624 research outputs found
Study of systematics effects on the Cross Power Spectrum of 21 cm Line and Cosmic Microwave Background using Murchison Widefield Array Data
Observation of the 21cm line signal from neutral hydrogen during the Epoch of
Reionization is challenging due to extremely bright Galactic and extragalactic
foregrounds and complicated instrumental calibration. A reasonable approach for
mitigating these problems is the cross correlation with other observables. In
this work, we present the first results of the cross power spectrum (CPS)
between radio images observed by the Murchison Widefield Array and the cosmic
microwave background (CMB), measured by the Planck experiment. We study the
systematics due to the ionospheric activity, the dependence of CPS on group of
pointings, and frequency. The resulting CPS is consistent with zero because the
error is dominated by the foregrounds in the 21cm observation. Additionally,
the variance of the signal indicates the presence of unexpected systematics
error at small scales. Furthermore, we reduce the error by one order of
magnitude with application of a foreground removal using a polynomial fitting
method. Based on the results, we find that the detection of the 21cm-CMB CPS
with the MWA Phase I requires more than 99.95% of the foreground signal
removed, 2000 hours of deep observation and 50% of the sky fraction coverage.Comment: 15 pages, 16 figures, accepted to MNRA
Constraints on the Evolution of the Primordial Magnetic Field from the Small-Scale Cosmic Microwave Background Angular Anisotropy
Recent observations of the cosmic microwave background (CMB) have extended
the measured power spectrum to higher multipoles 1000, and there
appears to be possible evidence for excess power on small angular scales. The
primordial magnetic field (PMF) can strongly affect the CMB power spectrum and
the formation of large scale structure. In this paper, we calculate the CMB
temperature anisotropies generated by including a power-law magnetic field at
the photon last-scattering surface (PLSS). We then deduce an upper limit on the
PMF based on our theoretical analysis of the power excess on small angular
scales. We have taken into account several important effects such as the
modified matter sound speed in the presence of a magnetic field. An upper limit
to the field strength of 4.7 nG at the present scale of 1
Mpc is deduced. This is obtained by comparing the calculated theoretical result
including the Sunyaev-Zeldovich (SZ) effect with recent observed data on the
small-scale CMB anisotropies from the
(WMAP), the Cosmic Background Imager (CBI), and the Arcminute Cosmology
Bolometer Array Receiver (ACBAR). We discuss several possible mechanisms for
the generation and evolution of the PMF.Comment: 27 pages, 4 figures, accepted to ApJ April 10, 200
Probing the Nature of the Weakest Intergalactic Magnetic Fields with the High Energy Emission of Gamma-Ray Bursts
We investigate the delayed, secondary GeV-TeV emission of gamma-ray bursts
and its potential to probe the nature of intergalactic magnetic fields.
Geometrical effects are properly taken into account for the time delay between
primary high energy photons and secondary inverse Compton photons from
electron-positron pairs, which are produced in - interactions
with background radiation fields and deflected by intervening magnetic fields.
The time-dependent spectra of the delayed emission are evaluated for a wide
range of magnetic field strengths and redshifts. The typical flux and delay
time of secondary photons from bursts at are respectively GeV cm s and s if the field strengths are
G, as might be the case in intergalactic void regions. We find
crucial differences between the cases of coherent and tangled magnetic fields,
as well as dependences on the field coherence length.Comment: 19 pages, 9 figures, formulation revised, accepted for publication in
Ap
Constraining the Primordial Magnetic Field from Cosmic Microwave Background Anisotropies at Higher Multipoles
The cosmological magnetic field is one of the important physical quantities
which affect strongly the cosmic microwave background (CMB) power spectrum.
Recent CMB observations have been extended to higher multipoles 1000,
and they resultantly exhibit an excess power than the standard model prediction
in cosmological theory which best fits the Wilkinson Microwave Anisotropy Probe
(WMAP) data at lower multipoles 900. We calculate the CMB
temperature anisotropies generated by the power-law magnetic field at the last
scattering surface (LSS) in order to remove the tension between theory and
observation at higher multipoles and also place an upper limit on primordial
magnetic field. In our present calculation we take account of the effect of
ionization ratio exactly without approximation. This effect is very crucial to
precisely estimate the effect of the magnetic field on CMB power spectrum. We
consider both effects of the scalar and vector modes of magnetic field on the
CMB anisotropies, where current data are known to be insensitive to the tensor
mode which we ignore in the present study. In order to constrain the primordial
magnetic field, we evaluate likelihood function of the WMAP data in a wide
range of parameters of the magnetic field strength and
the power-law spectral index , along with six cosmological parameters in
flat Universe models, using the technique of the Markov Chain Monte Carlo(MCMC)
method. We find that the upper limit at C.L. turns out to be
nG at 1 Mpc for any values, which is
obtained by comparing the calculated result including the Sunyaev-Zeldovich(SZ)
effect with recent WMAP data of the CMB anisotropies.Comment: 10 pages, 1 figures, 1 table, accepted to ApJ Letter April 13, 200
A Simulation Method to Resolve Hydrodynamic Interactions in Colloidal Dispersions
A new computational method is presented to resolve hydrodynamic interactions
acting on solid particles immersed in incompressible host fluids. In this
method, boundaries between solid particles and host fluids are replaced with a
continuous interface by assuming a smoothed profile. This enabled us to
calculate hydrodynamic interactions both efficiently and accurately, without
neglecting many-body interactions. The validity of the method was tested by
calculating the drag force acting on a single cylindrical rod moving in an
incompressible Newtonian fluid. This method was then applied in order to
simulate sedimentation process of colloidal dispersions.Comment: 7pages, 7 figure
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