7,281 research outputs found
Are all fast radio bursts repeating sources?
We present Monte-Carlo simulations of a cosmological population of repeating
fast radio burst (FRB) sources whose comoving density follows the cosmic star
formation rate history. We assume a power-law model for the intrinsic energy
distribution for each repeating FRB source located at a randomly chosen
position in the sky and simulate their dispersion measures (DMs) and
propagation effects along the chosen lines-of-sight to various telescopes. In
one scenario, an exponential distribution for the intrinsic wait times between
pulses is chosen, and in a second scenario we model the observed pulse arrival
times to follow a Weibull distribution. For both models we determine whether
the FRB source would be deemed a repeater based on the telescope sensitivity
and time spent on follow-up observations. We are unable to rule out the
existence of a single FRB population based on comparisons of our simulations
with the longest FRB follow-up observations performed. We however rule out the
possibility of FRBs 171020 and 010724 repeating with the same rate statistics
as FRB 121102 and also constrain the slope of a power-law fit to the FRB energy
distribution to be . All-sky simulations of repeating FRB
sources imply that the detection of singular events correspond to the bright
tail-end of the adopted energy distribution due to the combination of the
increase in volume probed with distance, and the position of the burst in the
telescope beam.Comment: 10 pages, 4 figures, accepted for publication in MNRA
Galactic Archaeology and Minimum Spanning Trees
Chemical tagging of stellar debris from disrupted open clusters and
associations underpins the science cases for next-generation multi-object
spectroscopic surveys. As part of the Galactic Archaeology project TraCD
(Tracking Cluster Debris), a preliminary attempt at reconstructing the birth
clouds of now phase-mixed thin disk debris is undertaken using a parametric
minimum spanning tree (MST) approach. Empirically-motivated chemical abundance
pattern uncertainties (for a 10-dimensional chemistry-space) are applied to
NBODY6-realised stellar associations dissolved into a background sea of field
stars, all evolving in a Milky Way potential. We demonstrate that significant
population reconstruction degeneracies appear when the abundance uncertainties
approach 0.1 dex and the parameterised MST approach is employed; more
sophisticated methodologies will be required to ameliorate these degeneracies.Comment: To appear in "Multi-Object Spectroscopy in the Next Decade: Big
Questions, Large Surveys and Wide Fields"; Held: Santa Cruz de La Palma,
Canary Islands, Spain, 2-6 Mar 2015; ed. I Skillen & S. Trager; ASP
Conference Series (Figures now optimised for B&W printing
Comparison of Preseason and In-Season Practice and Game Loads in National Collegiate Athletic Association Division i Football Players
Vibrational relaxation measurements in CO2 USING an induced fluorescence technique
Vibrational relaxation measurements in carbon dioxide using induced infrared fluorescence techniqu
M Dwarfs From Hubble Space Telescope Star Counts III: The Groth Strip
We analyze the disk M dwarfs found in 31 new fields observed with the Wide
Field Camera (WFC2) on the Hubble Space Telescope (HST) together with the
sample previously analyzed from 22 WFC2 fields and 162 prerepair Planetary
Camera (PC1) fields. The new observations, which include the 28 high-latitude
fields comprising the Large Area Multi-Color Survey (``Groth Strip''), increase
the total sample to 337 stars, and more than double the number of late M dwarfs
(M_V>13.5) from 23 to 47. The mass function changes slope at M~0.6 Msun, from a
near-Salpeter power-law index of \alpha=-1.21 to \alpha=0.44. In both regimes
the mass function at the Galactic plane is given by {d^3 N / d\log M d M_V d
V} = 8.1\times 10^{-2}\pc^{-3} ({M / 0.59 M_\odot})^{\alpha}. The correction
for secondaries in binaries changes the low-mass index from \alpha=0.44 to
\alpha\sim 0.1. If the Salpeter slope continued to the hydrogen-burning limit,
we would expect 500 stars in the last four bins (14.5<M_V<18.5), instead of the
25 actually detected. The explanation of the observed microlensing rate towards
the Galactic bulge requires either a substantial population of bulge brown
dwarfs or that the disk and bulge mass functions are very different for stars
with M~< 0.5 Msun.Comment: 17 pages including 3 embedded figure
Half-life of the electron-capture decay of 97Ru: Precision measurement shows no temperature dependence
We have measured the half-life of the electron-capture (ec) decay of 97Ru in
a metallic environment, both at low temperature (19K), and also at room
temperature. We find the half-lives at both temperatures to be the same within
0.1%. This demonstrates that a recent claim that the ec decay half-life for 7Be
changes by $0.9% +/- 0.2% under similar circumstances certainly cannot be
generalized to other ec decays. Our results for the half-life of 97Ru,
2.8370(14)d at room temperature and 2.8382(14)d at 19K, are consistent with,
but much more precise than, previous room-temperature measurements. In
addition, we have also measured the half-lives of the beta-emitters 103Ru and
105Rh at both temperatures, and found them also to be unchanged.Comment: 6 pages, 6 figure
Conventional clear-sky aerosol retrievals: Do they work for cloudy days?
