Low Frequency Instrument Research Articles

Wide-field imaging and polarimetry for the biggest and brightest in the 20-GHz southern sky

We present the wide-field imaging and polarimetry at ν = 20 GHz of seven of the most extended, bright (Stotal > 0.50 Jy), high-frequency selected radio sources in the southern sky with declinations δ < 30 ◦ . Accompanying the data are brief reviews of the literature for each source. The results presented here aid in the statistical completeness of the Australia Telescope 20GHz Survey (AT20G) Bright Source Sample (Massardi et al. 2008; Burke-Spolaor et al., in prep). The data is of crucial interest for future CMB missions as a collection of information about candidate calibrator sources. We were able to obtain data for seven of the nine sources identified by our selection criteria. We report that Pictor A is thus far the best extragalactic calibrator candidate for the Low Frequency Instrument (LFI) of the Planck ESA mission due to its high level of integrated polarized flux density (� 0.50±0.06 Jy) on a scale of 10 arcminutes. Six out of the seven sources have a clearly detected compact radio core in our images, with either a null detection or less than 2 percent detection of polarised emission from the nuclei. Most sources with detected jets have magnetic field alignments running in a longitudinal configuration, however PKS 1333-33 exhibits transverse fields and an orthogonal change in field geometry from nucleus to jets.

Matrix Filters for the Detection of Extragalactic Point Sources in Cosmic Microwave Background Images

In this paper we introduce a new linear filtering technique, the so-called matrix filters, that maximizes the signal-to-interference ratio of compact sources of unknown intensity embedded in a set of images by taking into account the cross-correlations between the different channels. By construction, the new filtering technique outperforms (or at least equals) the standard matched filter applied on individual images. An immediate application is the detection of extragalactic point sources in cosmic microwave background images obtained at different wavelengths. We test the new technique in two simulated cases: a simple two-channel case with ideal correlated color noise and more realistic simulations of the sky as it will be observed by the low-frequency instrument instrument of the upcoming ESA's Planck mission. In both cases, we observe an improvement with respect to the standard matched filter in terms of signal-to-noise interference, number of detections and number of false alarms.

WMAP five-year constraints on lepton asymmetry and radiation energy density:implications for Planck

In this paper we set bounds on the radiation content of the Universe and neutrinoproperties by using the WMAP (Wilkinson microwave anisotropy probe)five-year CMB (cosmic microwave background) measurements complementedwith most of the existing CMB and LSS (large scale structure) data(WMAP5+All), imposing also self-consistent BBN (big bang nucleosynthesis) constraints on theprimordial helium abundance.We consider lepton asymmetric cosmological models parametrized by the neutrino degeneracy parameterξν and the variation of the relativistic degrees of freedom,ΔNeffoth, due to possible other physical processes occurring between BBN and structure formationepochs.We get a mean value of the effective number of relativistic neutrino species ofNeff = 2.98 2.273.60 1.654.37, providing an important improvement over the similar result obtained fromWMAP5+BAO+SN+HST (BAO: baryonic acoustic oscillations; SN: supernovae; HST: Hubble Space Telescope) data(Komatsu et al (WMAP Collaboration), 2008 Astrophys. J. Suppl. submitted [0803.0547]).We also find a strong correlation betweenΩmh2 andzeq, showing that weobserve Neff mainlyvia the effect of zeq, rather than via neutrino anisotropic stress as claimed by the WMAP team (Komatsuet al (WMAP Collaboration), 2008 Astrophys. J. Suppl. submitted [0803.0547]).WMAP5+All data provide a strong bound on the helium mass fraction ofYp = 0.2486 ± 0.0085 (68% CL), thatrivals the bound on Yp obtained from the conservative analysis of the present data on helium abundance.For the neutrino degeneracy parameter we find a bound of−0.216≤ξν≤0.226 (68% CL), which represents an important improvement over the similar result obtained byusing the WMAP three-year data.The inclusion in the analysis of LSS data reduces the upper limit of the neutrino mass tomν<0.46 eV (95% CL) with respect to the values obtained from the analysis from WMAP5-only data(Dunkley et al (WMAP Collaboration), 2008 Astrophys. J. Suppl. submitted [0803.0586]) andWMAP5+BAO+SN+HST data (Komatsu et al (WMAP Collaboration), 2008 Astrophys. J. Suppl. submitted [0803.0547]).We forecast that the CMB temperature and polarization measurements observed with highangular resolutions and sensitivities by the future Planck satellite will reduce the errors onξν and Yp down to (68% CL) and σ(Yp) = 0.013 (68% CL) respectively, values fully consistent with the BBN bounds on these parameters.This work has been done on behalf of Planck-LFI (low frequency instrument)activities.

