arXiv subjects

R. J. Davis

Publications and source records attributed to R. J. Davis.

At least 163 records · Page 9Linked to original sources

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.

astro-ph.CO↗

arXiv2009

30 GHz observations of sources in the VSA fields

Small angular scale (high l) studies of cosmic microwave background anisotropies require accurate knowledge of the statistical properties of extragalactic sources at cm-mm wavelengths. We have used a 30 GHz dual-beam receiver (OCRA-p) on the Torun 32-m telescope to measure the flux densities of 121 sources in VSA fields selected at 15 GHz with the Ryle Telescope. We have detected 57 sources above a limiting flux density of 5mJy, of which 31 sources have a flux density greater than 10mJy, which is our effective completeness limit. From these measurements we derive a surface density of sources above 10mJy at 30 GHz of 2.0+/-0.4 per square degree. This is consistent with the surface density obtained by Mason et al. (2009) who observed a large sample of sources selected at a much lower frequency (1.4 GHz). We have also investigated the dependence of the spectral index distribution on flux density by comparing our results with those for sources above 1 Jy selected from the WMAP 22 GHz catalogue. We conclude that the proportion of steep spectrum sources increases with decreasing flux density, qualitatively consistent with the predictions of deZotti et al. (2005). We find no evidence for an unexpected population of sources whose spectra rise towards high frequencies, which would affect our ability to interpret current high resolution CMB observations at 30 GHz and above.

astro-ph.CO↗

arXiv2009

Double radio peak and non-thermal collimated ejecta in RS Ophiuchi following the 2006 outburst

We report MERLIN, VLA, OCRA-p, VLBA, Effelsberg and GMRT observations beginning 4.5 days after the discovery of RS Ophiuchi undergoing its 2006 recurrent nova outburst. Observations over the first 9 weeks are included, enabling us to follow spectral development throughout the three phases of the remnant development. We see dramatic brightening on days 4 to 7 at 6 GHz and an accompanying increase in other bands, particularly 1.46 GHz, consistent with transition from the initial "free expansion" phase to the adiabatic expansion phase. This is complete by day 13 when the flux density at 5 GHz is apparently declining from an unexpectedly early maximum (compared with expectations from observations of the 1985 outburst). The flux density recovered to a second peak by approximately day 40, consistent with behaviour observed in 1985. At all times the spectral index is consistent with mixed non-thermal and thermal emission. The spectral indices are consistent with a non-thermal component at lower frequencies on all dates, and the spectral index changes show that the two components are clearly variable. The estimated extent of the emission at 22 GHz on day 59 is consistent with the extended east and west features seen at 1.7 GHz with the VLBA on day 63 being entirely non-thermal. We suggest a two-component model, consisting of a decelerating shell seen in mixed thermal and non-thermal emission plus faster bipolar ejecta generating the non-thermal emission, as seen in contemporaneous VLBA observations. Our estimated ejecta mass of 4+/-2x10^{-7} M_\odot is consistent with a WD mass of 1.4 M_\odot. It may be that this ejecta mass estimate is a lower limit, in which case a lower WD mass would be consistent with the data.

astro-ph.SR↗

arXiv2008

Anomalous Microwave Emission from the HII region RCW175

We present evidence for anomalous microwave emission in the RCW175 \hii region. Motivated by 33 GHz $13\arcmin$ resolution data from the Very Small Array (VSA), we observed RCW175 at 31 GHz with the Cosmic Background Imager (CBI) at a resolution of $4\arcmin$. The region consists of two distinct components, G29.0-0.6 and G29.1-0.7, which are detected at high signal-to-noise ratio. The integrated flux density is $5.97\pm0.30$ Jy at 31 GHz, in good agreement with the VSA. The 31 GHz flux density is $3.28\pm0.38$ Jy ($8.6σ$) above the expected value from optically thin free-free emission based on lower frequency radio data and thermal dust constrained by IRAS and WMAP data. Conventional emission mechanisms such as optically thick emission from ultracompact \hii regions cannot easily account for this excess. We interpret the excess as evidence for electric dipole emission from small spinning dust grains, which does provide an adequate fit to the data.

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arXiv2008

Multi-frequency spectral analysis of extragalactic radio sources in the 33-GHz VSA catalogue: sources with flattening and upturn spectrum

We present an analysis of the spectral properties of the extragalactic radio sources in the nearly-complete VSA sample at 33GHz. Data from different surveys are used to study source spectra between 1.4 and 33GHz. We find that, in general, spectra can not be well described by a single power law in the range of frequencies considered. In particular, most of the VSA sources that are steep between 1.4 and 5GHz, show a spectral flattening at ν>5GHz. We identify 20 objects (19% of the sample) clearly characterized by an upturn spectrum, i.e., a spectrum falling at low frequencies (ν<5GHz) and inverted at higher frequencies. Spectra with high-frequency flattening or upturn shape are supposed to occur when the emission from the AGN compact core begins to dominate over the component from extended lobes. This picture fits well with the AGN unified scheme, for objects observed at intermediate viewing angles of the AGN jet. Finally, we discuss implications that this class of sources can have on future CMB observations at high resolution.

