A multifrequency angular power spectrum analysis of the Leiden polarization surveys

The Galactic synchrotron emission is expected to be the most relevant source of astrophysical contamination in cosmic microwave background polarization measurements, at least at frequencies 30'. We present a multifrequency analysis of the Leiden surveys, linear polarization surveys covering essentially the Northern Celestial Hemisphere at five frequencies between 408 MHz and 1411 MHz. By implementing specific interpolation methods to deal with these irregularly sampled data, we produced maps of the polarized diffuse Galactic radio emission with pixel size of 0.92 deg. We derived the angular power spectrum (APS) (PI, E, and B modes) of the synchrotron dominated radio emission as function of the multipole, l. We considered the whole covered region and some patches at different Galactic latitudes. By fitting the APS in terms of power laws (C_l = k l^a), we found spectral indices that steepen with increasing frequency: from a = -(1-1.5) at 408 MHz to a = -(2-3) at 1411 MHz for 10 < l < 100 and from a = -0.7 to a = -1.5 for lower multipoles (the exact values depending on the considered sky region and polarization mode). The bulk of this steepening can be interpreted in terms of Faraday depolarization effects. We then considered the APS at various fixed multipoles and its frequency dependence. Using the APSs of the Leiden surveys at 820 MHz and 1411 MHz, we determined possible ranges for the rotation measure, RM, in the simple case of an interstellar medium slab model. Taking also into account the polarization degree at 1.4 GHz, we could break the degeneracy between the identified RM intervals. The most reasonable of them turned out to be RM = 9-17 rad/m^2.Comment: 18 pages, 14 figures. Astronomy and Astrophysics, in pres

The GMRT EoR Experiment: Limits on Polarized Sky Brightness at 150 MHz

The GMRT reionization effort aims to map out the large scale structure of the Universe during the epoch of reionization (EoR). Removal of polarized Galactic emission is a difficult part of any 21 cm EoR program, and we present new upper limits to diffuse polarized foregrounds at 150 MHz. We find no high significance evidence of polarized emission in our observed field at mid galactic latitude (J2000 08h26m+26). We find an upper limit on the 2-dimensional angular power spectrum of diffuse polarized foregrounds of [l^2 C_l/(2 PI)]^{1/2}< 3K in frequency bins of width 1 MHz at 300<l<1000. The 3-dimensional power spectrum of polarized emission, which is most directly relevant to EoR observations, is [k^3 P_p(k)/(2 PI^2)]^{1/2} 0.03 h/Mpc, k < 0.1 h/Mpc. This can be compared to the expected EoR signal in total intensity of [k^3 P(k)/ (2 PI^2) ]^{1/2} ~ 10 mK. We find polarized structure is substantially weaker than suggested by extrapolation from higher frequency observations, so the new low upper limits reported here reduce the anticipated impact of these foregrounds on EoR experiments. We discuss Faraday beam and depth depolarization models and compare predictions of these models to our data. We report on a new technique for polarization calibration using pulsars, as well as a new technique to remove broadband radio frequency interference. Our data indicate that, on the edges of the main beam at GMRT, polarization squint creates ~ 3% leakage of unpolarized power into polarized maps at zero rotation measure. Ionospheric rotation was largely stable during these solar minimum night time observations.Comment: 17 pages, 6 figures, 2 tables; changed figures, added appendices. To appear in MNRA

Global spectral energy distribution of the Crab Nebula in the prospect of the Planck satellite polarisation calibration

Whithin the framework of the Planck satellite polarisation calibration, we present a study of the Crab Nebula spectral energy distribution (SED) over more than 6 decades in frequency ranging from 1 to $\rm 10^6 GHz$. The Planck satellite mission observes the sky from 30 to 857 GHz and therefore we focus on the millimetre region. We use radio and submillimetre data from the WMAP satellite between 23 and 94 GHz (from 13 to 3.18 mm) and from the Archeops balloon experiment between 143 (2.1 mm) and 545 GHz (0.55 mm), and a compendium of other Crab Nebula observations. The Crab SED is compared to models including three main components : synchrotron which is responsible for the emission at low and at high frequencies, dust which explains the excess of flux observed by the IRAS satellite and an extra component on the millimetre regime. From this analysis we conclude that the unpolarised emission of the Crab Nebula at microwave and millimetre wavelengths is the same synchrotron emission that the one observed in the radio domain. Therefore, we expect the millimetre emission of the Crab nebula to be polarised with the same degree of polarisation and orientation than the radio emission. We set upper limits on the possible errors induced by any millimetre extra component on the reconstruction of the degree and angle of polarisation at the percent level as a maximum. This result strongly supports the choice by the Planck collaboration of the Crab nebula emission for performing polarisation cross-checks in the range 30 (299 mm) to 353 GHz (0.849 mm)

The BaR-SPOrt Experiment

BaR-SPOrt (Balloon-borne Radiometers for Sky Polarisation Observations) is an experiment to measure the linearly polarized emission of sky patches at 32 and 90 GHz with sub-degree angular resolution. It is equipped with high sensitivity correlation polarimeters for simultaneous detection of both the U and Q stokes parameters of the incident radiation. On-axis telescope is used to observe angular scales where the expected polarization of the Cosmic Microwave Background (CMBP) peaks. This project shares most of the know-how and sophisticated technology developed for the SPOrt experiment onboard the International Space Station. The payload is designed to flight onboard long duration stratospheric balloons both in the Northern and Southern hemispheres where low foreground emission sky patches are accessible. Due to the weakness of the expected CMBP signal (in the range of microK), much care has been spent to optimize the instrument design with respect to the systematics generation, observing time efficiency and long term stability. In this contribution we present the instrument design, and first tests on some components of the 32 GHz radiometer.Comment: 12 pages, 10 figures, Astronomical Telescopes and Instrumentation (Polaimetry in Astronomy) Hawaii August 2002 SPIE Meetin

SPOrt: an Experiment Aimed at Measuring the Large Scale Cosmic Microwave Background Polarization

SPOrt (Sky Polarization Observatory) is a space experiment to be flown on the International Space Station during Early Utilization Phase aimed at measuring the microwave polarized emission with FWHM = 7deg, in the frequency range 22-90 GHz. The Galactic polarized emission can be observed at the lower frequencies and the polarization of Cosmic Microwave Background (CMB) at 90 GHz, where contaminants are expected to be less important. The extremely low level of the CMB Polarization signal (< 1 uK) calls for intrinsically stable radiometers. The SPOrt instrument is expressly devoted to CMB polarization measurements and the whole design has been optimized for minimizing instrumental polarization effects. In this contribution we present the receiver architecture based on correlation techniques, the analysis showing its intrinsic stability and the custom hardware development carried out to detect such a low signal.Comment: 9 pages, 5 figures, conference proceeding, to appear in "Polarimetry in Astronomy", SPIE Symposium on 'Astronomical Telescopes and Instrumentation', Waikoloa, August 22-28 200

ON THE EVOLUTION OF RADIO EMISSION OF THE TYCHO BRAHE'S SUPERNOVA REMNANT (3C10)

Basing on long-term (1966-1998) measurements at the frequency 86 MHz the mean rate of the secular decrease of Tycho Brahe's supernova remnant (SNR) radio emission flux density has been estimated as (0.92 ± 0.70)%yr -1. The decrease of Tycho Brahe's SNR radio emission is not uniform in time. The rate of the secular decrease of this SNR radio emission is frequency independent in the limits of the errors. The weighted mean value of the rate of the secular decrease of the Tycho Brahe's SNR radio emission in the frequency range 86-5000 MHz is (0.41 ± 0.02)%yr -1