Making maps from Planck LFI 30 GHz data

This paper is one of a series describing the performance and accuracy of map-making codes as assessed by the Planck CTP working group. We compare the performance of multiple codes written by different groups for making polarized maps from Planck-sized, all-sky cosmic microwave background (CMB) data. Three of the codes are based on a destriping algorithm, whereas the other three are implementations of a maximum-likelihood algorithm. Previous papers in the series described simulations at 100 GHz (Poutanen et al. 2006, A&A, 449, 1311) and 217 GHz (Ashdown et al. 2007, A&A, 467, 761). In this paper we make maps (temperature and polarisation) from the simulated one-year observations of four 30 GHz detectors of Planck Low Frequency Instrument (LFI). We used Planck Level S simulation pipeline to produce the observed time-ordered-data streams (TOD). Our previous studies considered polarisation observations for the CMB only. For this paper we increased the realism of the simulations and included polarized galactic foregrounds in our sky model, which is based on the version 0.1 of the PLANCK reference sky. Our simulated TODs comprised dipole, CMB, diffuse galactic emissions, extragalactic radio sources, and detector noise. The strong subpixel signal gradients arising from the foreground signals couple to the output map through the map-making and cause an error (signal error) in the maps. Destriping codes have smaller signal error than the maximum-likelihood codes. We examined a number of schemes to reduce this error. On the other hand, the maximum-likelihood map-making codes can produce maps with lower residual noise than destriping codes.

Tracking Solar Gravity Modes: The Dynamics of the Solar Core

Solar gravity modes have been actively sought because they directly probe the solar core (below 0.2 solar radius), but they have not been conclusively detected in the Sun because of their small surface amplitudes. Using data from the Global Oscillation at Low Frequency instrument, we detected a periodic structure in agreement with the period separation predicted by the theory for gravity dipole modes. When studied in relation to simulations including the best physics of the Sun determined through the acoustic modes, such a structure favors a faster rotation rate in the core than in the rest of the radiative zone.

The Mexican hat wavelet family: application to point-source detection in cosmic microwave background maps

We propose a new detection technique in the plane based on an isotropic wavelet family. This family is naturally constructed as an extension of the Gaussian–Mexican hat wavelet pair and for that reason we call it the Mexican hat wavelet family (MHWF). We show the performance of these wavelets when dealing with the detection of extragalactic point sources in cosmic microwave background (CMB) maps: a very important issue within the most general problem of the component separation of the microwave sky. Specifically, flat two-dimensional simulations of the microwave sky comprising all astrophysical components plus instrumental noise have been analysed for the channels at 30, 44 and 70 GHz of the forthcoming ESA Planck mission Low Frequency Instrument (LFI). We adopt up-to-date cosmological evolution models of extragalactic sources able to fit well the new data on high-frequency radio surveys and we discuss our current results on point-source detection by comparing them with those obtained using the Mexican hat wavelet (MHW) technique, which has been already proven a suitable tool for detecting point sources. By assuming a 5 per cent reliability level, the first new members of the MHWF, at their ‘optimal scale’, provide three point-source catalogues on half of the sky (at Galactic latitude |b| > 30°) at 30, 44 and 70 GHz of 639, 387 and 340 extragalactic sources, respectively. The corresponding flux detection limits are 0.38, 0.45 and 0.47 Jy. By using the same simulated sky patches and at the same frequencies as before, the MHW at its optimal scale provides 543, 322 and 311 sources with flux detection limits of 0.44, 0.51 and 0.50 Jy, respectively (5 per cent reliability level). These results show a clear improvement when we use the new members of the MHWF and, in particular, the MHW2 with respect to the MHW.