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arXiv2007

Silicate dust in the environment of RS Ophiuchi following the 2006 eruption

We present further Spitzer Space Telescope observations of the recurrent nova RS Ophiuchi, obtained over the period 208-430 days after the 2006 eruption. The later Spitzer IRS data show that the line emission and free-free continuum emission reported earlier is declining, revealing incontrovertible evidence for the presence of silicate emission features at 9.7 and 18microns. We conclude that the silicate dust survives the hard radiation impulse and shock blast wave from the eruption. The existence of the extant dust may have significant implications for understanding the propagation of shocks through the red giant wind and likely wind geometry.

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arXiv2007

V723 Cas (Nova Cassiopeiae 1995): MERLIN observations from 1996 to 2001

MERLIN observations of the unusually slow nova V723 Cas are presented. Nine epochs of 6-cm data between 1996 and 2001 are mapped, showing the initial expansion and brightening of the radio remnant, the development of structure and the final decline. A radio light curve is presented and fitted by the standard Hubble flow model for radio emission from novae in order to determine the values of various physical parameters for the shell. The model is consistent with the overall development of the radio emission. Assuming a distance of 2.39 (+/-0.38) kpc and a shell temperature of 17000 K, the model yields values for expansion velocity of 414 +/- 0.1 km s^-1 and shell mass of 1.13 +/- 0.04 * 10^-4 Msolar. These values are consistent with those derived from other observations although the ejected masses are rather higher than theoretical predictions. The structure of the shell is resolved by MERLIN and shows that the assumption of spherical symmetry in the standard model is unlikely to be correct.

astro-ph↗

arXiv2007

Spitzer and ground-based infrared observations of the 2006 eruption of RS Ophiuchi

We present Spitzer Space Telescope and complementary ground-based infrared observations of the recurrent nova RS Ophiuchi, obtained over the period 64-111 days after the 2006 eruption. The Spitzer IRS data show a rich emission line spectrum superimposed on a free-free continuum. The presence of fine structure and coronal infrared lines lead us to deduce that there are at least two temperatures (1.5e5K and 9e5K) in the ejecta/wind environment, and that the electron density in the `cooler' region is 2.2e5 cm-3. The determination of elemental abundances is not straightforward but on the assumption that the Ne and O fine structure lines arise in the same volume of the ejecta, the O/Ne ratio is >~0.6 by number.

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arXiv2007

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 $100 μ$m data indicate an average dust emissivity of $3.3\pm1.7 μ$K (MJy/sr)$^{-1}$, or a 95 per cent confidence limit of $<6.1 μ$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 ($3.3σ$) 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 $13.6\pm4.2 μ$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.

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arXiv2006

Millimetric observation of the SZ effect in the Corona Borealis supercluster

We have observed the Corona Borealis Supercluster with the Millimeter and Infrared Testa grigia Observatory (MITO), located in the Italian Alps, at 143, 214, 272, and 353 GHz. We present a description of the measurements, data analysis, and results of the observations together with a comparison with observations performed at 33 GHz with the Very Small Array (VSA) interferometer situated at the Teide Observatory (Tenerife-Spain). Observations have been made in the direction of the supercluster towards one Cosmic Microwave Background (CMB) cold spot previously detected in a VSA temperature map. Observational strategy and data analysis are described in detail, explaining the procedures used to disentangle primary and secondary anisotropies in the resulting maps. From a first level of data analysis we find evidence in MITO data of primary anisotropy, however still with room for the presence of secondary anisotropy, especially when VSA results are included. With a second level of data analysis using map-making and the maximum entropy method we claim a weak detection of a faint signal compatible with a SZ effect characterized at most by a Comptonization parameter $y=(7.8^{+5.3}_{-4.4})\times10^{-6}$ 68% CL. The low level of confidence in the presence of a SZ signal invite us to study this sky region with higher sensitivity and angular resolution experiments like the already planned upgraded versions of VSA and MITO.

astro-ph↗

arXiv2006

Infrared observations of the 2006 outburst of the recurrent nova RS Ophiuchi: the early phase

We present infrared spectroscopy of the recurrent nova RS Ophiuchi, obtained 11.81, 20.75 and 55.71 days following its 2006 eruption. The spectra are dominated by hydrogen recombination lines, together with HeI, OI and OII lines; the electron temperature of ~10^4 K implied by the recombination spectrum suggests that we are seeing primarily the wind of the red giant, ionized by the ultraviolet flash when RS Oph erupted. However, strong coronal emission lines (i.e. emission from fine structure transitions in ions having high ionization potential) are present in the last spectrum. These imply a temperature of 930000K for the coronal gas; this is in line with x-ray observations of the 2006 eruption. The emission line widths decrease with time in a way that is consistent with the shock model for the x-ray emission.

