533 research outputs found
Chasing the identification of ASCA Galactic Objects (ChIcAGO): An X-ray survey of unidentified sources in the galactic plane. I : Source sample and initial results
We present the Chasing the Identification of ASCA Galactic Objects (ChIcAGO) survey, which is designed to identify the unknown X-ray sources discovered during the ASCA Galactic Plane Survey (AGPS). Little is known about most of the AGPS sources, especially those that emit primarily in hard X-rays (2-10 keV) within the Fx 10-13 to 10-11 erg cm -2 s-1 X-ray flux range. In ChIcAGO, the subarcsecond localization capabilities of Chandra have been combined with a detailed multiwavelength follow-up program, with the ultimate goal of classifying the >100 unidentified sources in the AGPS. Overall to date, 93 unidentified AGPS sources have been observed with Chandra as part of the ChIcAGO survey. A total of 253 X-ray point sources have been detected in these Chandra observations within 3âČ of the original ASCA positions. We have identified infrared and optical counterparts to the majority of these sources, using both new observations and catalogs from existing Galactic plane surveys. X-ray and infrared population statistics for the X-ray point sources detected in the Chandra observations reveal that the primary populations of Galactic plane X-ray sources that emit in the Fx 10-13 to 10-11 erg cm -2 s-1 flux range are active stellar coronae, massive stars with strong stellar winds that are possibly in colliding wind binaries, X-ray binaries, and magnetars. There is also another primary population that is still unidentified but, on the basis of its X-ray and infrared properties, likely comprises partly Galactic sources and partly active galactic nuclei.Peer reviewedSubmitted Versio
Modeling Single Electron Transfer in Si:P Double Quantum Dots
Solid-state systems such as P donors in Si have considerable potential for
realization of scalable quantum computation. Recent experimental work in this
area has focused on implanted Si:P double quantum dots (DQDs) that represent a
preliminary step towards the realization of single donor charge-based qubits.
This paper focuses on the techniques involved in analyzing the charge transfer
within such DQD devices and understanding the impact of fabrication parameters
on this process. We show that misalignment between the buried dots and surface
gates affects the charge transfer behavior and identify some of the challenges
posed by reducing the size of the metallic dot to the few donor regime.Comment: 11 pages, 7 figures, submitted to Nanotechnolog
An Efficient Molecular Dynamics Scheme for the Calculation of Dopant Profiles due to Ion Implantation
We present a highly efficient molecular dynamics scheme for calculating the
concentration depth profile of dopants in ion irradiated materials. The scheme
incorporates several methods for reducing the computational overhead, plus a
rare event algorithm that allows statistically reliable results to be obtained
over a range of several orders of magnitude in the dopant concentration.
We give examples of using this scheme for calculating concentration profiles
of dopants in crystalline silicon. Here we can predict the experimental profile
over five orders of magnitude for both channeling and non-channeling implants
at energies up to 100s of keV.
The scheme has advantages over binary collision approximation (BCA)
simulations, in that it does not rely on a large set of empirically fitted
parameters. Although our scheme has a greater computational overhead than the
BCA, it is far superior in the low ion energy regime, where the BCA scheme
becomes invalid.Comment: 17 pages, 21 figures, 2 tables. See: http://bifrost.lanl.gov/~reed
Exploring the error characteristics of thin ice cloud property retrievals using a Markov chain Monte Carlo algorithm
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95385/1/jgrd15111.pd
Exploring the first aerosol indirect effect over Southeast Asia using a 10-year collocated MODIS, CALIOP, and model dataset
Satellite observations and model simulations cannot, by
themselves, give full insight into the complex relationships between aerosols
and clouds. This is especially true over Southeast Asia, an area that is
particularly sensitive to changes in precipitation yet poses some of the
world's largest observability and predictability challenges. We present a new
collocated dataset, the Curtain Cloud-Aerosol Regional A-Train dataset, or
CCARA. CCARA includes collocated satellite observations from Aqua's
Moderate-resolution Imaging Spectroradiometer (MODIS) and the Cloud-Aerosol
Lidar with Orthogonal Polarization (CALIOP) with the Navy Aerosol Analysis
and Prediction System (NAAPS). The CCARA dataset is designed with the
capability to investigate aerosolâcloud relationships in regions with
limited aerosol retrievals due to high cloud amounts by leveraging the NAAPS
model reanalysis of aerosol concentration in these regions. This combined
aerosol and cloud dataset provides coincident and vertically resolved cloud
and aerosol observations for 2006–2016. Using the model reanalysis aerosol
fields from the NAAPS and coincident cloud liquid effective radius retrievals
from MODIS (cirrus contamination using CALIOP), we investigate the first
aerosol indirect effect in Southeast Asia. We find that, as expected, aerosol
loading anti-correlates with cloud effective radius, with maximum sensitivity
in cumulous mediocris clouds with heights in the 3–4.5 km level. The
highest susceptibilities in droplet effective radius to modeled perturbations
in particle concentrations were found in the more remote and pristine regions
of the western Pacific Ocean and Indian Ocean. Conversely, there was much
less variability in cloud droplet size near emission sources over both land
and water. We hypothesize this is suggestive of a high aerosol background
already saturated with cloud condensation nuclei even during the relatively
clean periods, in contrast to the remote ocean regions, which have periods
where the aerosol concentrations are low enough to allow for larger droplet
growth.</p
Trimodal cloudiness and tropical stable layers in simulations of radiative convective equilibrium
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95176/1/grl24304.pd
The Thousand-Pulsar-Array programme on MeerKAT XIII: Timing, flux density, rotation measure and dispersion measure timeseries of 597 pulsars