Comparison of map-making algorithms for CMB experiments

We have compared the cosmic microwave background (CMB) temperature anisotropy maps made from one-year time ordered data (TOD) streams that simulated observations of the originally planned 100 GHz Planck Low Frequency Instrument (LFI). The maps were made with three different codes. Two of these, ROMA and MapCUMBA, were implementations of maximum-likelihood (ML) map-making, whereas the third was an implementation of the destriping algorithm. The purpose of this paper is to compare these two methods, ML and destriping, in terms of the maps they produce and the angular power spectrum estimates derived from these maps. The difference in the maps produced by the two ML codes was found to be negligible. As expected, ML was found to produce maps with lower residual noise than destriping. In addition to residual noise, the maps also contain an error which is due to the effect of subpixel structure in the signal on the map-making method. This error is larger for ML than for destriping. If this error is not corrected a bias will be introduced in the power spectrum estimates. This study is related to Planck activities.

Planck-LFI 44 GHz back end module

This work describes the principle of operation, assembly and performance of one branch of the 44 GHz back end module (BEM) for the Planck low frequency instrument (LFI). This subsystem constitutes a fully representative branch of the qualification-model version (QM). It includes waveguide to microstrip transition, GaAs pseudomorphic high electron mobility transistor (PHEMT) low noise amplifiers (LNA), bandpass filter, square-law detector and dc amplifier. The fundamentals of the design of the RF part are described and all of the components have been tested individually before integration. Using single tone and wideband noise stimuli, the output voltage has been measured for several input powers, in order to obtain the sensitivity factor of the complete BEM. The effective bandwidth and the equivalent noise temperature have been calculated from the measurements, taking into account the frequency dependence on the noise source and the BEM. Finally, the low frequency output power spectrum has been obtained and a maximum 1/f knee frequency around 200 Hz has been measured with a 3 dB output signal video bandwidth above 50 KHz.

Very low-noise differential radiometer at 30 GHz for the PLANCK LFI

The PLANCK mission of the European Space Agency is devoted to produce sky maps of the cosmic microwave background radiation. The low-frequency instrument is a wide-band cryogenic microwave radiometer array operating at 30, 44, and 70 GHz. The design, test techniques, and performance of the complete differential radiometer at 30 GHz are presented. This elegant breadboard 30-GHz radiometer is composed of a front-end module (FEM) assembled at the Jodrell Bank Observatory, Cheshire, U.K., and a back-end module assembled at the Universidad de Cantabria, Cantabria, Spain, and Telecomunicacio/spl acute/, Universitat Polite/spl acute/cnica de Catalunya, Barcelona, Spain. The system noise temperature was excellent, mainly due to the very low noise performance of the FEM amplifiers, which achieved an average noise temperature of 9.4 K.

Madam- a map-making method for CMB experiments

We present a new map-making method for cosmic microwave background (CMB) measurements. The method is based on the destriping technique, but it also utilizes information about the noise spectrum. The low-frequency component of the instrument noise stream is modelled as a superposition of a set of simple base functions, whose amplitudes are determined by means of maximum-likelihood analysis, involving the covariance matrix of the amplitudes. We present simulation results with 1/f noise and show a reduction in the residual noise with respect to ordinary destriping. This study is related to Planck Low Frequency Instrument (LFI) activities.

