Stafford Withington

HARP/ACSIS: a submillimetre spectral imaging system on the James Clerk Maxwell Telescope

This paper describes a new Heterodyne Array Receiver Programme (HARP) and AutoCorrelation Spectral Imaging System (ACSIS) that have recently been installed and commissioned on the James Clerk Maxwell Telescope (JCMT). The 16-element focal-plane array receiver, operating in the submillimetre from 325 to 375 GHz, offers high (three-dimensional) mapping speeds, along with significant improvements over single-detector counterparts in calibration and image quality. Receiver temperatures are 120 K across the whole band and system temperatures of 300K are reached routinely under good weather conditions. The system includes a single-sideband filter so these are SSB figures. Used in conjunction with ACSIS, the system can produce large-scale maps rapidly, in one or more frequency settings, at high spatial and spectral resolution. Fully-sampled maps of size 1 square degree can be observed in under 1 hour. The scientific need for array receivers arises from the requirement for programmes to study samples of objects of statistically significant size, in large-scale unbiased surveys of galactic and extra-galactic regions. Along with morphological information, the new spectral imaging system can be used to study the physical and chemical properties of regions of interest. Its three-dimensional imaging capabilities are critical for research into turbulence and dynamics. In addition, HARP/ACSIS will provide highly complementary science programmes to wide-field continuum studies, and produce the essential preparatory work for submillimetre interferometers such as the SMA and ALMA.

The SCUBA Bright Quasar Survey (SBQS): 850-μm observations of the z>≳ 4 sample

We present initial results of a new, systematic search for massive star formation in the host galaxies of the most luminous and probably most massive radio-quiet quasars . The survey, undertaken at 850 I¼m using SCUBA at the James Clerk Maxwell Telescope (JCMT), has a target sensitivity limit of , set to identify sources suitable for detailed follow-up, e.g. continuum mapping and molecular line diagnostics. A total of 38 radio-quiet quasars have been observed at 850 I¼m, of which eight were detected (>3Iƒ) with (submillimetre-loud). The new detections almost triple the number of optically selected, submillimetre-loud radio-quiet quasars known to date. We include a detailed description of how our quasar sample is defined in terms of radio and optical properties. As a by-product of our selection procedure, we have identified 17 radio-loud quasars with . There is no strong evidence for trends in either detectability or 850-I¼m flux with absolute magnitude, MB. We find that the weighted mean flux of the undetected sources is , consistent with an earlier estimate of ∼3 mJy based on more sensitive observations of a sample radio-quiet quasars. This corresponds to an inferred star formation rate of ∼1000 MâS™ yr−1, similar to Arp220. The typical star formation time-scale for the submillimetre-bright sources is ∼1 Gyr, 10 times longer than the typical accretion-driven e-folding time-scale of ∼ . Our 850-I¼m detection of the quasar PSS J1048+4407, when analysed in conjunction with 1.2-mm single-dish and interferometric observations, suggests that this source is resolved on angular scales of . In addition, we present a new optical spectrum of this source, identifying it as a broad absorption line (BAL) quasar. The new redshift is outside that covered in a recent CO line search, highlighting the need for accurate redshifts for the observation and interpretation of high-redshift line studies.

A FULL-HEIGHT WAVEGUIDE TO THIN-FILM MICROSTRIP TRANSITION WITH EXCEPTIONAL RF BANDWIDTH AND COUPLING EFFICIENCY

We describe a waveguide to thin-film microstrip transition for high-performance submillimetre wave and teraherz applications. The proposed constant-radius probe couples thin-film microstrip line, to full-height rectangular waveguide with better than 99% efficiency (VSWR ≤ 1.20) and 45% fractional bandwidth. Extensive HFSS simulations, backed by scale-model measurements, are presented in the paper. By selecting the substrate material and probe radius, any real impedance between ≈ 15-60 Ω can be achieved. The radial probe gives significantly improved performance over other designs discussed in the literature. Although our primary application is submillimetre wave superconducting mixers, we show that membrane techniques should allow broad-band waveguide components to be constructed for the THz frequency range.

