and its density as a function of redshift. We apply the method to recently compiled supernova data. Our results are consistent with the cosmological constant scenario, in agreement with other analyses that use traditional parameterizations, though we find marginal (2-sigma) evidence for w(z) < -1 at z < 0.2. In addition to easy interpretation, uncorrelated, localized band powers allow intuitive and powerful testing of the constancy of either the energy density or equation of state. While we have used relatively coarse redshift binning suitable for the current set of about 150 supernovae, this approach should reach its full potential in the future, when applied to thousands of supernovae found from ground and space, combined with complementary information from other cosmological probes.

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arXiv2004

LISA Measurement of Gravitational Wave Background Anisotropy: Hexadecapole Moment via a Correlation Analysis

We discuss spatial fluctuations in the gravitational wave background arising from unresolved Galactic binary sources, such as close white dwarf binaries, due to the fact the galactic binary source distribution is anisotropic. We introduce a correlation analysis of the two data streams of the Laser Interferometer Space Antenna (LISA) to extract spherical harmonic coefficients, in an independent manner, of the hexadecapole moment ($l=4$) related to the projected two-dimensional density distribution of the binary source population. The proposed technique complements and improves over previous suggestions in the literature to measure the gravitational wave background anisotropy based on the time modulation of data as LISA orbits around the Sun. Such techniques, however, are restricted only to certain combinations of spherical harmonic coefficients of the galaxy with no ability to separate them individually. With LISA, $m=2,3$ and 4 coefficients of the hexadecapole ($l=4$) can be measured with signal-to-noise ratios at the level of 10 and above in a certain coordinate system. In addition to the hexadecapole coefficients, when combined with the time modulation analysis, the correlation study can also be used, in principle, to measure quadrupole coefficients of the binary distribution.

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arXiv2004

Graviton Mass from Close White Dwarf Binaries Detectable with LISA

The arrival times of gravitational waves and optical light from orbiting binaries provide a mechanism to understand the propagation speed of gravity when compared to that of light or electromagnetic radiation. This is achieved with a measurement of any offset between optically derived orbital phase related to that derived from gravitational wave data, at a specified location of one binary component with respect to the other. Using a sample of close white dwarf binaries (CWDBs) detectable with the Laser Interferometer Space Antenna (LISA) and optical light curve data related to binary eclipses from meter-class telescopes for the same sample, we determine the accuracy to which orbital phase differences can be extracted. We consider an application of these measurements involving a variation to the speed of gravity, when compared to the speed of light, due to a massive graviton. For a subsample of $\sim$ 400 CWDBs with high signal-to-noise gravitational wave and optical data with magnitudes brighter than 25, the combined upper limit on the graviton mass is at the level of $\sim 6 \times 10^{-24}$ eV. This limit is two orders of magnitude better than the present limit derived by Yukawa-correction arguments related to the Newtonian potential and applied to the Solar-system.

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arXiv2004

Free-Free Emission at Low Radio Frequencies

We discuss free-free radio emission from ionized gas in the intergalactic medium. Because the emissivity is proportional to the square of the electron density, the mean background is strongly sensitive to the spatial clumping of free electrons. Using several existing models for the clumping of ionized gas, we find that the expected free-free distortion to the cosmic microwave background (CMB) blackbody spectrum is at a level detectable with upcoming experiments such as the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE). However, the dominant contribution to the distortion comes from clumpy gas at z < 3, and the integrated signal does not strongly constrain the epoch of reionization. In addition to the mean emission, we consider spatial fluctuations in the free-free background and the extent to which these anisotropies confuse the search for fluctuations in 21 cm line emission from neutral hydrogen during and prior to reionization. This background is smooth in frequency space and hence can be removed through frequency differencing, but only so long as the 21 cm signal and the free-free emission are uncorrelated. We show that, because the free-free background is generated primarily at low redshifts, the cross-correlation between the two fields is smaller than a few percent. Thus, multifrequency cleaning should be an effective way to eliminate the free-free confusion.

