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D. Burke

Publications and source records attributed to D. Burke.

33 records · Page 2Linked to original sources
arXiv2018

Thermal architecture for the QUBIC cryogenic receiver

QUBIC, the QU Bolometric Interferometer for Cosmology, is a novel forthcoming instrument to measure the B-mode polarization anisotropy of the Cosmic Microwave Background. The detection of the B-mode signal will be extremely challenging; QUBIC has been designed to address this with a novel approach, namely bolometric interferometry. The receiver cryostat is exceptionally large and cools complex optical and detector stages to 40 K, 4 K, 1 K and 350 mK using two pulse tube coolers, a novel 4He sorption cooler and a double-stage 3He/4He sorption cooler. We discuss the thermal and mechanical design of the cryostat, modelling and thermal analysis, and laboratory cryogenic testing.

astro-ph.IM↗
arXiv2018

Breaking the habit - the peculiar 2016 eruption of the unique recurrent nova M31N 2008-12a

Since its discovery in 2008, the Andromeda galaxy nova M31N 2008-12a has been observed in eruption every single year. This unprecedented frequency indicates an extreme object, with a massive white dwarf and a high accretion rate, which is the most promising candidate for the single-degenerate progenitor of a type-Ia supernova known to date. The previous three eruptions of M31N 2008-12a have displayed remarkably homogeneous multi-wavelength properties: (i) From a faint peak, the optical light curve declined rapidly by two magnitudes in less than two days; (ii) Early spectra showed initial high velocities that slowed down significantly within days and displayed clear He/N lines throughout; (iii) The supersoft X-ray source (SSS) phase of the nova began extremely early, six days after eruption, and only lasted for about two weeks. In contrast, the peculiar 2016 eruption was clearly different. Here we report (i) the considerable delay in the 2016 eruption date, (ii) the significantly shorter SSS phase, and (iii) the brighter optical peak magnitude (with a hitherto unobserved cusp shape). Early theoretical models suggest that these three different effects can be consistently understood as caused by a lower quiescence mass-accretion rate. The corresponding higher ignition mass caused a brighter peak in the free-free emission model. The less-massive accretion disk experienced greater disruption, consequently delaying re-establishment of effective accretion. Without the early refueling, the SSS phase was shortened. Observing the next few eruptions will determine whether the properties of the 2016 outburst make it a genuine outlier in the evolution of M31N 2008-12a.

astro-ph.SR↗
arXiv2017

CBETA Design Report, Cornell-BNL ERL Test Accelerator

This design report describes the construction plans for the world's first multi-pass SRF ERL. It is a 4-pass recirculating linac that recovers the beam's energy by 4 additional, decelerating passes. All beams are returned for deceleration in a single beam pipe with a large-momentum-aperture permanent magnet FFAG optics. Cornell University has been pioneering a new class of accelerators, Energy Recovery Linacs (ERLs), with a new characteristic set of beam parameters. Technology has been prototyped that is essential for any high brightness electron ERL. This includes a DC electron source and an SRF injector Linac with world-record current and normalized brightness in a bunch train, a high-current linac cryomodule, and a high-power beam stop, and several diagnostics tools for high-current and high-brightness beams. All these are now being used to construct a novel one-cryomodule ERL in Cornell's Wilson Lab. Brookhaven National Laboratory (BNL) has designed a multi-turn ERL for eRHIC, where beam is transported more than 20 times around the 4km long RHIC tunnel. The number of transport lines is minimized by using two arcs with strongly-focusing permanent magnets that can control many beams of different energies. A collaboration between BNL and Cornell has been formed to investigate this multi-turn eRHIC ERL design by building a 4-turn, one-cryomodule ERL at Cornell. It also has a return loop built with strongly focusing permanent magnets and is meant to accelerate 40mA beam to 150MeV. This high-brightness beam will have applications beyond accelerator research, in industry, in nuclear physics, and in X-ray science.