Submillimetre observations of luminous z > 4 radio-quiet quasars and the contribution of AGN to the submm source population

We present sensitive 850µm SCUBA photometry of a statistically-complete sample of six of the most luminous (MB 10 13 L⊙), z > 4 radio-quiet quasars, reaching noise levels (1� � 1.5mJy) comparable with the deep blank sky surveys. These observations probe the rest frame far infrared region (�150µm), at luminosity levels for thermal sources comparable with those that IRAS permitted for low redshift quasars. One quasar (BR2237 0607; z = 4.55) is detected at 850µm with a flux of 5.0±1.1mJy (4.5�), whilst a second (BR0019 1522; z=4.52) has a detection at the 2� level. When combined with our previous millimetre measurements of z>4 quasars, we find that there is a large range (5–10) in far infrared (FIR) luminosity (LFIR) at fixed UV luminosity, and that the typical quasar has a LFIR and mass of cool (50K) dust similar to that of the archetyepal low redshift (z=0.018) ultraluminous IRAS galaxy(ULIRG) Arp220 (LFIR � 5 × 10 12 L⊙; Md(cool) � 10 8 M⊙). If one assumes a fiducial FIR luminosity of 5 × 10 12 L⊙ for for all quasars with MB 15 per cent of the sources in the SCUBA deep surveys could be classical broad-lined radio-quiet AGN. Thus if one considers the observed ratio of Seyfert II to Seyfert I galaxies at low redshift and any contribution from totally optically obscured AGN, a significant fraction of the SCUBA source population will harbour AGN and hence the inferred star formation rates from submm fluxes may be overestimated if the active nuclei are bolometrically dominant or the IMF is top heavy.

Analysis of diagonal horns through Gaussian-Hermite modes

The radiation characteristics of diagonal horns are investigated by means of Gaussian-Hermite modes. It is shown that, for reasonably long horns, the beamwidths in the principal and 45 degrees planes are equal to within 10%, and all sidelobes are below -15 dB. It is also demonstrated that the phase center of a diffraction-limited horn is close to the aperture, whereas the phase center of a constant-beamwidth horn is behind the throat. The maximum coupling to the lowest order copolar Gaussian mode is 84%, and the total amount of power coupled into the cross-polarized lobes is 9.5%. More significantly, the aperture efficiency of a Cassegrain antenna fed by a diagonal horn has a maximum value of 81%, which compares with 87% for a corrugated horn. The maximum efficiency is achieved when the aperture of a diffraction-limited horn is placed at a confocal tertiary focus, although a secondary focus gives an aperture efficiency that is only 10% lower, suggesting that diagonal horns are suitable for focal-plane arrays. >

The Large-Scale Polarization Explorer (LSPE)

The LSPE is a balloon-borne mission aimed at measuring the polarization of the Cosmic Microwave Background (CMB) at large angular scales, and in particular to constrain the curl component of CMB polarization (B-modes) produced by tensor perturbations generated during cosmic inflation, in the very early universe. Its primary target is to improve the limit on the ratio of tensor to scalar perturbations amplitudes down to r = 0.03, at 99.7% confidence. A second target is to produce wide maps of foreground polarization generated in our Galaxy by synchrotron emission and interstellar dust emission. These will be important to map Galactic magnetic fields and to study the properties of ionized gas and of diffuse interstellar dust in our Galaxy. The mission is optimized for large angular scales, with coarse angular resolution (around 1.5 degrees FWHM), and wide sky coverage (25% of the sky). The payload will fly in a circumpolar long duration balloon mission during the polar night. Using the Earth as a giant solar shield, the instrument will spin in azimuth, observing a large fraction of the northern sky. The payload will host two instruments. An array of coherent polarimeters using cryogenic HEMT amplifiers will survey the sky at 43 and 90 GHz. An array of bolometric polarimeters, using large throughput multi-mode bolometers and rotating Half Wave Plates (HWP), will survey the same sky region in three bands at 95, 145 and 245 GHz. The wide frequency coverage will allow optimal control of the polarized foregrounds, with comparable angular resolution at all frequencies.

Representation of mirros in beam waveguides as inclined phase-transforming surfaces

Abstract We have developed an approximate technique, based on the principles of multi-mode Gaussian optics, for studying the behaviour of shaped off-axis mirrors. We describe a mirror as an inclined phase-transforming plane, where the phase transformation across the plane is determined by the depth of the mirror as a function of position. The scattering matrix is calculated in the usual way by evaluating the overlap integrals over some surface for which the amplitudes and phases of the incoming and outgoing fields are known; because, however, the modes are not orthogonal over the surface of interest, a system of linear equations has to be solved. We demonstrate the “thin-mirror” technique by studying the behaviour of paraboloidal and ellipsoidal mirros, and we show how the performance of measured and approximate surfaces can be assessed.

