arXiv subjects

R. C. Butler

Publications and source records attributed to R. C. Butler.

At least 127 records · Page 7Linked to original sources

arXiv2010

Dynamic validation of the Planck/LFI thermal model

The Low Frequency Instrument (LFI) is an array of cryogenically cooled radiometers on board the Planck satellite, designed to measure the temperature and polarization anisotropies of the cosmic microwave backgrond (CMB) at 30, 44 and 70 GHz. The thermal requirements of the LFI, and in particular the stringent limits to acceptable thermal fluctuations in the 20 K focal plane, are a critical element to achieve the instrument scientific performance. Thermal tests were carried out as part of the on-ground calibration campaign at various stages of instrument integration. In this paper we describe the results and analysis of the tests on the LFI flight model (FM) performed at Thales Laboratories in Milan (Italy) during 2006, with the purpose of experimentally sampling the thermal transfer functions and consequently validating the numerical thermal model describing the dynamic response of the LFI focal plane. This model has been used extensively to assess the ability of LFI to achieve its scientific goals: its validation is therefore extremely important in the context of the Planck mission. Our analysis shows that the measured thermal properties of the instrument show a thermal damping level better than predicted, therefore further reducing the expected systematic effect induced in the LFI maps. We then propose an explanation of the increased damping in terms of non-ideal thermal contacts.

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arXiv2010

Planck pre-launch status: Low Frequency Instrument calibration and expected scientific performance

We give the calibration and scientific performance parameters of the Planck Low Frequency Instrument (LFI) measured during the ground cryogenic test campaign. These parameters characterise the instrument response and constitute our best pre-launch knowledge of the LFI scientific performance. The LFI shows excellent $1/f$ stability and rejection of instrumental systematic effects; measured noise performance shows that LFI is the most sensitive instrument of its kind. The set of measured calibration parameters will be updated during flight operations through the end of the mission.

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arXiv2010

Planck pre-launch status: Design and description of the Low Frequency Instrument

In this paper we present the Low Frequency Instrument (LFI), designed and developed as part of the Planck space mission, the ESA program dedicated to precision imaging of the cosmic microwave background (CMB). Planck-LFI will observe the full sky in intensity and polarisation in three frequency bands centred at 30, 44 and 70 GHz, while higher frequencies (100-850 GHz) will be covered by the HFI instrument. The LFI is an array of microwave radiometers based on state-of-the-art Indium Phosphide cryogenic HEMT amplifiers implemented in a differential system using blackbody loads as reference signals. The front-end is cooled to 20K for optimal sensitivity and the reference loads are cooled to 4K to minimise low frequency noise. We provide an overview of the LFI, discuss the leading scientific requirements and describe the design solutions adopted for the various hardware subsystems. The main drivers of the radiometric, optical and thermal design are discussed, including the stringent requirements on sensitivity, stability, and rejection of systematic effects. Further details on the key instrument units and the results of ground calibration are provided in a set of companion papers.

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arXiv2010

Planck pre-launch status: the Planck-LFI programme

This paper provides an overview of the Low Frequency Instrument (LFI) programme within the ESA Planck mission. The LFI instrument has been developed to produce high precision maps of the microwave sky at frequencies in the range 27-77 GHz, below the peak of the cosmic microwave background (CMB) radiation spectrum. The scientific goals are described, ranging from fundamental cosmology to Galactic and extragalactic astrophysics. The instrument design and development are outlined, together with the model philosophy and testing strategy. The instrument is presented in the context of the Planck mission. The LFI approach to ground and inflight calibration is described. We also describe the LFI ground segment. We present the results of a number of tests demonstrating the capability of the LFI data processing centre (DPC) to properly reduce and analyse LFI flight data, from telemetry information to calibrated and cleaned time ordered data, sky maps at each frequency (in temperature and polarization), component emission maps (CMB and diffuse foregrounds), catalogs for various classes of sources (the Early Release Compact Source Catalogue and the Final Compact Source Catalogue). The organization of the LFI consortium is briefly presented as well as the role of the core team in data analysis and scientific exploitation. All tests carried out on the LFI flight model demonstrate the excellent performance of the instrument and its various subunits. The data analysis pipeline has been tested and its main steps verified. In the first three months after launch, the commissioning, calibration, performance, and verification phases will be completed, after which Planck will begin its operational life, in which LFI will have an integral part.

