Rotary antenna attenuator

Radio frequency attenuator, having negligible insertion loss at minimum attenuation, can be used for making precise antenna gain measurements. It is small in size compared to a rotary-vane attenuator

Producing graphite with desired properties

Isotropic or anisotropic graphite is synthesized with precise control of particle size, distribution, and shape. The isotropic graphites are nearly perfectly isotropic, with thermal expansion coefficients two or three times those of ordinary graphites. The anisotropic graphites approach the anisotropy of pyrolytic graphite

Rectenna system design

The function of the rectenna in the solar power satellite system is described and the basic design choices based on the desired microwave field concentration and ground clearance requirements are given. One important area of concern, from the EMI point of view, harmonic reradiation and scattering from the rectenna is also designed. An optimization of a rectenna system design to minimize costs was performed. The rectenna cost breakdown for a 56 w installation is given as an example

Structure of Micro-instabilities in Tokamak Plasmas: Stiff Transport or Plasma Eruptions?

Solutions to a model 2D eigenmode equation describing micro-instabilities in tokamak plasmas are presented that demonstrate a sensitivity of the mode structure and stability to plasma profiles. In narrow regions of parameter space, with special plasma profiles, a maximally unstable mode is found that balloons on the outboard side of the tokamak. This corresponds to the conventional picture of a ballooning mode. However, for most profiles this mode cannot exist and instead a more stable mode is found that balloons closer to the top or bottom of the plasma. Good quantitative agreement with a 1D ballooning analysis is found provided the constraints associated with higher order profile effects, often neglected, are taken into account. A sudden transition from this general mode to the more unstable ballooning mode can occur for a critical flow shear, providing a candidate model for why some experiments observe small plasma eruptions (Edge Localised Modes, or ELMs) in place of large Type I ELMs.Comment: 11 pages, 3 figure

ALMA observations of 99 GHz free-free and H40α\alpha line emission from star formation in the centre of NGC 253

We present Atacama Large Millimeter/submillimeter Array observations of 99.02 GHz free-free and H40$\alpha$ emission from the centre of the nearby starburst galaxy NGC 253. We calculate electron temperatures of 3700-4500 K for the photoionized gas, which agrees with previous measurements. We measure a photoionizing photon production rate of $(3.2\pm0.2)\times10^{53}$ s$^{-1}$ and a star formation rate of $1.73\pm0.12$ M$_\odot$ yr$^{-1}$ within the central 20$\times$10 arcsec, which fall within the broad range of measurements from previous millimetre and radio observations but which are better constrained. We also demonstrate that the dust opacities are ~3 dex higher than inferred from previous near-infrared data, which illustrates the benefits of using millimetre star formation tracers in very dusty sources.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter

Tests of star formation metrics in the low metallicity galaxy NGC 5253 using ALMA observations of H30α\alpha line emission

We use Atacama Large Millimeter/submillimeter Array (ALMA) observations of H30$\alpha$ (231.90 GHz) emission from the low metallicity dwarf galaxy NGC 5253 to measure the star formation rate (SFR) within the galaxy and to test the reliability of SFRs derived from other commonly-used metrics. The H30$\alpha$ emission, which originates mainly from the central starburst, yields a photoionizing photon production rate of (1.9$\pm$0.3)$\times$10$^{52}$ s$^{-1}$ and an SFR of 0.087$\pm$0.013 M$_\odot$ yr$^{-1}$ based on conversions that account for the low metallicity of the galaxy and for stellar rotation. Among the other star formation metrics we examined, the SFR calculated from the total infrared flux was statistically equivalent to the values from the H30$\alpha$ data. The SFR based on previously-published versions of the H$\alpha$ flux that were extinction corrected using Pa$\alpha$ and Pa$\beta$ lines were lower than but also statistically similar to the H30$\alpha$ value. The mid-infrared (22 $\mu$m) flux density and the composite star formation tracer based on H$\alpha$ and mid-infrared emission give SFRs that were significantly higher because the dust emission appears unusually hot compared to typical spiral galaxies. Conversely, the 70 and 160 $\mu$m flux densities yielded SFR lower than the H30$\alpha$ value, although the SFRs from the 70 $\mu$m and H30$\alpha$ data were within 1-2$\sigma$ of each other. While further analysis on a broader range of galaxies are needed, these results are instructive of the best and worst methods to use when measuring SFR in low metallicity dwarf galaxies like NGC 5253.Comment: 14 pages, 5 figures, accepted for publication in MNRA

