Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe power-law behaviours of angular spectra of polarized Galactic synchrotron
52
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We study the angular power spectra of polarized Galactic synchrotron in the range 10<l<800, at several frequencies between 0.4 and 2.7 GHz and at several Galactic latitudes up to near the North Galactic Pole. Electric- and magnetic-parity polarization spectra are found to have slopes around alpha _{E,B} = 1.4 - 1.5 in the Parkes and Effelsberg Galactic-Plane surveys, but strong local fluctuations of alpha_{E,B} are found at | b | ~ 10 degree from the 1.4 GHz Effelsberg survey. The C_{PIl} spectrum, which is insensitive to the polarization direction, is somewhat steeper, being alpha_{PI} = 1.6 - 1.8 for the same surveys. The low-resolution multifrequency survey of Brouw and Spoelstra (1976) shows some flattening of the spectra below 1 GHz, more intense for C_{E,Bl} than for C_{PIl}. In no case we find evidence for really steep spectra. The extrapolation to the cosmological window shows that at 90 GHz the detection of E-mode harmonics in the cosmic background radiation should not be disturbed by synchrotron, even around l~10 for a reionization optical depth tau _{ri}>~0.05.
rate research
Read More
We study the angular power spectra of the polarized component of the Galactic synchrotron emission in the 28-deg^2 Test Region of the Southern Galactic Plane Survey at 1.4 GHz. These data were obtained by the Australia Telescope Compact Array and allow us to investigate angular power spectra down to arcminute scales. We find that, at this frequency, the polarization spectra for E- and B-modes seem to be affected by Faraday rotation produced in compact foreground screens. A different behavior is shown by the angular spectrum of the polarized intensity PI=sqrt{Q^2+U^2}. This is well fitted by a power law with slope ~1.7, which agrees with higher frequency results and can probably be more confidently extrapolated to the cosmological window.
We develop an analytic model for the power spectra of polarized filamentary structures as a way to study the Galactic polarization foreground to the Cosmic Microwave Background. Our approach is akin to the cosmological halo-model framework, and reproduces the main features of the Planck 353 GHz power spectra. We model the foreground as randomly-oriented, three-dimensional, spheroidal filaments, accounting for their projection onto the sky. The main tunable parameters are the distribution of filament sizes, the filament physical aspect ratio, and the dispersion of the filament axis around the local magnetic field direction. The abundance and properties of filaments as a function of size determine the slopes of the foreground power spectra, as we show via scaling arguments. The filament aspect ratio determines the ratio of $B$-mode power to $E$-mode power, and specifically reproduces the Planck-observed dust ratio of one-half when the short axis is roughly one-fourth the length of the long axis. Filament misalignment to the local magnetic field determines the $TE$ cross-correlation, and to reproduce Planck measurements, we need a (three-dimensional) misalignment angle with a root mean squared dispersion of about 50 degrees. These parameters are not sensitive to the particular filament density profile. By artificially skewing the distribution of the misalignment angle, this model can reproduce the Planck-observed (and parity-violating) $TB$ correlation. The skewing of the misalignment angle necessary to explain $TB$ will cause a yet-unobserved, positive $EB$ dust correlation, a possible target for future experiments.
Galactic synchrotron and free-free foregrounds angular spectra are analytically estimated with account for interstellar turbulence and radiating process physics. Unknown parameters of the spectra are obtained by fitting to observational data.
Earlier papers introduced a method of accurately estimating the angular cosmic microwave background (CMB) temperature power spectrum based on Gibbs sampling. Here we extend this framework to polarized data. All advantages of the Gibbs sampler still apply, and exact analysis of mega-pixel polarized data sets is thus feasible. These advantages may be even more important for polarization measurements than for temperature measurements. While approximate methods can alias power from the larger E-mode spectrum into the weaker B-mode spectrum, the Gibbs sampler (or equivalently, exact likelihood evaluations) allows for a statistically optimal separation of these modes in terms of power spectra. To demonstrate the method, we analyze two simulated data sets: 1) a hypothetical future CMBPol mission, with the focus on B-mode estimation; and 2) a Planck-like mission, to highlight the computational feasibility of the method.
We have analyzed the available polarization surveys of the Galactic emission to estimate to what extent it may be a serious hindrance to forthcoming experiments aimed at detecting the polarized component of Cosmic Microwave Background (CMB) anisotropies. Regions were identified for which independent data consistently indicate that depolarization must be small. The power spectrum of the polarized emission, in terms of antenna temperature, was found to be described by $C_{ell}simeq (1.2pm 0.8)cdot 10^{-9}cdot (ell / 450)^{-1.8pm 0.3}cdot ( u/ 2.4{rm GHz})^{-5.8}$ K$^{2}$, from arcminute to degree scales. Data on larger angular scales ($ellle 100$) indicate a steeper slope $sim ell^{-3}$. We conclude that polarized Galactic emission is unlikely to be a serious limitation to CMB polarization measurements at the highest frequencies of the MAP and {sc Planck}/LFI instruments, at least for $ellge 50$ and standard cosmological models. The weak correlation between polarization and total power and the low polarization degree of radio emission close to the Galactic plane, found also in low-depolarization regions, is interpreted as due to large contributions to the observed intensity from unpolarized sources, primarily strong HII regions, concentrated on the Galactic plane. Thus estimates of the power spectrum of total intensity at low Galactic latitudes are not representative of the spatial distribution of Galactic emission far from the plane. Both total power and polarized emissions show highly significant deviations from a Gaussian distribution.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