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arXiv2004

The Planck Low Frequency Instrument

Planck is the third generation of mm-wave instruments designed for space observations of the cosmic microwave background (CMB) anisotropies within the new Cosmic Vision 2020 ESA Science Program. Planck will map the whole sky with unprecedented sensitivity, angular resolution, and frequency coverage, and it likely leads us to the final comprehension of the CMB anisotropies. The Low Frequency Instrument (LFI), operating in the 30-70 GHz range, is one of the two instruments onboard Planck satellite, sharing the focal region of a 1.5 meter off-axis dual reflector telescope together with the High Frequency Instrument (HFI) operating at 100-857 GHz. We present LFI and discuss the major instrumental systematic effects that could degrade the measurements and the solutions adopted in the design and data analysis phase in order to adequately reduce and control them.

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arXiv2003

The Low Frequency Instrument in the ESA Planck mission

Measurements of the cosmic microwave background (CMB) allow high precision observation of the cosmic plasma at redshift z~1100. After the success of the NASA satellite COBE, that in 1992 provided the first detection of the CMB anisotropy, results from many ground-based and balloon-borne experiments have showed a remarkable consistency between different results and provided quantitative estimates of fundamental cosmological properties. During the current year the team of the NASA WMAP satellite has released the first improved full-sky maps of the CMB since COBE, leading to a deeper insight in the origin and evolution of the Universe. The ESA satellite Planck, scheduled for launch in 2007, is designed to provide the ultimate measurement of the CMB temperature anisotropy over the full sky, with an accuracy that will be limited only by astrophysical foregrounds, and robust detection of polarisation anisotropy. Planck will observe the sky with two instruments over a wide spectral band (the Low Frequency Instrument, based on coherent radiometers, from 30 to 70 GHz and the High Frequency Instrument, based on bolometric detectors, from 100 to 857 GHz). The mission performances will improve dramatically the scientific return compared to WMAP. Furthermore the LFI radiometers (as well as some of the HFI bolometers) are intrinsically sensitive to polarisation so that by combining the data from different receivers it will be possible to measure accurately the E mode and to detect the B mode of the polarisation power spectrum. Planck sensitivity will offer also the possibility to detect the non-Gaussianities imprinted in the CMB.

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arXiv2003

Advanced pseudo-correlation radiometers for the Planck-LFI instrument

The LFI (Low Frequency Instrument) on board the ESA Planck satellite is constituted by an array of radiometric detectors actively cooled at 20 K in the 30-70 GHz frequency range in the focal plane of the Planck telescope. In this paper we present an overview of the LFI instrument, with a particular focus on the radiometer design. The adopted pseudo-correlation scheme uses a software balancing technique (with a tunable parameter called gain modulation factor) which is effective in reducing the radiometer susceptibility to amplifier instabilities also in presence of small non-idealities in the radiometric chain components, provided that the gain modulation factor is estimated with an accuracy of the order of 0.2%. These results have been recently confirmed by experimental laboratory measurements conducted on the LFI prototype radiometers at 30, 70 and 100 GHz.

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arXiv2003

PLANCK/LFI: an Advanced Multi-beam High Performance mm-wave Optics for Space Applications

The Low Frequency Instrument (LFI) is one of the two instruments onboard the ESA PLANCK satellite foreseen to be launched in 2007. The LFI will image the Cosmic Microwave Background (CMB) anisotropies and the polarization in four different bands, with an unprecedented combination of sky coverage, calibration accuracy, control of systematic errors, and sensitivity. LFI is coupled to the PLANCK Telescope by an array of 23 high performance dual profiled corrugated feed horns. The dual reflector off axis-design of the 1.5 meter projected aperture telescope is the most advanced optical design ever conceived for accommodating large multi-beam and multi-frequency focal plane units. The location and the design of the feed horns, for both instruments, have been optimised in order to maintain good beam symmetry, even far from the optical axis, and an excellent straylight rejection. In this paper we describe the LFI instrument with emphasis on its optical performance and coupling with the PLANCK telescope.