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arXiv2004

First Star Signature in Infrared Background Anisotropies

(Abridged) Recent cosmic microwave background anisotropy results from the Wilkinson Microwave Anisotropy Probe suggest that the universe was reionized at a redshift around 20. Such an early reionization could arise through the ionizing radiation emitted by metal-free population III stars at redshifts of 10 and higher. We discuss infrared background (IRB) surface brightness spatial fluctuations from such a generation of early star formation. We show that the spatial clustering of these stars at tens of arcminute scales generates a contribution to the angular power spectrum of the IRB anisotropies at the same angular scales. This excess can be potentially detected when resolved foreground galaxies out to a redshift of a few is removed from the clustering analysis. Additionally, assuming a population III stellar spectrum, we suggest that the clustering excess related to the first generation of stars can be separated from brightness fluctuations resulting from other foreground sources and galaxies using multifrequency observations in the wavelength range of ~ 1 to 5 microns. In addition to removing the low-redshift galaxy population, the multifrequency data are essential to account for certain foreground contaminants such zodiacal light, which, if varying spatially over degree scales, can be a significant source of confusion for the proposed study. Using various instruments, we study the extent to which spatial fluctuations of the IRB can be studied in the near future.

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arXiv2003

Non-linear Galaxy Power Spectrum and Cosmological Parameters

The galaxy power spectrum is now a well-known tool of precision cosmology. In addition to the overall shape, baryon oscillations and the small-scale suppression of power by massive neutrinos capture complimentary information on cosmological parameters when compared to the angular power spectrum of cosmic microwave background anisotropies. We study both the real space and redshift space galaxy power spectra in the context of non-linear effects and model them based on the halo approach to large scale structure clustering. We consider potential systematic in the cosmological parameter determination when non-linear effects are ignored and the galaxy power spectrum is described with the linear power spectrum scaled by a constant bias factor. We suggest that significant improvements can be made when non-linear effects are taken into account as a power-law contribution with two additional parameters to be determined from the data. In addition to cosmological parameters through galaxy clustering, such an approach allow a determination of useful information related to astrophysics on how galaxies occupy dark matter halos.

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arXiv2003

Polarized CMB: Reionization and Primordial Tensor modes

We discuss upcoming opportunities with cosmic microwave background (CMB) observations during the post-WMAP era. The curl-modes of CMB polarization probe inflationary gravitational waves (IGWs). While a significant source of confusion is expected from cosmic shear conversion of polarization related to density perturbations, higher resolution observations of CMB anisotropies can be used for a lensing reconstruction and to separate gravitational-wave polarization signature from that of lensing. Separations based on current lensing reconstruction techniques allow the possibility to probe inflationary energy scales below 10^15 GeV in a range that includes grand unified theories. The observational detection of primordial curl-modes is aided by rescattering at late times during the reionized epoch with optical depth to electron scattering at the level of 0.1 and above. An improved measurement of this optical depth is useful to optimize experimental parameters of a post-WMAP mission attempting to target the IGW background.

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arXiv2003

Optical Identification of Close White Dwarf Binaries in the LISA Era

The Laser Interferometer Space Antenna (LISA) is expected to detect close white dwarf binaries (CWDBs) through their gravitational radiation. Around 3000 binaries will be spectrally resolved at frequencies > 3 mHz, and their positions on the sky will be determined to an accuracy ranging from a few tens of arcminutes to a degree or more. Due to the small binary separation, the optical light curves of >~ 30% of these CWDBs are expected to show eclipses, giving a unique signature for identification in follow-up studies of the LISA error boxes. While the precise optical location improves binary parameter determination with LISA data, the optical light curve captures additional physics of the binary, including the individual sizes of the stars in terms of the orbital separation. To optically identify a substantial fraction of CWDBs and thus localize them very accurately, a rapid monitoring campaign is required, capable of imaging a square degree or more in a reasonable time, at intervals of 10--100 seconds, to magnitudes between 20 and 25. While the detectable fraction can be up to many tens of percent of the total resolved LISA CWDBs, the exact fraction is uncertain due to unknowns related to the white dwarf spatial distribution, and potentially interesting physics, such as induced tidal heating of the WDs due to their small orbital separation.

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arXiv2003

Cross-correlation between the soft X-ray background and SZ Sky

While both X-ray emission and Sunyaev-Zel'dovich (SZ) temperature fluctuations are generated by the warm-hot gas in dark matter halos, the two observables have different dependence on the underlying physical properties, including the gas distribution. A cross-correlation between the soft X-ray background (SXRB) and the SZ sky may allow an additional probe on the distribution of warm-hot gas at intermediate angular scales and redshifts complementing studies involving clustering within SXRB and SZ separately. Using a halo approach, we investigate this cross-correlation analytically. The two contributions are correlated mildly with a correlation coefficient of $\sim0.3$, and this relatively low correlation presents a significant challenge for its detection. The correlation, at small angular scales, is affected by the presence of radiative cooling or preheating and provides a probe on the thermal history of the hot gas in dark halos. While the correlation remains undetectable with CMB data from the WMAP satellite and X-ray background data from existing catalogs, upcoming observations with CMB missions such as Planck, for the SZ side, and an improved X-ray map of the large scale structure, such as the one planned with DUET mission, may provide a first opportunity for a reliable detection of this cross-correlation.