Superconducting kinetic inductance detectors for astrophysics

The kinetic inductance detector (KID) is an exciting new device that promises high-sensitivity, large-format, submillimetre to x-ray imaging arrays for astrophysics. KIDs comprise a superconducting thin-film microwave resonator capacitively coupled to a probe transmission line. By exciting the electrical resonance with a microwave probe signal, the transmission phase of the resonator can be monitored, allowing the deposition of energy or power to be detected. We describe the fabrication and low-temperature testing, down to 26 mK, of a number of devices, and confirm the basic principles of operation. The KIDs were fabricated on r-plane sapphire using superconducting niobium and aluminium as the resonator material, and tantalum as the x-ray absorber. KID quality factors of up to Q = (741 ± 15) × 103 were measured for niobium at 1 K, and quasiparticle effective recombination times of τ*R = 30 µs after x-ray absorption. Al/Ta quasiparticle traps were combined with resonators to make complete detectors. These devices were operated at 26 mK with quality factors of up Q = (187.7 ± 3.5) × 103 and a phase-shift responsivity of ∂θ/∂Nqp = (5.06 ± 0.23) × 10−6 degrees per quasiparticle. Devices were characterized both at thermal equilibrium and as x-ray detectors. A range of different x-ray pulse types was observed. Low phase-noise readout measurements on Al/Ta KIDs gave a minimum NEP = 1.27 × 10−16 W Hz−1/2 at a readout frequency of 550 Hz and NEP = 4.60 × 10−17 W Hz−1/2 at 95 Hz, for effective recombination times τ*R = 100 µs and τ*R = 350 µs respectively. This work demonstrates that high-sensitivity detectors are possible, encouraging further development and research into KIDs.

Analytical expression for the input impedance of a microstrip probe in waveguide

We present a closed-form expression for the the input impedance of a microstrip probe in a rectangular waveguide. The probe extends only part way across the waveguide and is therefore compatible with RF components that require an open circuit at low frequencies. Our analysis is based on the spectral-domain method and is able to take into account the orientation of the antenna with respect to the direction of propagation. We have examined the validity of our model by carrying out extensive impedance measurements at 5GHz. In those cases where the probe did not extend more than half way across the waveguide, excellent agreement was obtained. We show that the bandwidth of a probe that stretches only part way cross the waveguide is very much greater than the bandwidth of a probe that stretches all of the way across the waveguide and that is earthed at both ends. Moreover, the input resistance is lower and more suited to submillimetre-wave detectors such as SIS tunnel junctions. Our expression suggests that it should be possible to develop low-impedance, wideband probes for nearlydouble-height waveguide, and this implies that the upper frequency limit to which probes and waveguides can be manufactured can be extended well into the THz frequency range. A related, and often neglected consideration, is that the ohmic loss associated with an oversized waveguide is very much smaller than the ohmic loss associated with a reduced-height waveguide.

SWIPE: a bolometric polarimeter for the Large-Scale Polarization Explorer

The balloon-borne LSPE mission is optimized to measure the linear polarization of the Cosmic Microwave Background at large angular scales. The Short Wavelength Instrument for the Polarization Explorer (SWIPE) is composed of 3 arrays of multi-mode bolometers cooled at 0.3K , with optical components and filters cryogenically cooled below 4K to reduce the background on the detectors. Polarimetry is achieved by means of large rotating half-wave plates and wire-grid polarizers in front of the arrays. The polarization modulator is the first component of the optical chain, reducing significantly the effect of instrumental polarization. In SWIPE we trade angular resolution for sensitivity. The diameter of the entrance pupil of the refractive telescope is 45 cm, while the field optics is optimized to collect tens of modes for each detector, thus boosting the absorbed power. This approach results in a FWHM resolution of 1.8, 1.5, 1.2 degrees at 95, 145, 245 GHz respectively. The expected performance of the three channels is limited by photon noise, resulting in a final sensitivity around 0.1-0.2 μK per beam, for a 13 days survey covering 25% of the sky.