IR-correlated 31 GHz radio emission from Orion East

Lynds dark cloud LDN1622 represents one of the best examples of anomalous dust emission, possibly originating from small spinning dust grains. We present Cosmic Background Imager (CBI) 31 GHz data of LDN1621, a diffuse dark cloud to the north of LDN1622 in a region known as Orion East. A broken ring with diameter g\approx 20 arcmin of diffuse emission is detected at 31 GHz, at \approx 20-30 mJy beam$^{-1}$ with an angular resolution of \approx 5 arcmin. The ring-like structure is highly correlated with Far Infra-Red emission at $12-100 \mu$m with correlation coefficients of r \approx 0.7-0.8, significant at $\sim10\sigma$. Multi-frequency data are used to place constraints on other components of emission that could be contributing to the 31 GHz flux. An analysis of the GB6 survey maps at 4.85 GHz yields a $3\sigma$ upper limit on free-free emission of 7.2 mJy beam$^{-1}$ (\la 30 per cent of the observed flux) at the CBI resolution. The bulk of the 31 GHz flux therefore appears to be mostly due to dust radiation. Aperture photometry, at an angular resolution of 13 arcmin and with an aperture of diameter 30 arcmin, allowed the use of IRAS maps and the {\it WMAP} 5-year W-band map at 93.5 GHz. A single modified blackbody model was fitted to the data to estimate the contribution from thermal dust, which amounts to \sim$10 per cent at 31 GHz. In this model, an excess of 1.52\pm 0.66 Jy (2.3\sigma) is seen at 31 GHz. Future high frequency$\sim$100-1000 GHz data, such as those from the {\it Planck} satellite, are required to accurately determine the thermal dust contribution at 31 GHz. Correlations with the IRAS$100 \mu$m gave a coupling coefficient of$18.1\pm4.4 \mu$K (MJy/sr)$^{-1}$, consistent with the values found for LDN1622.Comment: 8 pages, 3 figures, 3 tables, submitted to MNRA

The joint large-scale foreground-CMB posteriors of the 3-year WMAP data

Using a Gibbs sampling algorithm for joint CMB estimation and component separation, we compute the large-scale CMB and foreground posteriors of the 3-yr WMAP temperature data. Our parametric data model includes the cosmological CMB signal and instrumental noise, a single power law foreground component with free amplitude and spectral index for each pixel, a thermal dust template with a single free overall amplitude, and free monopoles and dipoles at each frequency. This simple model yields a surprisingly good fit to the data over the full frequency range from 23 to 94 GHz. We obtain a new estimate of the CMB sky signal and power spectrum, and a new foreground model, including a measurement of the effective spectral index over the high-latitude sky. A particularly significant result is the detection of a common spurious offset in all frequency bands of ~ -13muK, as well as a dipole in the V-band data. Correcting for these is essential when determining the effective spectral index of the foregrounds. We find that our new foreground model is in good agreement with template-based model presented by the WMAP team, but not with their MEM reconstruction. We believe the latter may be at least partially compromised by the residual offsets and dipoles in the data. Fortunately, the CMB power spectrum is not significantly affected by these issues, as our new spectrum is in excellent agreement with that published by the WMAP team. The corresponding cosmological parameters are also virtually unchanged.Comment: 5 pages, 4 figures, submitted to ApJL. Background data are available at http://www.astro.uio.no/~hke under the Research ta

The relationship between classified difficulty and implausible distractors in multiple-choice questions

Published banks of multiple-choice questions are ubiquitous, the questions in those banks often being classified into levels of difficulty. The specific level of difficulty into which a question is classified might or should be a function of the question’s substance. Possibly, though, insubstantive aspects of the question, such as the incidence of incorrect answers that are readily dismissed, also affect the difficulty level into which a question is classified. The present research investigates the relationship between classified question difficulty and the incidence of implausible incorrect answer options

CBI limits on 31 GHz excess emission in southern HII regions

We have mapped four regions of the southern Galactic plane at 31 GHz with the Cosmic Background Imager. From the maps, we have extracted the flux densities for six of the brightest \hii regions in the southern sky and compared them with multi-frequency data from the literature. The fitted spectral index for each source was found to be close to the theoretical value expected for optically thin free-free emission, thus confirming that the majority of flux at 31 GHz is due to free-free emission from ionised gas with an electron temperature of $\approx 7000-8000$ K. We also found that, for all six sources, the 31 GHz flux density was slightly higher than the predicted value from data in the literature. This excess emission could be due to spinning dust or another emission mechanism. Comparisons with $100 \mu$m data indicate an average dust emissivity of $3.3\pm1.7 \mu$K (MJy/sr)$^{-1}$, or a 95 per cent confidence limit of $<6.1 \mu$K (MJy/sr)$^{-1}$. This is lower than that found in diffuse clouds at high Galactic latitudes by a factor of $\sim 3-4$. The most significant detection ($3.3\sigma$) was found in $G284.3-0.3$ (RCW49) and may account for up to $\approx 30$ per cent of the total flux density observed at 31 GHz. Here, the dust emissivity of the excess emission is $13.6\pm4.2 \mu$K (MJy/sr)$^{-1}$ and is within the range observed at high Galactic latitudes. Low level polarised emission was observed in all six sources with polarisation fractions in the range $0.3-0.6$ per cent. This is likely to be mainly due to instrumental leakage and is therefore upper an upper limit to the free-free polarisation. It corresponds to an upper limit of $\sim1$ per cent for the polarisation of anomalous emission.Comment: Accepted in MNRAS. 12 pages, 10 figures, 5 table