astro-ph↗

arXiv2006

An asymmetric shock wave in the 2006 outburst of the recurrent nova RS Ophiuchi

Nova outbursts take place in binary star systems comprising a white dwarf and either a low-mass Sun-like star or, as in the case of the recurrent nova RS Ophiuchi, a red giant. Although the cause of these outbursts is known to be thermonuclear explosion of matter transferred from the companion onto the surface of the white dwarf, models of the previous (1985) outburst of RS Ophiuchi failed to adequately fit the X-ray evolution and there was controversy over a single-epoch high-resolution radio image, which suggested that the remnant was bipolar rather than spherical as modelled. Here we report the detection of spatially resolved structure in RS Ophiuchi from two weeks after its 12 February 2006 outburst. We track an expanding shock wave as it sweeps through the red giant wind, producing a remnant similar to that of a type II supernova but evolving over months rather than millennia. As in supernova remnants, the radio emission is non-thermal (synchrotron emission), but asymmetries and multiple emission components clearly demonstrate that contrary to the assumptions of spherical symmetry in models of the 1985 explosion, the ejection is jet-like, collimated by the central binary whose orientation on the sky can be determined from these observations.

astro-ph↗

arXiv2006

A determination of the Spectra of Galactic components observed by WMAP

WMAP data when combined with ancillary data on free-free, synchrotron and dust allow an improved understanding of the spectrum of emission from each of these components. Here we examine the sky variation at intermediate latitudes using a cross-correlation technique. In particular, we compare the observed emission in 15 selected sky regions to three ``standard'' templates. The free-free emission of the diffuse ionised gas is fitted by a well-known spectrum at K and Ka band, but the derived emissivity corresponds to a mean electron temperature of ~4000-5000K. This is inconsistent with estimates from galactic HII regions. The origin of the discrepancy is unclear. The anomalous emission associated with dust is clearly detected in most of the 15 fields studied; it correlates well with the Finkbeiner et al. model 8 predictions (FDS8) at 94 GHz, with an effective spectral index between 20 and 60GHz of -2.85. Furthermore, the emissivity varies by a factor of ~2 from cloud to cloud. A modestly improved fit to the anomalous dust at K-band is provided by modulating the template by an estimate of the dust colour temperature, specifically FDS8*T^n. We find a preferred value n~1.6. The synchrotron emission steepens between GHz frequencies and the WMAP bands. There are indications of spectral index variations across the sky but the current data are not precise enough to accurately quantify this from region to region. Our analysis of the WMAP data indicates strongly that the dust-correlated emission at the low WMAP frequencies has a spectrum which is compatible with spinning dust; we find no evidence for a synchrotron component correlated with dust (abridged).

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arXiv2005

CMB observations from the CBI and VSA: A comparison of coincident maps and parameter estimation methods

We present coincident observations of the Cosmic Microwave Background (CMB) from the Very Small Array (VSA) and Cosmic Background Imager (CBI) telescopes. The consistency of the full datasets is tested in the map plane and the Fourier plane, prior to the usual compression of CMB data into flat bandpowers. Of the three mosaics observed by each group, two are found to be in excellent agreement. In the third mosaic, there is a 2 sigma discrepancy between the correlation of the data and the level expected from Monte Carlo simulations. This is shown to be consistent with increased phase calibration errors on VSA data during summer observations. We also consider the parameter estimation method of each group. The key difference is the use of the variance window function in place of the bandpower window function, an approximation used by the VSA group. A re-evaluation of the VSA parameter estimates, using bandpower windows, shows that the two methods yield consistent results.

astro-ph↗

arXiv2004

White dwarfs with jets as non-relativistic analogues of quasars and microquasars?

We explore the similarities between accreting white dwarfs (CH Cyg and MWC 560) and the much more energetic jet sources - quasars and microquasars. To-date we have identified several common attributes: (1) they exhibit collimated outflows (jets); (2) the jets are precessing; (3) these two symbiotic stars exhibit quasar-like emission line spectra; (4) there is a disk-jet connection like that observed in microquasars. Additionally they may have a similar energy source (extraction of rotational energy from the accreting object). Study of the low energy analogues could have important implications for our understanding of their higher energy cousins.

astro-ph↗

arXiv2003

Spectroscopy of the Millennium Outburst and Recent Variability of the Yellow Hypergiant Rho Cassiopeiae