Trade-off between angular resolution and straylight contamination in the PLANCK Low Frequency Instrument

The study of cosmic microwave background (CMB) anisotropies represents one of the most powerful cosmological tools. After the great success of the two NASA satellite missions COBE and WMAP, PLANCK is the third generation of mm-wave instruments designed for space observations of CMB anisotropies within the new Cosmic Vision 2020 ESA Science Programme. PLANCK will map the whole sky with unprecedented sensitivity, angular resolution and frequency coverage, using two instruments that share the focal region of a 1.5 m off-axis dual reflector telescope: the Low Frequency Instrument (LFI) and the High Frequency Instrument (HFI). In the optimisation of the optical interfaces of the LFI two concurrent demands have to be satisfied: the very good angular resolution (which affects the ability to reconstruct the angular power spectrum of the CMB anisotropies at high multipoles) and a very low level of straylight contamination (which may be one of the most serious sources of systematic effects). We present the results of the optical simulations aimed at establishing the trade-off between angular resolution and straylight rejection, carried out for the 100 GHz channel of the PLANCK LFI. Antenna patterns of different models of dual profiled corrugated conical feed horns have been simulated using advanced simulation techniques, considering the whole spacecraft geometry in order to obtain reliable sidelobe predictions. We show the optical computation accuracy necessary to provide strong straylight evaluation in reasonable computational time and demonstrate the inadequacy of a Gaussian feed model in realistic far beam predictions.

Trade-off between angular resolution and straylight contamination in the PLANCK Low Frequency Instrument

The last generation of CMB anisotropy experiments operating either from space, like the WMAP and PLANCK satellite, from the atmosphere, such as balloons, or from the ground, like interferometers, make use of complex multi-frequency instruments at the focus of meter class telescopes to allow the joint study of CMB and foreground anisotropies, necessary to achieve an accurate component separation. Between ∼70 GHz and ∼300 GHz, where foreground contamination is minimum, it is extremely important to reach the best trade-off between the improvement of the angular resolution, necessary for measuring the high order acoustic peaks of CMB anisotropy, and the minimization of the straylight contamination mainly due to the Galactic emission. This is one ol the most critical systematic effects at large and intermediate angular scales (i.e. at multipoles f less than 100) and consists in unwanted radiation entering the beam at large angles from the direction of the antenna boresight direction. We consider here the 30 and 100 GHz channels of the PLANCK Low Frequency Instrument (LFI). Assuming the nominal PLANCK scanning strategy, we evaluate the straylight contamination introduced by the most relevant Galactic foreground components for a reference set of optical configurations, accurately simulated as described in Sandri et al. (2004, A&A, 428, 299) (hereafter Paper 1). Given the overall constraints to the LFI optical design, we show that it is possible to improve the angular resolution by 5-7% by keeping the overall peak-to-peak signal of the Galaxy straylight contamination (GSC) below the level of few μK (and about 10 times smaller in terms of rms). A comparison between the level of straylight introduced by the different Galactic components for different beam regions (intermediate and far beam) is presented. We provide approximate relations, both for the intermediate and the far beam, for the rms and the peak-to-peak levels of the GSC as functions of the corresponding contributions to the integrated beam or of the spillover. For some reference cases we compare the results based on Galactic foreground maps derived from radio, IR, and Hα templates with those based on WMAP maps including CMB and extragalactic source fluctuations. The implications for the GSC in the PLANCK LFI polarization data are discussed. Finally, we compare the results obtained at 100 GHz with those at 30 GHz, where the GSC is more critical.

A maximum likelihood approach to the destriping technique

The destriping technique is a viable tool for removing different kinds of systematic effects in CMB related experiments. It has already been proven to work for gain instabilities that produce the so-called 1/f noise and periodic fluctuations due to e.g. thermal instability. Both effects when coupled with the observing strategy result in stripes on the observed sky region. Here we present a maximum-likelihood approach to this type of technique and provide also a useful generalization. As a working case we consider a data set similar to what the Planck satellite will produce in its Low Frequency Instrument (LFI). We compare our method to those presented in the literature and find some improvement in performance. Our approach is also more general and allows for different base functions to be used when fitting the systematic effect under consideration. We study the effect of increasing the number of these base functions on the quality of signal cleaning and reconstruction. This study is related to Planck LFI activities.

