arXiv subjects

G. Savini

Publications and source records attributed to G. Savini.

At least 37 records · Page 2Linked to original sources

arXiv2016

The NIKA2 instrument, a dual-band kilopixel KID array for millimetric astronomy

NIKA2 (New IRAM KID Array 2) is a camera dedicated to millimeter wave astronomy based upon kilopixel arrays of Kinetic Inductance Detectors (KID). The pathfinder instrument, NIKA, has already shown state-of-the-art detector performance. NIKA2 builds upon this experience but goes one step further, increasing the total pixel count by a factor $\sim$10 while maintaining the same per pixel performance. For the next decade, this camera will be the resident photometric instrument of the Institut de Radio Astronomie Millimetrique (IRAM) 30m telescope in Sierra Nevada (Spain). In this paper we give an overview of the main components of NIKA2, and describe the achieved detector performance. The camera has been permanently installed at the IRAM 30m telescope in October 2015. It will be made accessible to the scientific community at the end of 2016, after a one-year commissioning period. When this happens, NIKA2 will become a fundamental tool for astronomers worldwide.

astro-ph.IM↗

arXiv2015

First polarised light with the NIKA camera

NIKA is a dual-band camera operating with 315 frequency multiplexed LEKIDs cooled at 100 mK. NIKA is designed to observe the sky in intensity and polarisation at 150 and 260 GHz from the IRAM 30-m telescope. It is a test-bench for the final NIKA2 camera. The incoming linear polarisation is modulated at four times the mechanical rotation frequency by a warm rotating multi-layer Half Wave Plate. Then, the signal is analysed by a wire grid and finally absorbed by the LEKIDs. The small time constant (< 1ms ) of the LEKID detectors combined with the modulation of the HWP enables the quasi-simultaneous measurement of the three Stokes parameters I, Q, U, representing linear polarisation. In this paper we present results of recent observational campaigns demonstrating the good performance of NIKA in detecting polarisation at mm wavelength.

astro-ph.IM↗

arXiv2015

Pressure distribution of the high-redshift cluster of galaxies CL J1226.9+3332 with NIKA

The thermal Sunyaev-Zel'dovich (tSZ) effect is expected to provide a low scatter mass proxy for galaxy clusters since it is directly proportional to the cluster thermal energy. The tSZ observations have proven to be a powerful tool for detecting and studying them, but high angular resolution observations are now needed to push their investigation to a higher redshift. In this paper, we report high angular (< 20 arcsec) resolution tSZ observations of the high-redshift cluster CL J1226.9+3332 (z=0.89). It was imaged at 150 and 260 GHz using the NIKA camera at the IRAM 30-meter telescope. The 150 GHz map shows that CL J1226.9+3332 is morphologically relaxed on large scales with evidence of a disturbed core, while the 260 GHz channel is used mostly to identify point source contamination. NIKA data are combined with those of Planck and X-ray from Chandra to infer the cluster's radial pressure, density, temperature, and entropy distributions. The total mass profile of the cluster is derived, and we find $M_{500} = 5.96^{+1.02}_{-0.79} $ x $10^{14} M_{\odot}$ within the radius $R_{500} = 930^{+50}_{-43}$ kpc, at a 68% confidence level. ($R_{500}$ is the radius within which the average density is 500 times the critical density at the cluster's redshift.) NIKA is the prototype camera of NIKA2, a KIDs (kinetic inductance detectors) based instrument to be installed at the end of 2015. This work is, therefore, part of a pilot study aiming at optimizing tSZ NIKA2 large programs.

astro-ph.CO↗

arXiv2014

PILOT: a balloon-borne experiment to measure the polarized FIR emission of dust grains in the interstellar medium

Future cosmology space missions will concentrate on measuring the polarization of the Cosmic Microwave Background, which potentially carries invaluable information about the earliest phases of the evolution of our universe. Such ambitious projects will ultimately be limited by the sensitivity of the instrument and by the accuracy at which polarized foreground emission from our own Galaxy can be subtracted out. We present the PILOT balloon project which will aim at characterizing one of these foreground sources, the polarization of the dust continuum emission in the diffuse interstellar medium. The PILOT experiment will also constitute a test-bed for using multiplexed bolometer arrays for polarization measurements. We present the results of ground tests obtained just before the first flight of the instrument.

