arXiv subjects

A. Smith

Publications and source records attributed to A. Smith.

At least 73 records · Page 4Linked to original sources

arXiv2012

HerMES: A Statistical Measurement of the Redshift Distribution of Herschel-SPIRE Sources Using the Cross-correlation Technique

The wide-area imaging surveys with the {\it Herschel} Space Observatory at sub-mm wavelengths have now resulted in catalogs of order one hundred thousand dusty, star-burst galaxies. We make a statistical estimate of $N(z)$ using a clustering analysis of sub-mm galaxies detected at each of 250, 350 and 500 $μ$m from the Herschel Multi-tiered Extragalactic Survey (HerMES) centered on the Boötes field. We cross-correlate {\it Herschel} galaxies against galaxy samples at optical and near-IR wavelengths from the Sloan Digital Sky Survey (SDSS), the NOAO Deep Wide Field Survey (NDWFS) and the Spitzer Deep Wide Field Survey (SDWFS). We create optical and near-IR galaxy samples based on their photometric or spectroscopic redshift distributions and test the accuracy of those redshift distributions with similar galaxy samples defined with catalogs of the Cosmological Evolution Survey (COSMOS), as the COSMOS field has superior spectroscopy coverage. We model-fit the clustering auto and cross-correlations of {\it Herschel} and optical/IR galaxy samples to estimate $N(z)$ and clustering bias factors. The $S_{350} > 20$ mJy galaxies have a bias factor varying with redshift as $b(z)=1.0^{+1.0}_{-0.5}(1+z)^{1.2^{+0.3}_{-0.7}}$. This bias and the redshift dependence is broadly in agreement with galaxies that occupy dark matter halos of mass in the range of 10$^{12}$ to 10$^{13}$ M$_{\sun}$. We find that the redshift distribution peaks around $z \sim 0.5$ to 1 for galaxies selected at 250 $μ$m with an average redshift of $< z > = 1.8 \pm 0.2$. For 350 and 500 $μ$m-selected SPIRE samples the peak shifts to higher redshift, but the average redshift remains the same with a value of $1.9 \pm 0.2$.

astro-ph.CO↗

arXiv2012

Radio-assay of Titanium samples for the LUX Experiment

We report on the screening of samples of titanium metal for their radio-purity. The screening process described in this work led to the selection of materials used in the construction of the cryostats for the Large Underground Xenon (LUX) dark matter experiment. Our measurements establish titanium as a highly desirable material for low background experiments searching for rare events. The sample with the lowest total long-lived activity was measured to contain <0.25 mBq/kg of U-238, <0.2 mBq/kg of Th-232, and <1.2 mBq/kg of K-40. Measurements of several samples also indicated the presence of short-lived (84 day half life) Sc-46, likely produced cosmogenically via muon initiated (n,p) reactions.

physics.ins-det↗

arXiv2012

LOFT: Large Observatory For X-ray Timing

High-time-resolution X-ray observations of compact objects provide direct access to strong field gravity, black hole masses and spins, and the equation of state of ultra-dense matter. LOFT, the large observatory for X-ray timing, is specifically designed to study the very rapid X-ray flux and spectral variability that directly probe the motion of matter down to distances very close to black holes and neutron stars. A 10 m^2-class instrument in combination with good spectral resolution (<260 eV @ 6 keV) is required to exploit the relevant diagnostics and holds the potential to revolutionise the study of collapsed objects in our Galaxy and of the brightest supermassive black holes in active galactic nuclei. LOFT will carry two main instruments: a Large Area Detector (LAD), to be built at MSSL/UCL with the collaboration of the Leicester Space Research Centre for the collimator) and a Wide Field Monitor (WFM). The ground-breaking characteristic of the LAD (that will work in the energy range 2-30 keV) is a mass per unit surface in the range of ~10 kg/m^2, enabling an effective area of ~10 m^2 (@10 keV) at a reasonable weight and improving by a factor of ~20 over all predecessors. This will allow timing measurements of unprecedented sensitivity, allowing the capability to measure the mass and radius of neutron stars with ~5% accuracy, or to reveal blobs orbiting close to the marginally stable orbit in active galactic nuclei. In this contribution we summarise the characteristics of the LOFT instruments and give an overview of the expectations for its capabilities.

astro-ph.IM↗

arXiv2011

The Cosmic Infrared Background Experiment (CIBER): The Low Resolution Spectrometer