Cosmic microwave background power spectrum estimation with the destriping technique

Extraction of the cosmic microwave background (CMB) angular power spectrum is a challenging task for current and future CMB experiments due to the large data sets involved. Here we describe an implementation of Monte Carlo apodized spherical transform estimator (MASTER) described in Hivon et al., which exploits the destriping technique as a map-making method. In this method a noise estimate based on destriped noise-only Monte Carlo (MC) simulations is subtracted from the pseudo-angular power spectrum. As a working case we use realistic simulations of the Planck low-frequency instrument (LFI). We found that the effect of destriping on a pure sky signal is minimal and requires no correction. Instead we found an effect related to the distribution of detector pointings, which affects the high-ℓ part of the power spectrum. We correct for this by subtracting a ‘signal bias’ estimated by MC simulations. We also give analytical estimates for this signal bias. Our method is fast and accurate enough (the estimator is unbiased and errors are close to theoretical expectations for maximal accuracy) to estimate the CMB angular power spectra for current and future CMB space missions. This study is related to Planck LFI activities.

Tracing the “Acoustic” Solar Cycle: A Direct Comparison of BiSON and GOLF Low‐lp‐Mode Variations

We present a detailed, comparative study of low angular degree solar p-mode variations extracted by analyses of two sets of observational data. These were collected by the ground-based Birmingham Solar-Oscillations Network (BiSON) and the Global Oscillations at Low Frequency (GOLF) instrument on board the ESA/NASA SOHO satellite. The ~5.5 yr period analyzed covers the complete rising phase of solar activity cycle 23 (1996-2002). We find an excellent level of agreement in the uncovered variations, indicating that the two data sets are highly correlated and dominated by the same mode realization noise (the signature of the stochastic forcing of the resonances). The results lend further support to the surmise that changes in damping alone may account for observed variations in mode power and damping. While significant variations in peak asymmetry are uncovered in the near-continuous GOLF set, a similar analysis of the BiSON database yields a null result. This reflects the deleterious impact of its ground-based window function on the precision with which the asymmetry can be determined. As such we are unable to rule out the possibility that variations of magnitude similar to those in GOLF may be present in the BiSON observations.

Analysis of the radiometer—reference load system on board the Planck/LFI instrument

The Low Frequency Instrument aboard the Planck satellite will employ pseudo-correlation radiometers, operating over three broad bands centered at 30, 44, 70GHz. This choice is oriented to maximize the instrumental stability reducing non-white noise effects produced inside the radiometers. The radiometer scheme allows the simultaneous comparison of two input signals, one coming from the telescope and the other coming from a reference blackbody (the “reference load” RL) at a stable temperature near 4K. In order to minimize non-white noise, typically exhibiting a 1/f spectrum, the Reference Load temperature must be as close as possible to the sky temperature (about 2.7K). The accuracy of the measurement is tightly related to the ability of keeping under control and minimizing all systematic effects potentially perturbing the RL signal, particularly those not easily removable in software during data analysis. A model of the radiometer and RL system interface, based on the finite elements method, has been developed, together with a detailed thermal modeling, to characterize the physical system and to optimize its performance. The function of the radiometric chain, the modeling and the changes made in the baseline configuration to improve the system performances are described. The impact of systematic effects on the optimized configuration are also quantitatively investigated.