astro-ph.IM↗

arXiv2014

Latest NIKA results and the NIKA-2 project

NIKA (New IRAM KID Arrays) is a dual-band imaging instrument installed at the IRAM (Institut de RadioAstronomie Millimetrique) 30-meter telescope at Pico Veleta (Spain). Two distinct Kinetic Inductance Detectors (KID) focal planes allow the camera to simultaneously image a field-of-view of about 2 arc-min in the bands 125 to 175 GHz (150 GHz) and 200 to 280 GHz (240 GHz). The sensitivity and stability achieved during the last commissioning Run in June 2013 allows opening the instrument to general observers. We report here the latest results, in particular in terms of sensitivity, now comparable to the state-of-the-art Transition Edge Sensors (TES) bolometers, relative and absolute photometry. We describe briefly the next generation NIKA-2 instrument, selected by IRAM to occupy, from 2015, the continuum imager/polarimeter slot at the 30-m telescope.

astro-ph.IM↗

arXiv2014

High resolution SZ observations at the IRAM 30-m telescope with NIKA

High resolution observations of the thermal Sunyaev-Zel'dovich (tSZ) effect are necessary to allow the use of clusters of galaxies as a probe for large scale structures at high redshifts. With its high resolution and dual-band capability at millimeter wavelengths, the NIKA camera can play a significant role in this context. NIKA is based on newly developed Kinetic Inductance Detectors (KIDs) and operates at the IRAM 30m telescope, Pico Veleta, Spain. In this paper, we give the status of the NIKA camera, focussing on the KID technology. We then present observations of three galaxy clusters: RX J1347.5-1145 as a demonstrator of the NIKA capabilities and the recent observations of CL J1226.9+3332 (z = 0.89) and MACS J0717.5+3745 (z = 0.55). We also discuss prospects for the final NIKA2 camera, which will have a 6.5 arcminute field of view with about 5000 detectors in two bands at 150 and 260 GHz.

astro-ph.CO↗

arXiv2014

First observation of the thermal Sunyaev-Zel'dovich effect with kinetic inductance detectors

Clusters of galaxies provide valuable information on the evolution of the Universe and large scale structures. Recent cluster observations via the thermal Sunyaev-Zel'dovich (tSZ) effect have proven to be a powerful tool to detect and study them. In this context, high resolution tSZ observations (~ tens of arcsec) are of particular interest to probe intermediate and high redshift clusters. Observations of the tSZ effect will be carried out with the millimeter dual-band NIKA2 camera, based on Kinetic Inductance Detectors (KIDs) to be installed at the IRAM 30-meter telescope in 2015. To demonstrate the potential of such an instrument, we present tSZ observations with the NIKA camera prototype, consisting of two arrays of 132 and 224 detectors that observe at 140 and 240 GHz with a 18.5 and 12.5 arcsec angular resolution, respectively. The cluster RX J1347.5-1145 was observed simultaneously at 140 and 240 GHz. We used a spectral decorrelation technique to remove the atmospheric noise and obtain a map of the cluster at 140 GHz. The efficiency of this procedure has been characterized through realistic simulations of the observations. The observed 140 GHz map presents a decrement at the cluster position consistent with the tSZ nature of the signal. We used this map to study the pressure distribution of the cluster by fitting a gNFW model to the data. Subtracting this model from the map, we confirm that RX J1347.5-1145 is an ongoing merger, which confirms and complements previous tSZ and X-ray observations. For the first time, we demonstrate the tSZ capability of KID based instruments. The NIKA2 camera with ~ 5000 detectors and a 6.5 arcmin field of view will be well-suited for in-depth studies of the intra cluster medium in intermediate to high redshifts, which enables the characterization of recently detected clusters by the Planck satellite.

astro-ph.CO↗

arXiv2014

Performance and calibration of the NIKA camera at the IRAM 30 m telescope

The New IRAM KID Array (NIKA) instrument is a dual-band imaging camera operating with Kinetic Inductance Detectors (KID) cooled at 100 mK. NIKA is designed to observe the sky at wavelengths of 1.25 and 2.14 mm from the IRAM 30 m telescope at Pico Veleta with an estimated resolution of 13\,arcsec and 18 arcsec, respectively. This work presents the performance of the NIKA camera prior to its opening to the astrophysical community as an IRAM common-user facility in early 2014. NIKA is a test bench for the final NIKA2 instrument to be installed at the end of 2015. The last NIKA observation campaigns on November 2012 and June 2013 have been used to evaluate this performance and to improve the control of systematic effects. We discuss here the dynamical tuning of the readout electronics to optimize the KID working point with respect to background changes and the new technique of atmospheric absorption correction. These modifications significantly improve the overall linearity, sensitivity, and absolute calibration performance of NIKA. This is proved on observations of point-like sources for which we obtain a best sensitivity (averaged over all valid detectors) of 40 and 14 mJy.s$^{1/2}$ for optimal weather conditions for the 1.25 and 2.14 mm arrays, respectively. NIKA observations of well known extended sources (DR21 complex and the Horsehead nebula) are presented. This performance makes the NIKA camera a competitive astrophysical instrument.