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arXiv2003

Advanced simulation techniques for straylight prediction of high performance mm-wave reflecting telescope

The prediction of the antenna radiation pattern can be extremely complex for antenna system composed of several reflector surfaces. Typically, at mm-wavelengths, telescopes are composed of mirrors and shields, and an accurate prediction of the whole system response is therefore complicated. This is the case for the optical system of the ESA Planck mission. Planck represents the third generation of mm-wave instruments designed for space observations of CMB anisotropies over the whole sky, and has unprecedented sensitivity and angular resolution. The Low Frequency Instrument (LFI) is one of the two instruments onboard the satellite. LFI consists of an array of 46 cooled HEMT-based receivers coupled to the 1.5 meter off-axis dual reflector telescope by an array of 23 dual profiled corrugated feed horns working at four frequencies between 30 and 100 GHz. Owing to the small level of CMB anisotropies, the control of the systematic effects is mandatory, and this is assured by means of a dedicated instrument design and by an accurate knowledge of instrument characteristics. The telescope is a source of systematic errors both in the main beam region (aberrations) and in the sidelobes region (straylight contamination) and efficient and accurate radiation pattern simulations of the telescope-feed system are needed. We report on the simulations of the full pattern applied to the Planck/LFI-Telescope system. These simulations, based on advanced GTD methods, are used to evaluate the straylight rejection of the entire optical system, by applying a new approach for evaluating the sidelobes in a rigorous way, even at very low levels.

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arXiv2003

Planck Low Frequency Instrument: Beam Patterns

The Low Frequency Instrument on board the ESA Planck satellite is coupled to the Planck 1.5 meter off-axis dual reflector telescope by an array of 27 corrugated feed horns operating at 30, 44, 70, and 100 GHz. We briefly present here a detailed study of the optical interface devoted to optimize the angular resolution (10 arcmin at 100 GHz as a goal) and at the same time to minimize all the systematics coming from the sidelobes of the radiation pattern. Through optical simulations, we provide shapes, locations on the sky, angular resolutions, and polarization properties of each beam.

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arXiv2002

Sources Variability With Planck LFI

Planck LFI (Low Frequency Instrument) will produce a complete survey of the sky at millimeter wavelengths. Data stream analysis will provide the possibility to reveal unexpected millimeter sources and to study their flux evolution in time at different frequencies. We describe here the main implications and discuss data analysis methods. Planck sensitivities typical for this kind of detection are taken into account. We present also preliminary results of our simulation activity.

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arXiv2001

The Planck Telescope

In this paper we present an overview of the Telescope designed for ESA's mission dedicated to map the Cosmic Microwave Background Anisotropies and Polarization. Two instruments, LFI and HFI, operate in an overall frequency range between 25 and 900 GHz and share the focal region of the 1.5 meter optimized telescope. The optimization techniques adopted for the optical design and the telescope characteristic are reported and discussed.

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arXiv2001

BeppoSAX measurements of the bright gamma-ray burst 010222

We analyze the BeppoSAX measurements of the prompt and afterglow emission of the gamma-ray burst GRB010222. Among 45 GRBs detected with the Wide Field Cameras on BeppoSAX, the 40-700 keV fluence of (9.3+/-0.3)E-5 erg cm-2 is only surpassed by GRB990123. In terms of the isotropic 20-2000 keV energy output of 7.8E53 erg, it ranks third of all GRBs with measured distances. Since this burst is so bright, the data provide complete and valuable coverage up to 65 hr after the event, except for a gap between 3.5 and 8.0 hr. The 2-10 keV flux history shows clear signs of a break which is consistent with a break seen in the optical, and provides supporting evidence for the achromatic nature of the break. An explanation for the break in the context of a collimated expansion is not straightforward. Rather, a model is favored whereby the fireball is braked to the non-relativistic regime quickly (within a fraction of day) by a dense 1E6 cm-3 circumburst medium. This implies that, after a mild beaming correction, GRB010222 may be the most energetic burst observed thus far. The X-ray decay index after the break is 1.33+/-0.04, the spectral index 0.97+/-0.05. The decay is, with unprecedented accuracy, identical to that observed in the optical.