Thermal stability in precision cosmology experiments: the Planck LFI case

After the great success of NASA's satellite missions COBE and WMAP, the Planck mission represents the third generation of mm-wave instruments designed for space observations of CMB anisotropies. Two instruments, the Low-Frequency Instrument (LFI) and the High-Frequency Instrument (HFI) will produce CMB maps with unprecedented angular resolution, sensitivity and frequency coverage. This ambitious task will be achieved by using low noise HEMT detectors cryogenically cooled at ∼20K for the LFI and bolometric detectors cooled at 0.1K for the HFI; in particular, the LFI is based on pseudo-correlation receivers in which the sky signal is continuously compared to a cryogenic reference load in thermal contact with the HFI 4K stage. Such high sensitivity in Planck detectors calls for a strict control of systematic effects, which must be kept at μK level in the final maps; this in turn imposes tight requirements on the thermal and electrical stability of the different stages in the instrument. In this paper we discuss a study of the impact of thermal fluctuations at the level of the 20K cooler cold-end on the Planck-LFI measurements and present some viable solutions that have been adopted to keep the residual systematic error within the required values for Planck-LFI.

The Planck High Frequency Instrument, a third generation CMB experiment, and a full sky submillimeter survey

The High Frequency Instrument (HFI) of Planck is the most sensitive CMB experiment ever planned. Statistical fluctuations (photon noise) of the CMB itself will be the major limitation to the sensitivity of the CMB channels. Higher frequency channels will measure galactic foregrounds. Together with the Low Frequency Instrument, this will make a unique tool to measure the full sky and to separate the various components of its spectrum. Measurement of the polarization of these various components will give a new picture of the CMB. In addition, HFI will provide the scientific community with new full sky maps of intensity and polarization at six frequencies, with unprecedented angular resolution and sensitivity. This paper describes the logics that prevailed to define the HFI and the performances expected from this instrument. It details several features of the HFI design that has not been published up to now.

The Planck milestone

Planck, a European Space Agency satellite to be launched in 2007, is dedicated to surveying the full sky at sub-millimetre and millimetre wavelength. The primary goal of the mission is the final mapping of the Cosmic Microwave Background Anisotropies (CMBA). With an angular resolution of 5 arcmin and a sensitivity of Δ T CMB / T CMB =2×10 −6, the Planck mission will be about 1000 times more sensitive than COBE-DMR and at least 20 times more than WMAP. Planck has also very good capabilites for measurements of polarization, although it will not exhaust the information contained in the CMBA polarization pattern. Two instruments share the Planck focal plane; the High Frequency Instrument (HFI) covers the wavelength ranging from 300 μm to 3 mm by using 48 bolometers cooled to 100 mK. This instrument is realized by an international collaboration, led by the IAS at Orsay. The other part of the relevant electromagnetic spectrum is covered by the Low Frequency Instrument (LFI) using HEMT radiometers cooled at 18 K and realized by a consortium led by the CNR in Milano. The first part of this article presents expected results of Planck on CMBA, both in intensity and polarization. In a second part, the global design of the Planck mission will be presented. We describe in particular the implications of Planck scientific goals on the instruments design, and especially on HFI that is the most sensitive Planck instrument. To cite this article: F.R. Bouchet et al., C. R. Physique 4 (2003).

On the loss of telemetry data in full-sky surveys from space

In this paper we discuss the issue of loosing telemetry (TM) data due to different reasons (e.g. spacecraft–ground transmissions) while performing a full-sky survey with space-borne instrumentation. This is a particularly important issue considering the current and future space missions (like Planck from ESA and MAP from NASA) operating from an orbit far from Earth with short periods of visibility from ground stations. We consider, as a working case, the Low Frequency Instrument (LFI) on-board the Planck satellite albeit the approach developed here can be easily applied to any kind of experiment that makes use of an observing (scanning) strategy which assumes repeated pointings of the same region of the sky on different time scales. The issue is addressed by means of a Monte Carlo approach. Our analysis clearly shows that, under quite general conditions, it is better to cover the sky more times with a lower fraction of TM retained than less times with a higher guaranteed TM fraction. In the case of Planck, an extension of mission time to allow a third sky coverage with 95% of the total TM guaranteed provides a significant reduction of the probability to loose scientific information with respect to an increase of the total guaranteed TM to 98% with the two nominal sky coverages.