astro-ph.IM↗

arXiv2014

The NIKA 2012-2014 observing campaigns: control of systematic effects and results

The New IRAM KID Array (NIKA) is a dual-band camera operating with frequency multiplexed arrays of Lumped Element Kinetic Inductance Detectors (LEKIDs) cooled to 100 mK. NIKA is designed to observe the intensity and polarisation of the sky at 1.25 and 2.14 mm from the IRAM 30 m telescope. We present the improvements on the control of systematic effects and astrophysical results made during the last observation campaigns between 2012 and 2014.

astro-ph.IM↗

arXiv2014

Attitude determination for balloon-borne experiments

An attitude determination system for balloon-borne experiments is presented. The system provides pointing information in azimuth and elevation for instruments flying on stratospheric balloons over Antarctica. In-flight attitude is given by the real-time combination of readings from star cameras, a magnetometer, sun sensors, GPS, gyroscopes, tilt sensors and an elevation encoder. Post-flight attitude reconstruction is determined from star camera solutions, interpolated by the gyroscopes using an extended Kalman Filter. The multi-sensor system was employed by the Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry (BLASTPol), an experiment that measures polarized thermal emission from interstellar dust clouds. A similar system was designed for the upcoming flight of SPIDER, a Cosmic Microwave Background polarization experiment. The pointing requirements for these experiments are discussed, as well as the challenges in designing attitude reconstruction systems for high altitude balloon flights. In the 2010 and 2012 BLASTPol flights from McMurdo Station, Antarctica, the system demonstrated an accuracy of <5' rms in-flight, and <5" rms post-flight.

astro-ph.IM↗

arXiv2014

The Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry-BLASTPol: Performance and results from the 2012 Antarctic flight

The Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry (BLASTPol) is a suborbital mapping experiment, designed to study the role played by magnetic fields in the star formation process. BLASTPol observes polarized light using a total power instrument, photolithographic polarizing grids, and an achromatic half-wave plate to modulate the polarization signal. During its second flight from Antarctica in December 2012, BLASTPol made degree scale maps of linearly polarized dust emission from molecular clouds in three wavebands, centered at 250, 350, and 500 microns. The instrumental performance was an improvement over the 2010 BLASTPol flight, with decreased systematics resulting in a higher number of confirmed polarization vectors. The resultant dataset allows BLASTPol to trace magnetic fields in star-forming regions at scales ranging from cores to entire molecular cloud complexes.

astro-ph.IM↗

arXiv2014

Thermal design and performance of the balloon-borne large aperture submillimeter telescope for polarimetry BLASTPol

We present the thermal model of the Balloon-borne Large-Aperture Submillimeter Telescope for Polarimetry (BLASTPol). This instrument was successfully flown in two circumpolar flights from McMurdo, Antarctica in 2010 and 2012. During these two flights, BLASTPol obtained unprecedented information about the magnetic field in molecular clouds through the measurement of the polarized thermal emission of interstellar dust grains. The thermal design of the experiment addresses the stability and control of the payload necessary for this kind of measurement. We describe the thermal modeling of the payload including the sun-shielding strategy. We present the in-flight thermal performance of the instrument and compare the predictions of the model with the temperatures registered during the flight. We describe the difficulties of modeling the thermal behavior of the balloon-borne platform and establish landmarks that can be used in the design of future balloon-borne instruments.

astro-ph.IM↗

arXiv2014

BLASTbus electronics: general-purpose readout and control for balloon-borne experiments

We present the second generation BLASTbus electronics. The primary purposes of this system are detector readout, attitude control, and cryogenic housekeeping, for balloon-borne telescopes. Readout of neutron transmutation doped germanium (NTD-Ge) bolometers requires low noise and parallel acquisition of hundreds of analog signals. Controlling a telescope's attitude requires the capability to interface to a wide variety of sensors and motors, and to use them together in a fast, closed loop. To achieve these different goals, the BLASTbus system employs a flexible motherboard-daughterboard architecture. The programmable motherboard features a digital signal processor (DSP) and field-programmable gate array (FPGA), as well as slots for three daughterboards. The daughterboards provide the interface to the outside world, with versions for analog to digital conversion, and optoisolated digital input/output. With the versatility afforded by this design, the BLASTbus also finds uses in cryogenic, thermometry, and power systems. For accurate timing control to tie everything together, the system operates in a fully synchronous manner. BLASTbus electronics have been successfully deployed to the South Pole, and flown on stratospheric balloons.

astro-ph.IM↗

arXiv2014

Comparison of Prestellar Core Elongations and Large-Scale Molecular Cloud Structures in the Lupus I Region