We report here on the timing of 597 pulsars over the last four years with the
MeerKAT telescope. We provide Times-of-Arrival, pulsar ephemeris files and
per-epoch measurements of the flux density, dispersion measure (DM) and
rotation measure (RM) for each pulsar. In addition we use a Gaussian process to
model the timing residuals to measure the spin frequency derivative at each
epoch. We also report the detection of 11 glitches in 9 individual pulsars. We
find significant DM and RM variations in 87 and 76 pulsars respectively. We
find that the DM variations scale approximately linearly with DM, which is
broadly in agreement with models of the ionised interstellar medium. The
observed RM variations seem largely independent of DM, which may suggest that
the RM variations are dominated by variations in the interstellar magnetic
field on the line of sight, rather than varying electron density. We also find
that normal pulsars have around 5 times greater amplitude of DM variability
compared to millisecond pulsars, and surmise that this is due to the known
difference in their velocity distributions.Comment: This is a pre-copyedited, author-produced PDF of an article accepted
for publication in MNRAS following peer review. 12 pages 7 figure
Aerosol meteorology of Maritime Continent for the 2012 7SEAS southwest monsoon intensive study - Part 2: Philippine receptor observations of fine-scale aerosol behavior
Abstract. The largest 7 Southeast Asian Studies (7SEAS) operations period within the Maritime Continent (MC) occurred in the AugustâSeptember 2012 biomass burning season. Data included were observations aboard the M/Y Vasco, dispatched to the Palawan Archipelago and Sulu Sea of the Philippines for September 2012. At these locations, the Vasco observed MC smoke and pollution entering the southwest monsoon (SWM) monsoonal trough. Here we describe the research cruise findings and the finer-scale aerosol meteorology of this convectively active region. This 2012 cruise complemented a 2-week cruise in 2011 and was generally consistent with previous findings in terms of how smoke emission and transport related to monsoonal flows, tropical cyclones (TC), and the covariance between smoke transport events and the atmosphere's thermodynamic structure. Biomass burning plumes were usually mixed with significant amounts of anthropogenic pollution. Also key to aerosol behavior were squall lines and cold pools propagating across the South China Sea (SCS) and scavenging aerosol particles in their path. However, the 2012 cruise showed much higher modulation in aerosol frequency than its 2011 counterpart. Whereas in 2011 large synoptic-scale aerosol events transported high concentrations of smoke into the Philippines over days, in 2012 measured aerosol events exhibited a much shorter-term variation, sometimes only 3â12âŻh. Strong monsoonal flow reversals were also experienced in 2012. Nucleation events in cleaner and polluted conditions, as well as in urban plumes, were observed. Perhaps most interestingly, several cases of squall lines preceding major aerosol events were observed, as opposed to 2011 observations where these lines largely scavenged aerosol particles from the marine boundary layer. Combined, these observations indicate pockets of high and low particle counts that are not uncommon in the region. These perturbations are difficult to observe by satellite and very difficult to model. Indeed, the Navy Aerosol Analysis and Prediction System (NAAPS) simulations captured longer period aerosol events quite well but largely failed to capture the timing of high-frequency phenomena. Ultimately, the research findings of these cruises demonstrate the real world challenges of satellite-based missions, significant aerosol life cycle questions such as those the future Aerosol/Clouds/Ecosystems (ACE) will investigate, and the importance of small-scale phenomena such as sea breezes, squall lines, and nucleation events embedded within SWM patterns in dominating aerosol life cycle and potential relationships to clouds
Confronting the Challenge of Modeling Cloud and Precipitation Microphysics
In the atmosphere, microphysics refers to the microscale processes that affect cloud and precipitation particles and is a key linkage among the various components of Earth\u27s atmospheric water and energy cycles. The representation of microphysical processes in models continues to pose a major challenge leading to uncertainty in numerical weather forecasts and climate simulations. In this paper, the problem of treating microphysics in models is divided into two parts: (i) how to represent the population of cloud and precipitation particles, given the impossibility of simulating all particles individually within a cloud, and (ii) uncertainties in the microphysical process rates owing to fundamental gaps in knowledge of cloud physics. The recently developed Lagrangian particleâbased method is advocated as a way to address several conceptual and practical challenges of representing particle populations using traditional bulk and bin microphysics parameterization schemes. For addressing critical gaps in cloud physics knowledge, sustained investment for observational advances from laboratory experiments, new probe development, and nextâgeneration instruments in space is needed. Greater emphasis on laboratory work, which has apparently declined over the past several decades relative to other areas of cloud physics research, is argued to be an essential ingredient for improving processâlevel understanding. More systematic use of natural cloud and precipitation observations to constrain microphysics schemes is also advocated. Because it is generally difficult to quantify individual microphysical process rates from these observations directly, this presents an inverse problem that can be viewed from the standpoint of Bayesian statistics. Following this idea, a probabilistic framework is proposed that combines elements from statistical and physical modeling. Besides providing rigorous constraint of schemes, there is an added benefit of quantifying uncertainty systematically. Finally, a broader hierarchical approach is proposed to accelerate improvements in microphysics schemes, leveraging the advances described in this paper related to process modeling (using Lagrangian particleâbased schemes), laboratory experimentation, cloud and precipitation observations, and statistical methods
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