physics.acc-ph↗
arXiv2017

Forward Global Photometric Calibration of the Dark Energy Survey

Many scientific goals for the Dark Energy Survey (DES) require calibration of optical/NIR broadband $b = grizY$ photometry that is stable in time and uniform over the celestial sky to one percent or better. It is also necessary to limit to similar accuracy systematic uncertainty in the calibrated broadband magnitudes due to uncertainty in the spectrum of the source. Here we present a "Forward Global Calibration Method (FGCM)" for photometric calibration of the DES, and we present results of its application to the first three years of the survey (Y3A1). The FGCM combines data taken with auxiliary instrumentation at the observatory with data from the broad-band survey imaging itself and models of the instrument and atmosphere to estimate the spatial- and time-dependence of the passbands of individual DES survey exposures. "Standard" passbands are chosen that are typical of the passbands encountered during the survey. The passband of any individual observation is combined with an estimate of the source spectral shape to yield a magnitude $m_b^{\mathrm{std}}$ in the standard system. This "chromatic correction" to the standard system is necessary to achieve sub-percent calibrations. The FGCM achieves reproducible and stable photometric calibration of standard magnitudes $m_b^{\mathrm{std}}$ of stellar sources over the multi-year Y3A1 data sample with residual random calibration errors of $σ=5-6\,\mathrm{mmag}$ per exposure. The accuracy of the calibration is uniform across the $5000\,\mathrm{deg}^2$ DES footprint to within $σ=7\,\mathrm{mmag}$. The systematic uncertainties of magnitudes in the standard system due to the spectra of sources are less than $5\,\mathrm{mmag}$ for main sequence stars with $0.5<g-i<3.0$.

astro-ph.IM↗
arXiv2015

DES J0454-4448: Discovery of the First Luminous z > 6 Quasar from the Dark Energy Survey

We present the first results of a survey for high redshift, z $\ge$ 6, quasars using izY multi-colour photometric observations from the Dark Energy Survey (DES). Here we report the discovery and spectroscopic confirmation of the $\rm z_{AB}, Y_{AB}$ = 20.2, 20.2 (M$_{1450}$ = $-$26.5) quasar DES J0454$-$4448 with an emission line redshift of z = 6.10$\pm$0.03 and a HI near zone size of 4.6 $\pm$ 1.7 Mpc.The quasar was selected as an i-band drop out with i$-$z = 2.46 and z$_{AB} < 21.5$ from an area of $\rm \sim$300 deg$^2$. It is the brightest of our 43 candidates and was identified for follow-up spectroscopically solely based on the DES i$-$z and z$-$Y colours. The quasar is detected by WISE and has $W1_{AB} = 19.68$. The discovery of one spectroscopically confirmed quasar with 5.7 $<$ z $<$ 6.5 and z$_{AB} \leq$ 20.2 is consistent with recent determinations of the luminosity function at z $\sim$ 6. DES when completed will have imaged $\rm \sim$5000 deg$^2$ to $Y_{AB}$ = 23.0 ($5σ$ point source) and we expect to discover $>$ 50-100 new quasars with z $>$ 6 including 3-10 with z $>$ 7 dramatically increasing the numbers of quasars currently known that are suitable for detailed studies including determination of the neutral HI fraction of the intergalactic medium (IGM) during the epoch of Hydrogen reionization.

astro-ph.GA↗
arXiv2014

Photometric redshift analysis in the Dark Energy Survey Science Verification data

We present results from a study of the photometric redshift performance of the Dark Energy Survey (DES), using the early data from a Science Verification (SV) period of observations in late 2012 and early 2013 that provided science-quality images for almost 200 sq.~deg.~at the nominal depth of the survey. We assess the photometric redshift performance using about 15000 galaxies with spectroscopic redshifts available from other surveys. These galaxies are used, in different configurations, as a calibration sample, and photo-$z$'s are obtained and studied using most of the existing photo-$z$ codes. A weighting method in a multi-dimensional color-magnitude space is applied to the spectroscopic sample in order to evaluate the photo-$z$ performance with sets that mimic the full DES photometric sample, which is on average significantly deeper than the calibration sample due to the limited depth of spectroscopic surveys. Empirical photo-$z$ methods using, for instance, Artificial Neural Networks or Random Forests, yield the best performance in the tests, achieving core photo-$z$ resolutions $σ_{68} \sim 0.08$. Moreover, the results from most of the codes, including template fitting methods, comfortably meet the DES requirements on photo-$z$ performance, therefore, providing an excellent precedent for future DES data sets.

astro-ph.IM↗
arXiv2007

Spatially resolved kinematics and stellar populations of brightest cluster and group galaxies