Turbulence and magnetic fields are expected to be important for regulating molecular cloud formation and evolution. However, their effects on subparsec to 100 parsec scales, leading to the formation of starless cores, is not well understood. We investigate the prestellar core structure morphologies obtained from analysis of the Herschel-SPIRE 350 $μ$m maps of the Lupus I cloud. This distribution is first compared on a statistical basis to the large scale shape of the main filament. We find the distribution of the elongation position angle of the cores to be consistent with a random distribution, which means no specific orientation of the morphology of the cores is observed with respect to a large-scale filament shape model for Lupus I, or relative to a large-scale bent filament model. This distribution is also compared to the mean orientation of the large-scale magnetic fields probed at 350 $μ$m with the Balloon-borne Large Aperture Telescope for Polarimetry (BLASTPol) during its 2010 campaign. Here again we do not find any correlation between the core morphology distribution and the average orientation of the magnetic fields on parsec scales. Our main conclusion is that the local filament dynamics - including secondary filaments that often run orthogonally to the primary filament - and possibly small-scale variations in the local magnetic field direction, could be the dominant factors for explaining the final orientation of each core.

astro-ph.GA↗

arXiv2013

The NIKA instrument: results and perspectives towards a permanent KID based camera for the Pico Veleta observatory

The New IRAM KIDs Array (NIKA) is a pathfinder instrument devoted to millimetric astronomy. In 2009 it was the first multiplexed KID camera on the sky; currently it is installed at the focal plane of the IRAM 30-meters telescope at Pico Veleta (Spain). We present preliminary data from the last observational run and the ongoing developments devoted to the next NIKA-2 kilopixels camera, to be commissioned in 2015. We also report on the latest laboratory measurements, and recent improvements in detector cosmetics and read-out electronics. Furthermore, we describe a new acquisition strategy allowing us to improve the photometric accuracy, and the related automatic tuning procedure.

astro-ph.IM↗

arXiv2013

Detection of the tSZ effect with the NIKA camera

We present the first detection of the thermal Sunyaev-Zel'dovich (tSZ) effect from a cluster of galaxies performed with a KIDs (Kinetic Inductance Detectors) based instrument. The tSZ effect is a distortion of the black body CMB (Cosmic Microwave Background) spectrum produced by the inverse Compton interaction of CMB photons with the hot electrons of the ionized intra-cluster medium. The massive, intermediate redshift cluster RX J1347.5-1145 has been observed using NIKA (New IRAM KIDs arrays), a dual-band (140 and 240 GHz) mm-wave imaging camera, which exploits two arrays of hundreds of KIDs: the resonant frequencies of the superconducting resonators are shifted by mm-wave photons absorption. This tSZ cluster observation demonstrates the potential of the next generation NIKA2 instrument, being developed for the 30m telescope of IRAM, at Pico Veleta (Spain). NIKA2 will have 1000 detectors at 140GHz and 2x2000 detectors at 240GHz, providing in that band also a measurement of the linear polarization. NIKA2 will be commissioned in 2015.

astro-ph.CO↗

arXiv2011

The Shear Mode of Multi-Layer Graphene

We uncover the interlayer shear mode of multi-layer graphene samples, ranging from bilayer-graphene (BLG) to bulk graphite, and show that the corresponding Raman peak measures the interlayer coupling. This peak scales from~43cm-1 in bulk graphite to~31cm-1 in BLG. Its low energy makes it a probe of near-Dirac point quasi-particles, with a Breit-Wigner-Fano lineshape due to resonance with electronic transitions. Similar shear modes are expected in all layered materials, providing a direct probe of interlayer interactions

cond-mat.mes-hall↗

arXiv2011

Warm water vapour in the sooty outflow from a luminous carbon star

In 2001, the discovery of circumstellar water vapour around the ageing carbon star IRC+10216 was announced. This detection challenged the current understanding of chemistry in old stars, since water vapour was predicted to be absent in carbon-rich stars. Several explanations for the occurrence of water vapour were postulated, including the vaporization of icy bodies (comets or dwarf planets) in orbit around the star, grain surface reactions, and photochemistry in the outer circumstellar envelope. However, the only water line detected so far from one carbon-rich evolved star can not discriminate, by itself, between the different mechanisms proposed. Here we report on the detection by the Herschel satellite of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC+10216, including some high-excitation lines with energies corresponding to ~1000 K. The emission of these high-excitation water lines can only be explained if water vapour is present in the warm inner region of the envelope. A plausible explanation for the formation of warm water vapour appears to be the penetration of ultraviolet (UV) photons deep into a clumpy circumstellar envelope. This mechanism triggers also the formation of other molecules such as ammonia, whose observed abundances are much higher than hitherto predicted.

astro-ph.SR↗