Faraday rotation measures (RMs) and more general Faraday structures are key parameters for studying cosmic magnetism and also are sensitive probes of faint ionized thermal gas. There is a need to define what derived quantities are required for various scientific studies, and then to address the challenges in determining Faraday structures. A wide variety of algorithms have been proposed to reconstruct these structures. In preparation for the Polarization Sky Survey of the Universe's Magnetism (POSSUM) to be conducted with the Australian Square Kilometre Array Pathfinder (ASKAP) and the ongoing Galactic Arecibo L-band Feeds Array Continuum Transit Survey (GALFACTS), we run a Faraday structure determination data challenge to benchmark the currently available algorithms including Faraday synthesis (previously called RM synthesis in the literature), wavelet, compressive sampling and QU -fitting. The input models include sources with one Faraday thin component, two Faraday thin components and one Faraday thick component. The frequency set is similar to POS-SUM/GALFACTS with a 300-MHz bandwidth from 1.1 to 1.4 GHz. We define three figures of merit motivated by the underlying science: a) an average RM weighted by polarized intensity, RM wtd , b) the separation ∆φ of two Faraday components and c) the reduced chi-squared χ 2 r . Based on the current test data of signal to noise ratio of about 32, we find that: (1) When only one Faraday thin component is present, most methods perform as expected, with occasional failures where two components are incorrectly found; (2) For two Faraday thin components, QU -fitting routines perform the best, with errors close to the theoretical ones for RM wtd , but with significantly higher errors for ∆φ. All other methods including standard Faraday synthesis frequently identify only one component when ∆φ is below or near the width of the Faraday point spread function; (3) No methods, as currently implemented, work well for Faraday thick components due to the narrow bandwidth; (4) There exist combinations of two Faraday components which produce a large range of acceptable fits and hence large uncertainties in the derived single RMs; in these cases, different RMs lead to the same Q, U behavior, so no method can recover a unique input model. Further exploration of all these issues is required before upcoming surveys will be able to provide reliable results on Faraday structures.

Unveiling the intergalactic magnetic field (IGMF) in filaments of galaxies is a very important and challenging subject in modern astronomy. In order to probe the IGMF from rotation measures (RMs) of extragalactic radio sources, we need to separate RMs due to other origins such as the source, intervening galaxies, and our Galaxy. In this paper, we discuss observational strategies for the separation by means of Faraday tomography (Faraday RM Synthesis). We consider an observation of a single radio source such as a radio galaxy or a quasar viewed through the Galaxy and the cosmic web. We then compare the observation with another observation of a neighbor source with a small angular separation. Our simulations with simple models of the sources suggest that it would be not easy to detect the RM due to the IGMF of order ∼ 1 rad m −2 , an expected value for the IGMF through a single filament. Contrary to it, we find that the RM of at least ∼ 10 rad m −2 could be detected with the SKA or its pathfinders/precursors, if we achieve selections of ideal sources. These results would be improved if we incorporate decomposition techniques such as RMCLEAN and QU-fitting. We discuss feasibility of the strategies for cases with complex Galactic emissions as well as with effects of observational noise and radio frequency interferences.

Faraday tomography is a powerful method to diagnose polarizations and Faraday rotations along the line of sight. The quality of Faraday tomography is, however, limited by several conditions. Recently, it is reported that Faraday tomography indicates false signals in some specific situations. In this paper, we systematically investigate the condition of the appearance of false signals in Faraday tomography. We study this by pseudo-observing two sources within a beam, and change in the intrinsic polarization angles, rotation measures, intensities, and frequency coverage. We find that false signals arise when rotation measure between the sources is less than 1.5 times the full width at half maximum of the rotation measure spread function. False signals also depend on the intensity ratio between the sources and are reduced for large ratio. On the other hand, the appearance of false signals does not depend on frequency coverage, meaning that the uncertainty should be correctly understood and taken into consideration even with future wide-band observations such as Square Kilometer Array (SKA).

We investigate the capability of ongoing radio telescopes for probing Faraday rotation measure (RM) due to the intergalactic magnetic field (IGMF) in the large-scale structure of the universe which is expected to be of order O(1) rad/m 2 . We consider polarization observations of a compact radio source such as quasars behind a diffuse source such as the Galaxy, and calculate Stokes parameters Q and U assuming a simple model of the Faraday dispersion functions with Gaussian shape. Then, we perform the Fisher analysis to estimate the expected errors in the model parameters from QU-fitting of polarization intensity, accounting for sensitivities and frequency bands of Australian Square Kilometer Array Pathfinder, Low Frequency Array, and the Giant Meterwave Radio Telescope. Finally, we examine the condition on the source intensities which are required to detect the IGMF. Our analysis indicates that the QU-fitting is promising for forthcoming wideband polarimetry to explore RM due to the IGMF in filaments of galaxies.Subject headings: magnetic fields -polarization -intergalactic mediumlarge-scale structure of universe

Abstract. A possible origin of the anomalous dip and bump in the primordial power spectrum, which are reconstructed from WMAP data corresponding to the multipole ℓ = 100 ∼ 140 by using the inversion method, is investigated as a consequence of modification of scalar field dynamics in the inflation era. Utilizing an analytic formula to handle higher order corrections to the slow-roll approximation, we evaluate the relation between a detailed shape of inflaton potential and a fine structure in the primordial power spectrum. We conclude that it is unlikely to generate the observed dip and bump in the power spectrum by adding any features in the inflaton potential. Though we can make a fine enough shape in the power spectrum by controlling the feature of the potential, the amplitude of the dip and bump becomes too small in that case.