This presentation highlights different approaches to determine the aerosol properties between clouds and covers a broad range of related topics, including the passive aerosol remote sensing from space, in situ observations of aerosol aloft and at surface and numerical modeling of aerosol and cloud properties. Some of these approaches, which are still in research phase, can reduce substantially the impact of cloud-induced contamination on the cloudy-sky aerosol retrievals, while other can reduce uncertainties associated with aerosol hygroscopicity and enhanced relative humidity near cloud edges. The combination of these approaches for addressing outstanding issues of the cloudy-sky aerosol retrievals is also discussed
Sky cover from MFRSR observations
The diffuse all-sky surface irradiances measured at two nearby wavelengths in the visible spectral range and their modeled clear-sky counterparts are the main components of a new method for estimating the fractional sky cover of different cloud types, including cumuli. The performance of this method is illustrated using 1-min resolution data from a ground-based Multi-Filter Rotating Shadowband Radiometer (MFRSR). The MFRSR data are collected at the US Department of Energy Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) Southern Great Plains (SGP) site during the summer of 2007 and represent 13 days with cumuli. Good agreement is obtained between estimated values of the fractional sky cover and those provided by a well-established independent method based on broadband observations
The relevance of aerosol optical depth to cumulus fraction changes: a five-year climatology at the ACRF SGP site
International audienceThe objective of this study is to investigate, by observational means, the magnitude and sign of the actively discussed relationship between cloud fraction N and aerosol optical depth ?a. Collocated and coincident ground-based measurements and Terra/Aqua satellite observations at the Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) Southern Great Plains (SGP) site form the basis of this study. The N??a relationship occurred in a specific 5-year dataset of fair-weather cumulus (FWC) clouds and mostly non-absorbing aerosols. To reduce possible contamination of the aerosols on the cloud properties estimation (and vice versa), we use independent datasets of ?a and N obtained from the Multi-filter Rotating Shadowband Radiometer (MFRSR) measurements and from the ARM Active Remotely Sensed Clouds Locations (ARSCL) value-added product, respectively. Optical depth of the FWC clouds ?cld and effective radius of cloud droplets re are obtained from the MODerate resolution Imaging Spectroradiometer (MODIS) data. We found that relationships between cloud properties (N,?cld, re) and aerosol optical depth are time-dependent (morning versus afternoon). Observed time-dependent changes of cloud properties, associated with aerosol loading, control the variability of surface radiative fluxes. In comparison with pristine clouds, the polluted clouds are more transparent in the afternoon due to smaller cloud fraction, smaller optical depth and larger droplets. As a result, the corresponding correlation between the surface radiative flux and ?a is positive (warming effect of aerosol). Also we found that relationship between cloud fraction and aerosol optical depth is cloud size dependent. The cloud fraction of large clouds (larger than 1 km) is relatively insensitive to the aerosol amount. In contrast, cloud fraction of small clouds (smaller than 1 km) is strongly positively correlated with ?a. This suggests that an ensemble of polluted clouds tends to be composed of smaller clouds than a similar one in a pristine environment. One should be aware of these time- and size-dependent features when qualitatively comparing N??a relationships obtained from the satellite observations, surface measurements, and model simulations
B meson form factors from HQET simulations
We use simulations of heavy quark effective field theory to calculate the
Isgur-Wise function, and we demonstrate the feasibility of calculating the
matrix element for the B \to \pi + \leptons decay in the lattice heavy quark
effective theory (HQET).Comment: 3 pages, 2 figures, talk presented at the lattice 97 conferenc
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