In the summer and fall of 2000 the yellow hypergiant Rho Cassiopeiae dimmed by more than a visual magnitude, while its effective temperature decreased from \~7000 K to below 4000 K over ~200 d. We observed the highest mass-loss rate of ~5 % of the solar mass per year in a single stellar eruption so far (Lobel et al. 2003, ApJ, 583, 923). We determine from newly formed TiO bands a gas mass-loss rate of M_dot ~ 5.4 10^{-2} Msun/yr, which is of the same order of magnitude as has been proposed for the giant outbursts of the Luminous Blue Variable Eta Carinae. Over the past two years since the outburst event we observe a very prominent inverse P Cygni profile in Balmer H alpha. Strong Halpha emission has not before been observed in the hypergiant over this long period of time, signaling an unusual strong collapse of the upper hydrogen atmosphere, which we also observed in the months before the 2000 outburst. Since the brightness decrease of March 2003 we observe a transformation of the H alpha profile into a P Cygni profile, signaling a strong expansion of Rho Cas's upper atmosphere, which could possibly result in a new outburst event. High-resolution spectroscopic observations reveal that the lower photosphere rapidly expanded until May-June 2003. Very recent high-resolution observations of September 2003 show however that the photospheric absorption lines did not shift far blueward as observed in July 2000 before the strong brightness decrease of the outburst. The Fe I 5572 A is redshifted, signaling the collapse of the lower photosphere. A new strong brightness decrease by more than a magnitude in V for the fall and winter of 2003 is therefore not expected.

astro-ph↗

arXiv2003

Reappraising foreground contamination in the COBE-DMR data

With the advent of all-sky H-Alpha surveys it is possible to determine a reliable free-free template of the diffuse interstellar medium (Dickinson, Davies & Davis 2003) which can be used in conjunction with the synchrotron and dust templates to correct CMB observations for diffuse Galactic foregrounds. We have used the COBE-DMR data at 31.5, 53 and 90 GHz and employed cross- correlation techniques to re-evaluate the foreground contributions, particularly that due to dust which is known to be correlated with H-Alpha (and free-free) emission.The DMR microwave maps are found to contain, as well as the expected synchrotron and free-free components, a component tightly correlated to the 140 micron dust maps of DIRBE. At 31.5, 53 and 90 GHz this emission is 6.3 +/- 0.6, 2.4 +/- 0.4 and 2.2 +/- 0.4 microK/(MJy/sr) at 140 microns respectively. When corrected for the contribution from thermal dust following model 7 of Finkbeiner, Davis & Schlegel (1999), a strong anomalous dust emission component remains, which is well-fitted by a frequency spectrum of the form $ν^{-β}$ where beta ~ 2.5 in the DMR frequency range; this is the dominant foreground at 31.5 GHz. The result implies the presence of an emission component with a dust-like morphology but a synchrotron-like spectrum. We discuss the possible origins of this component and compare it with the recent WMAP interpretation. The better knowledge of the individual foregrounds provided by the present study enables a larger area of the sky (|b|>15) to be used to re-appraise the CMB quadrupole normalisation, Qrms-PS, and the scalar perturbations spectral index, n. We find Qrms-PS=15.2^{+2.8}_{-2.3} with a power-law spectral index of n=1.2 +/- 0.2. These values are consistent with previous COBE-DMR analyses and the WMAP 1-year analysis.

astro-ph↗

arXiv2003

Towards a free-free template for CMB foregrounds

A full-sky template map of the Galactic free-free foreground emission component is increasingly important for high sensitivity CMB experiments. We use the recently published \ha data of both the northern and southern skies as the basis for such a template. The first step is to correct the \ha maps for dust absorption using the 100 $μ$m dust maps of Schlegel, Finkbeiner & Davis (1998). We show that for a range of longitudes, the Galactic latitude distribution of absorption suggests that it is 33 per cent of the full extragalactic absorption. A reliable absorption-corrected \ha map can be produced for $\sim 95$ per cent of the sky; the area for which a template cannot be recovered is the Galactic plane area $|b| < 5^{\circ}$, $l=260^{\circ}-0^{\circ}-160^{\circ}$ and some isolated dense dust clouds at intermediate latitudes. The second step is to convert the dust-corrected \ha data into a predicted radio surface brightness. The free-free emission formula is revised to give an accurate expression (1 per cent) for the radio emission covering the frequency range 100 MHz to 100 GHz and the electron temperature range 3000 to 20000 K. The main uncertainty when applying this expression is the variation of electron temperature across the sky. The emission formula is verified in several extended H{\sc ii} regions using data in the range 408 to 2326 MHz. A full-sky free-free template map is presented at 30 GHz; the scaling to other frequencies is given. The Haslam et al. all-sky 408 MHz map of the sky can be corrected for this free-free component, which amounts to a $\approx 6$ per cent correction at intermediate and high latitudes....

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