We present an examination of the kinematics and stellar populations of a sample of 3 Brightest Group Galaxies (BGGs) and 3 Brightest Cluster Galaxies (BCGs) in X-ray groups and clusters. We have obtained high signal-to-noise Gemini/GMOS (Gemini South Multi-Object Spectrograph) long-slit spectra of these galaxies and use Lick indices to determine ages, metallicities and alpha-element abundance ratios out to at least their effective radii. We find that the BGGs and BCGs have very uniform masses, central ages and central metallicities. Examining the radial dependence of their stellar populations, we find no significant velocity dispersion, age, or alpha-enhancement gradients. However, we find a wide range of metallicity gradients, suggesting a variety of formation mechanisms. The range of metallicity gradients observed is surprising given the homogeneous environment these galaxies probe and their uniform central stellar populations. However, our results are inconsistent with any single model of galaxy formation and emphasize the need for more theoretical understanding of both the origins of metallicity gradients and galaxy formation itself. We postulate two possible physical causes for the different formation mechanisms.

astro-ph↗
arXiv2007

Properties of Ellipticity Correlation with Atmospheric Structure from Gemini South

Cosmic shear holds great promise for a precision independent measurement of $Ω\rm_m$, the mass density of the universe relative to the critical density. The signal is expected to be weak, so a thorough understanding of systematic effects is crucial. An important systematic effect is the atmosphere: shear power introduced by the atmosphere is larger than the expected signal. Algorithms exist to extract the cosmic shear from the atmospheric component, though a measure of their success applied to a range of seeing conditions is lacking. To gain insight into atmospheric shear, Gemini South imaging in conjunction with ground condition and satellite wind data were obtained. We find that under good seeing conditions Point-Spread-Function (PSF) correlations persist well beyond the separation typical of high-latitude stars. Under these conditions, ellipticity residuals based on a simple PSF interpolation can be reduced to within a factor of a few of the shot-noise induced ellipticity floor. We also find that the ellipticity residuals are highly correlated with wind direction. Finally, we correct stellar shapes using a more sophisticated procedure and generate shear statistics from stars. Under all seeing conditions in our data set the residual correlations lie everywhere below the target signal level. For good seeing we find that the systematic error attributable to atmospheric turbulence is comparable in magnitude to the statistical error (shape noise) over angular scales relevant to present lensing surveys.

astro-ph↗
arXiv2005

AGN Feedback and Evolution of Radio Sources: Discovery of an X-ray Cluster Associated with z=1 Quasar

We report the first significant detection of an X-ray cluster associated with a powerful (L(bol) ~1e47 erg/sec) radio-loud quasar at high redshift (z=1.06). Diffuse X-ray emission is detected out to ~120 kpc from the CSS quasar 3C 186. A strong Fe-line emission at the z(rest)=1.06 confirms its thermal nature. We find that the CSS radio source is highly overpressured with respect to the thermal cluster medium by ~2-3 orders of magnitude. This provides direct observational evidence that the radio source is not thermally confined as posited in the ``frustrated'' scenario for CSS sources. Instead, the radio source may be young and at an early stage of its evolution. This source provides the first detection of the AGN in outburst in the center of a cooling flow cluster. Powerful radio sources are thought to be triggered by the cooling flows. The evidence for the AGN activity and intermittent outbursts comes from the X-ray morphology of low redshift clusters, which usually do not harbour quasars. 3C186 is a young active radio source which can supply the energy into the cluster and potentially prevent its cooling. We discuss energetics related to the quasar activity and the cluster cooling flow, and possible feedback between the evolving radio source and the cluster.

astro-ph↗
arXiv2005

Environmental Dependence of the Structure of Brightest Cluster Galaxies

We measure the Petrosian structural properties of 33 brightest cluster galaxies (BCGs) at redshifts z<0.1 in X-ray selected clusters with a wide range of X-ray luminosities. We find that some BCGs show distinct signatures in their Petrosian profiles, likely to be due to cD haloes. We also find that BCGs in high X-ray luminosity clusters have shallower surface brightness profiles than those in low X-ray luminosity clusters. This suggests that the BCGs in high X-ray luminosity clusters have undergone up to twice as many equal-mass mergers in their past as those in low X-ray luminosity clusters. This is qualitatively consistent with the predictions of hierarchical structure formation.

astro-ph↗
arXiv2003

A serendipitous survey for galaxy clusters by the XMM-Newton Survey Science Center