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arXiv2003

Lensing Studies with Diffuse Backgrounds

The current weak lensing measurements of the large scale structure are mostly related to statistical study of background galaxy ellipticities. We consider a possibility to extend lensing studies with intrinsically unresolved sources and suggest that spatial fluctuations in the integrated diffuse emission from these sources can be used for a lensing reconstruction. Examples of upcoming possibilities include the diffuse background generated by dusty starburst galaxies at far-infrared wavelengths, first stars and galaxies in near-infrared wavelengths, and the background related to 21 cm emission by neutral gas in the general intergalactic medium prior to reionization. While methods developed to extract lensing information from cosmic microwave background (CMB) temperature and polarization data can be easily modified to study lensing properties using diffuse backgrounds at other wavelengths, we suggest that the lensing extraction from these backgrounds using higher order non-Gaussian clustering information alone may not be the best approach. In contrast to CMB anisotropies, reasons for this include the lack of features in the clustering power spectrum such that the resulting lensing modification to the angular power spectrum of low-redshift diffuse backgrounds, at arcminute angular scales, is insignificant. While the use of low redshift backgrounds for lensing studies will be challenging, due to confusing foregrounds among other reasons, the use of suggested backgrounds will extend the reconstruction of the integrated matter power spectrum out to redshifts of 15 to 30, and will bridge the gap between current and upcoming galaxy lensing studies out to, at most, a redshift of a few and planned weak lensing studies with CMB out to the last scattering surface at a redshift of 1100.

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arXiv2003

Can Non-Gaussian Cosmological Models Explain the WMAP's High Optical Depth for Reionization?

The first-year Wilkinson Microwave Anisotropy Probe data suggest a high optical depth for Thomson scattering of 0.17 +/- 0.04, implying that the universe was reionized at an early epoch, z ~ 20. Such early reionization is likely to be caused by UV photons from first stars, but it appears that the observed high optical depth can be reconciled within the standard structure formation model only if star-formation in the early universe was extremely efficient. With normal star-formation efficiencies, cosmological models with non-Gaussian density fluctuations may circumvent this conflict as high density peaks collapse at an earlier epoch than in models with Gaussian fluctuations. We study cosmic reionization in non-Gaussian models and explore to what extent, within available constraints, non-Gaussianities affect the reionization history. For mild non-Gaussian fluctuations at redshifts of 30 to 50, the increase in optical depth remains at a level of a few percent and appears unlikely to aid significantly in explaining the measured high optical depth. On the other hand, within available observational constraints, increasing the non-Gaussian nature of density fluctuations can easily reproduce the optical depth and may remain viable in underlying models of non-Gaussianity with a scale-dependence.

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arXiv2003

Can Cosmic Shear Shed Light on Low Cosmic Microwave Background Multipoles?

The lowest multipole moments of the cosmic microwave background (CMB) are smaller than expected for a scale-invariant power spectrum. One possible explanation is a cutoff in the primordial power spectrum below a comoving scale of $k_c \simeq 5.0 \times 10^{-4}$ Mpc$^{-1}$. This would affect not only the CMB but also the cosmic-shear (CS) distortion of the CMB. Such a cutoff increases significantly the cross-correlation between the large-angle CMB and cosmic-shear patterns. The cross-correlation may be detectable at $> 2σ$ which, when combined with the low CMB moments, may tilt the balance between a $2σ$ result and a firm detection of a large-scale power-spectrum cutoff. As an aside, we also note that the cutoff increases the large-angle cross-correlation between the CMB and low-redshift tracers of the mass distribution.

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arXiv2003

Distances and Cosmology From Galaxy Cluster CMB Data

The measurement of angular diameter distance to galaxy clusters, through combined Sunyaev-Zel'dovich (SZ) effect data with X-ray emission observations, is now a well-known probe of cosmology. Using a combination of SZ data and a map of the lensed CMB anisotropies by the galaxy cluster potential, we propose an alternative geometric technique to measure distance information primarily through cluster related multi-frequency CMB measurements. We discuss necessary requirements to implement this measurement, potential errors including systematic biases, and the extent to which cosmological parameters can be extracted. While individual cluster distances are not likely to be precise, with upcoming subarcminute resolution wide-area CMB observations, useful information on certain cosmological parameters, such as the equation of state of dark energy, can be obtained from a large sample of galaxy clusters.

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