Absolute spectrophotometric measurements of diffuse radiation at 1 μm to 2 μm are crucial to our understanding of the radiative content of the Universe from nucleosynthesis since the epoch of reionization, the composition and structure of the Zodiacal dust cloud in our solar system, and the diffuse galactic light arising from starlight scattered by interstellar dust. The Low Resolution Spectrometer (LRS) on the rocket-borne Cosmic Infrared Background Experiment (CIBER) is a λ/ Δλ\sim 15-30 absolute spectrophotometer designed to make precision measurements of the absolute near-infrared sky brightness between 0.75 μm < λ< 2.1 μm. This paper presents the optical, mechanical and electronic design of the LRS, as well as the ground testing, characterization and calibration measurements undertaken before flight to verify its performance. The LRS is shown to work to specifications, achieving the necessary optical and sensitivity performance. We describe our understanding and control of sources of systematic error for absolute photometry of the near-infrared extragalactic background light.

astro-ph.IM↗

arXiv2011

The distribution of interplanetary dust between 0.96 and 1.04 AU as inferred from impacts on the STEREO spacecraft observed by the Heliospheric Imagers

The distribution of dust in the ecliptic plane between 0.96 and 1.04 AU has been inferred from impacts on the two STEREO spacecraft through observation of secondary particle trails and unexpected off-points in the Heliospheric Imager (HI) cameras. This study made use of analysis carried out by members of a distributed web-based project, Solar Stormwatch. A comparison between observations of the brightest particle trails and a survey of fainter trails shows consistent distributions. While there is no obvious correlation between this distribution and the occurrence of individual meteor streams at Earth, there are some broad longitudinal features in these distributions that are also observed in sources of the sporadic meteor population. The asymmetry in the number of trails seen by each spacecraft and the fact that there are many more unexpected off-points in the HI-B than in HI-A, indicates that the majority of impacts are coming from the apex direction. For impacts causing off-points in the HI-B camera these dust particles are estimated to have masses in excess of 10-17 kg with radii exceeding 0.1 μm. For off-points observed in the HI-A images, which can only have been caused by particles travelling from the anti-apex direction, the distribution is consistent with that of secondary 'storm' trails observed by HI-B, providing evidence that these trails also result from impacts with primary particles from an anti-apex source. It is apparent that the differential mass index of particles from the apex direction is consistently above 2. This indicates that the majority of the mass is within the smaller particles of this population. In contrast, the differential mass index of particles from the anti-apex direction (causing off-points in HI-A) is consistently below 2, indicating that the majority of the mass is to be found in larger particles of this distribution.

astro-ph.EP↗

arXiv2009

Terrestrial Planet Finder Coronagraph (TPF-C) Flight Baseline Concept

The Terrestrial Planet Finder Coronagraph (TPF-C) mission presented here is an existence proof for a flagship-class internal coronagraph space mission capable of detecting and characterizing Earth-like planets and planetary systems at visible wavelengths around nearby stars, using an existing launch vehicle. TPF-C will use spectroscopy to measure key properties of exoplanets including the presence of atmospheric water or oxygen, powerful signatures in the search for habitable worlds.

astro-ph.IM↗

arXiv2009

A Topological Trigger System for Imaging Atmospheric-Cherenkov Telescopes

A fast trigger system is being designed as a potential upgrade to VERITAS, or as the basis for a future array of imaging atmospheric-Cherenkov telescopes such as AGIS. The scientific goal is a reduction of the energy threshold by a factor of 2 over the current threshold of VERITAS of around 130 GeV. The trigger is being designed to suppress both accidentals from the night-sky background and cosmic rays. The trigger uses field-programmable gate arrays (FPGAs) so that it is adaptable to different observing modes and special physics triggers, e.g. pulsars. The trigger consists of three levels: The level 1 (L1.5) trigger operating on each telescope camera samples the discriminated pixels at a rate of 400 MHz and searches for nearest-neighbor coincidences. In L1.5, the received discriminated signals are delay-compensated with an accuracy of 0.078 ns, facilitating a short coincidence time-window between any nearest neighbor of 5 ns. The hit pixels are then sent to a second trigger level (L2) that parameterizes the image shape and transmits this information along with a GPS time stamp to the array-level trigger (L3) at a rate of 10 MHz via a fiber optic link. The FPGA-based event analysis on L3 searches for coincident time-stamps from multiple telescopes and carries out a comparison of the image parameters against a look-up table at a rate of 10 kHz. A test of the single-telescope trigger was carried out in spring 2009 on one VERITAS telescope.