We describe the initial results of a programme to detect and identify extended X-ray sources found serendipitously in XMM-Newton observations. We have analysed 186 EPIC-PN images at high galactic latitude with a limiting flux of $1\times 10^{-14}$ \ergcms and found 62 cluster candidates. Thanks to the enhanced sensitivity of the XMM-Newton telescopes, the new clusters found in this pilot study are on the average fainter, more compact, and more distant than those found in previous X-ray surveys. At our survey limit the surface density of clusters is about 5 deg$^{-2}$. We also present the first results of an optical follow-up programme aiming at the redshift measurement of a large sample of clusters. The results of this pilot study give a first glimpse on the potential of serendipitous cluster science with XMM-Newton based on real data. The largest, yet to be fulfilled promise is the identification of a large number of high-redshift clusters for cosmological studies up to $z=1$ or 1.5.

astro-ph↗
arXiv2003

Chandra Temperature Map of Abell 754 and Constraints on Thermal Conduction

We use Chandra data to derive a detailed gas temperature map of the nearby, hot, merging galaxy cluster A754. Combined with the X-ray and optical images, the map reveals a more complex merger geometry than previously thought, possibly involving more than two subclusters or a cool gas cloud sloshing independently from its former host subcluster. In the cluster central region, we detect spatial variations of the gas temperature on all linear scales, from 100 kpc (the map resolution) and up, which likely remain from a merger shock passage. These variations are used to derive an upper limit on effective thermal conductivity on a 100 kpc scale, which is at least an order of magnitude lower than the Spitzer value. This constraint pertains to the bulk of the intracluster gas, as compared to the previously reported estimates for cold fronts (which are rather peculiar sites). If the conductivity in a tangled magnetic field is at the recently predicted higher values (i.e., about 1/5 of the Spitzer value), the observed suppression can be achieved, for example, if the intracluster gas consists of magnetically isolated domains.

astro-ph↗
arXiv2002

The Far Ultraviolet Spectroscopic picture of the Missing Baryons in the Local Group

The number of detected baryons in the low-redshift Universe (z < 1) is far small er than the corresponding number of baryons observed at higher redshift. According to hydrodynamical simulations for the formation of structure in the Universe, up to 2/3 of the 'missing baryons' may have escaped identification so far, due to their high temperature and low density. One of the few ways to detect this matter is to look for its UV absorption signature on the light from background sources. Here we show that the amplitude of the average velocity vector of 'high velocity' OVI absorbers detected in a UV survey of AGN decreases significantly in moving from the Local Standard of Rest, to the Galactic Standard of Rest and Local Group frames. At least 82 % of these absorbers are not associated with any 'high velocity' HI complex (HV-HI) in our Galaxy, and so are strongly implicated as being due to primordial Warm-Hot Intergalactic Medium (WHIM) pervading an extended Galactic Corona or our Local Group. We estimate that the total baryonic mass contained in this medium is as large as ~ 1e12 solar masses, similar to the combined mass of the two most massive virialized constituents of the Local Group (the Andromeda and the Milky Way galaxies), and of the order of the mass required to dynamically stabilize the Local Group.

astro-ph↗
arXiv2002

XMM-Newton observation of the distant (z=0.6) galaxy cluster RX J1120.1+4318

We report on a 20 ksec XMM observation of the distant cluster RXJ1120.1+4318, discovered at z=0.6 in the SHARC survey. The cluster has a regular spherical morphology, suggesting it is in a relaxed state. The combined fit of the EPIC/MOS&pn camera gives a cluster mean temperature of kT=5.3\pm0.5 keV with an iron abundance of 0.47\pm0.19. The temperature profile, measured for the first time at such a redshift, is consistent with an isothermal atmosphere up to half the virial radius. The surface brightness profile, measured nearly up to the virial radius, is well fitted by a beta-model, with beta =0.78[+0.06,-0.04] and a core radius of thetac = 0.44[+0.06,-0.04] arcmin. We compared the properties of RXJ1120.1+4318 with the properties of nearby clusters for two cosmological models: an Einstein - de Sitter Universe and a flat low density Universe with Omega0=0.3. For both models, the scaled emission measure profile beyond the core, the gas mass fraction and luminosity are consistent with the expectations of the self-similar model of cluster formation, although a slightly better agreement is obtained for a low density Universe. There is no evidence of a central cooling flow, in spite of the apparent relaxed state of the cluster. This is consistent with its estimated cooling time, larger than the age of the Universe at the cluster redshift. The entropy profile shows a flat core with a central entropy of ~ 140 keV cm^2, remarkably similar to the entropy floor observed in nearby clusters, and a rising profile beyond typically 0.1 virial radius. Implications of our results, in terms of non-gravitational physics in cluster formation, are discussed.

astro-ph↗