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arXiv2000

Discovery of a redshifted Iron K-line in the X-ray afterglow of GRB 000214

We report the detection (3 sigma significance level) of a strong iron emission line in the X-ray spectrum of the afterglow of GRB 000214 (``Valentine's Day Burst'') observed by BeppoSAX. An emission line feature was observed with a centroid energy of 4.7+/-0.2 keV which, if interpreted as K-alpha emission from hydrogen-like iron, corresponds to a redshift of z=0.47. The observed intensity (EW ~2 keV) and duration (tens of hours) of the line give information on the distance, from the burst region, of the emitting material (R>3x10^15 cm) and its mass (M>1.4 solar mass). These results are not easily reconciled with the binary merger and hypernova models for gamma ray bursts.

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arXiv2000

BeppoSAX spectrum of GRB971214: evidence of a substantial energy output during afterglow

We report the X/gamma-ray spectrum of GRB971214 and of its afterglow. The afterglow was measured few hours after the main event and for an elapsed time of more than two days. The measure of this GRB and afterglow is relevant due to its extreme, cosmological distance (z=3.42). The prompt event shows a hard photon spectrum, consistent with a broken power law with photon indices Gamma_X~0.1 below ~20 keV and Gamma_g~1.3 above 60 keV. The afterglow spectrum, measured with the MECS and LECS BeppoSAX telescopes, is consistent with a power law with spectral photon index Gamma=1.6. Within the statistical accuracy of our measure no spectral evolution is detected during the observation of the afterglow. When integrated during the time span covered by BeppoSAX observations, the power in the afterglow emission, even with very conservative assumptions, is at least comparable with the power in the main event. The IR-to-X rays broad band spectrum is also presented, collecting data from the literature and adding them to the BeppoSAX measure. It shows that the predictions from synchrotron emission models is qualitatively confirmed. The BeppoSAX measurement of the X and gamma ray spectrum of this GRB/afterglow is discussed in the framework of current theoretical models.

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arXiv1999

BeppoSAX Observations of GRB980425: Detection of the Prompt Event and Monitoring of the Error Box

We present BeppoSAX follow-up observations of GRB980425 obtained with the Narrow Field Instruments (NFI) in April, May, and November 1998. The first NFI observation has detected within the 8' radius error box of the GRB an X-ray source positionally consistent with the supernova 1998bw, which exploded within a day of GRB980425, and a fainter X-ray source, not consistent with the position of the supernova. The former source is detected in the following NFI pointings and exhibits a decline of a factor of two in six months. If it is associated with SN 1998bw, this is the first detection of X-ray emission from a Type I supernova above 2 keV. The latter source exhibits only marginally significant variability. The X-ray spectra and variability of the supernova are compared with thermal and non-thermal models of supernova high energy emission. Based on the BeppoSAX data, it is not possible to firmly establish which of the two detected sources is the GRB X-ray counterpart, although probability considerations favor the supernova.

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arXiv1999

The 0.1-100 keV spectral shape and variability of Mkn421 in high state

The results of a BeppoSAX TOO observation of the BL Lac object Mkn421 during a high intensity state are reported and compared with monitoring X-ray data collected with the BeppoSAX Wide Field Cameras (WFC) and the RXTE All Sky Monitor(ASM). The 0.1-100 keV spectrum of Mkn421 shows continuous convex curvature that can be interpreted as the high-energy end of the synchrotron emission. The source shows significant short-term temporal and spectral variability, which can be interpreted in terms of synchrotron cooling. The comparison of our results with those of previous observations when the source was a factor 3-5 fainter shows evidence for strong spectral variability, with the maximum of the synchrotron power shifting to higher energy during high states. This behaviour suggest an increase in the number of energetic electrons during high states.

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arXiv1999

BeppoSAX Detection and Follow-up of GRB980425

We present BeppoSAX GRBM and WFC light curves of GRB980425 and NFI follow-up data taken in 1998 April, May, and November. The first NFI observation has detected within the 8' radius error box of the GRB an X-ray source positionally consistent with the supernova SN 1998bw, exploded within a day of GRB980425, and a fainter X-ray source, not consistent with the position of the supernova. The former source is detected in the following NFI pointings and exhibits a decline of a factor of two in six months. If it is associated with SN 1998bw, this is the first detection of hard X-ray emission from a Type I supernova. The latter source exhibits only marginally significant variability. Based on these data, it is not possible to select either source as a firm candidate for the GRB counterpart.

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