astro-ph.IM↗

arXiv2009

Magnetic cooling at Risoe DTU

Magnetic refrigeration at room temperature is of great interest due to a long-term goal of making refrigeration more energy-efficient, less noisy and free of any environmentally hostile materials. A refrigerator utilizing an active magnetic regenerator (AMR) is based on the magnetocaloric effect, which manifests itself as a temperature change in magnetic materials when subjected to a varying magnetic field. In this work we present the current state of magnetic refrigeration research at Risoe DTU with emphasis on the numerical modeling of an existing AMR test machine. A 2D numerical heat-transfer and fluid-flow model that represents the experimental setup is presented. Experimental data of both no-heat load and heat load situations are compared to the model. Moreover, results from the numerical modeling of the permanent magnet design used in the system are presented.

cond-mat.mtrl-sci↗

arXiv2008

Long Term X-ray Monitoring Of The TeV Binary LS I +61 303 with RXTE

We report on the results of a long term X-ray monitoring campaign of the galactic binary LS I +61 303 performed by the Rossi X-ray Timing Explorer. This dataset consists of 1 ks pointings taken every other day between 2007 August 28 until 2008 February 2. The observations covered six full cycles of the 26.496 day binary period and constitute the largest continuous X-ray monitoring dataset on LS I +61 303 to date with this sensitivity. There is no statistically strong detection of modulation of flux or photon index with orbital phase; however, we do find a strong correlation between flux and photon index, with the spectrum becoming harder at higher fluxes. The dataset contains three large flaring episodes, the largest of these reaching a flux level of 7.2 (+0.1,-0.2)*10^-11 erg cm^-2 s^-1 in the 3-10 keV band, which is a factor of three times larger than flux levels typically seen in the system. Analysis of these flares shows the X-ray emission from LS I +61 303 changing by up to a factor of six over timescales of several hundred seconds as well as doubling times as fast as 2 seconds. This is the fastest variability ever observed from LS I +61 303 at this wavelength and places constraints on the size of the X-ray emitting region.

astro-ph↗

arXiv2008

Technology Section of the White Paper on the Status and Future of Ground-based TeV Gamma-ray Astronomy

This is a report on the findings of the technology working group for the white paper on the status and future of TeV gamma-ray astronomy. The white paper is an APS commissioned document, and the overall version has also been released and can be found on astro-ph. This detailed section of the white paper discusses different technology opportunities and the technical feasibility for substantially improving IACTS and ground based particle detectors to achieve an order of magnitude better sensitivity than the instruments employed today as well as their planned upgrades. A technology roadmap for improving IACTS and ground based particle detectors is presented.

astro-ph↗

arXiv2008

Galactic Compact Objects Section of the White Paper on the Status and Future of Ground-based TeV Gamma-ray Astronomy

This is a report on the findings of the Galactic compact objects working group for the white paper on the status and future of TeV gamma-ray astronomy. The white paper is an APS commissioned document, and the full version has also been released and can be found on astro-ph. This section of the white paper discusses the potential of future ground-based gamma-ray experiments to advance our understanding of the physics of Galactic compact objects including pulsars, pulsar wind nebulae, and X-ray binaries.

astro-ph↗

arXiv2007

Simulation Study of TenTen: A new Multi-TeV IACT array

TenTen is a proposed array of Imaging Atmospheric Cherenkov Telescopes (IACT) optimized for the gamma ray energy regime of 10 TeV to 100 TeV, but with a threshold of ~1 to a few TeV. It will offer a collecting area of 10 km2 above energies of 10 TeV. In the initial phase, a cell of 3 to 5 modest-sized telescopes, each with 10-30 m2 mirror area, is suggested for an Australian site. A possible expansion of the array could comprise many such cells. Here we present work on configuration and technical issues from our simulation studies of the array. Working topics include array layout, telescope size and optics, camera field of view, telescope trigger system, electronics, and site surveys.

astro-ph↗

arXiv2007

TeV and X-ray Monitoring of LS I +61 303 With VERITAS, Swift, and RXTE

Between September 2006 and February 2007, the galactic binary LS I +61 303 was monitored in the TeV band with the VERITAS array of imaging Cherenkov telescopes. These observations confirm LS I +61 303 as a variable TeV gamma-ray source, with emission peaking between orbital phase 0.6 and 0.7. During this observational period, monitoring in the X-ray regime was also carried out using both the RXTE and Swift detectors, which offered complementary coverage of the source. Outbursts in the 0.2-10 keV band were observed by both satellites at close to the same orbital phase as the TeV peak during the 2 orbital cycles covered simultaneously in both bands. While this source has been extensively studied in the X-ray band in the past, this is the first observational campaign to utilize contemporaneous X-ray and TeV data on LS I +61 303.

astro-ph↗

arXiv2005

The Cleo Rich Detector

We describe the design, construction and performance of a Ring Imaging Cherenkov Detector (RICH) constructed to identify charged particles in the CLEO experiment. Cherenkov radiation occurs in LiF crystals, both planar and ones with a novel ``sawtooth''-shaped exit surface. Photons in the wavelength interval 135--165 nm are detected using multi-wire chambers filled with a mixture of methane gas and triethylamine vapor. Excellent pion/kaon separation is demonstrated.

physics.ins-det↗

arXiv2004

HAWC: A Next Generation All-Sky VHE Gamma-Ray Telescope

The study of the universe at energies above 100 GeV is a relatively new and exciting field. The current generation of pointed instruments have detected TeV gamma rays from at least 10 sources and the next generation of detectors promises a large increase in sensitivity. We have also seen the development of a new type of all-sky monitor in this energy regime based on water Cherenkov technology (Milagro). To fully understand the universe at these extreme energies requires a highly sensitive detector capable of continuously monitoring the entire overhead sky. Such an instrument could observe prompt emission from gamma-ray bursts and probe the limits of Lorentz invariance at high energies. With sufficient sensitivity it could detect short transients ($\sim$15 minutes) from active galaxies and study the time structure of flares at energies unattainable to space-based instruments. Unlike pointed instruments a wide-field instrument can make an unbiased study of all active galaxies and enable many multi-wavelength campaigns to study these objects. This paper describes the design and performance of a next generation water Cherenkov detector. To attain a low energy threshold and have high sensitivity the detector should be located at high altitude ($>$ 4km) and have a large area ($\sim$40,000 m$^2$). Such an instrument could detect gamma ray bursts out to a redshift of 1, observe flares from active galaxies as short as 15 minutes in duration, and survey the overhead sky at a level of 50 mCrab in one year.

astro-ph↗

arXiv2003

Observation of the Charge Symmetry Breaking d + d -> 4He + pi0 Reaction Near Threshold

We report the first observation of the charge symmetry breaking d + d -> 4He + pi0 reaction near threshold at the Indiana University Cyclotron Facility. Kinematic reconstruction permitted the separation of 4He + pi0 events from double radiative capture 4He + gamma + gamma events. We measured total cross sections for neutron pion production of 12.7 +- 2.2 pb at 228.5 MeV and 15.1 +- 3.1 pb at 231.8 MeV. The uncertainty is dominated by statistical errors.

nucl-ex↗

arXiv2003

An Initial Survey of White Dwarfs in the Sloan Digital Sky Survey

An initial assessment is made of white dwarf and hot subdwarf stars observed in the Sloan Digital Sky Survey. In a small area of sky (190 square degrees), observed much like the full survey will be, 269 white dwarfs and 56 hot subdwarfs are identified spectroscopically where only 44 white dwarfs and 5 hot subdwarfs were known previously. Most are ordinary DA (hydrogen atmosphere) and DB (helium) types. In addition, in the full survey to date, a number of WDs have been found with uncommon spectral types. Among these are blue DQ stars displaying lines of atomic carbon; red DQ stars showing molecular bands of C_2 with a wide variety of strengths; DZ stars where Ca and occasionally Mg, Na, and/or Fe lines are detected; and magnetic WDs with a wide range of magnetic field strengths in DA, DB, DQ, and (probably) DZ spectral types. Photometry alone allows identification of stars hotter than 12000 K, and the density of these stars for 15<g<20 is found to be ~2.2 deg^{-2} at Galactic latitudes 29-62 deg. Spectra are obtained for roughly half of these hot stars. The spectra show that, for 15<g<17, 40% of hot stars are WDs and the fraction of WDs rises to ~90% at g=20. The remainder are hot sdB and sdO stars.

astro-ph↗

arXiv2003

A Concept for an STJ-based Spectrograph

We describe a multi-order spectrograph concept suitable for 8m-class telescopes, using the intrinsic spectral resolution of Superconducting Tunneling Junction detectors to sort the spectral orders. The spectrograph works at low orders, 1-5 or 1-6, and provides spectral coverage with a resolving power of R~8000 from the atmospheric cutoff at 320 nm to the long wavelength end of the infrared H or K band at 1800 nm or 2400 nm. We calculate that the spectrograph would provide substantial throughput and wavelength coverage, together with high time resolution and sufficient dynamic range. The concept uses currently available technology, or technologies with short development horizons, restricting the spatial sampling to two linear arrays; however an upgrade path to provide more spatial sampling is identified. All of the other challenging aspects of the concept - the cryogenics, thermal baffling and magnetic field biasing - are identified as being feasible.

astro-ph↗