207 research outputs found
Family Effects in Youth Employment
The authors begin with the hypothesis that parental contacts play a major role in finding jobs for youth. This hypothesis is tested with a model of youth employment that includes characteristics of other family members in addition to a large set of control variables. Particular attention is paid to parental characteristics that might indicate a parent's ability to assist the youth in finding a job, including occupation, industry and education. The effects of such variables are generally not significant and do not support the initial hypothesis. However, the employment probability of a youth is significantly affected by the presence of employed siblings, indicating the presence of some intrafamily effects.
High-purity microwave generation using a dual-frequency hybrid integrated semiconductor-dielectric waveguide laser
We present an integrated semiconductor-dielectric hybrid dual-frequency laser
operating in the 1.5 m wavelength range for microwave and terahertz (THz)
generation. Generating a microwave beat frequency near 11 GHz, we observe a
record-narrow intrinsic linewidth as low as about 2 kHz. This is realized by
hybrid integration of a single diode amplifier based on indium phosphide (InP)
with a long, low-loss silicon nitride (SiN) feedback circuit to extend
the cavity photon lifetime, resulting in a cavity optical roundtrip length of
about 30 cm on a chip. Simultaneous lasing at two frequencies is enabled by
introducing an external control parameter for balancing the feedback from two
tunable, frequency-selective Vernier mirrors on the SiN chip. Each
frequency can be tuned with a wavelength coverage of about 80 nm, potentially
allowing for the generation of a broad range of frequencies in the microwave
range up to the THz range
Radiation from relativistic jets in blazars and the efficient dissipation of their bulk energy via photon breeding
High-energy photons propagating in the magnetised medium with large velocity
gradients can mediate energy and momentum exchange. Conversion of these photons
into electron-positron pairs in the field of soft photons with the consequent
isotropization and emission of new high-energy photons by Compton scattering
can lead to the runaway cascade of the high-energy photons and
electron-positron pairs fed by the bulk energy of the flow. This is the essence
of the photon breeding mechanism. We study the problem of high-energy emission
of relativistic jets in blazars via photon breeding mechanism using 2D
ballistic model for the jet with the detailed treatment of particle propagation
and interactions. The gamma-ray background of similar energy density as
observed at Earth is sufficient to trigger the photon breeding. As a result, a
jet can convert up to 80 per cent of its total power into radiation. Photon
breeding produces a population of high-energy pairs and predicts the spectra in
agreement with observations of blazars (e.g. the blazar sequence). It also
decelerates the jet at subparsec scales and induces the transversal gradient of
the Lorentz factor which reconcile the discrepancy between the high Doppler
factors determined from the spectra of TeV blazars and the low apparent
velocities observed at VLBI scales. The broad angular distribution of radiation
predicted by the mechanism reconciles the observed statistics and luminosity
ratio of FR I and BL Lac objects with the large Lorentz factors of the jets as
well as explains the high level of the TeV emission in the radio galaxy M87.
(abridged)Comment: 18 pages, 12 figure; replaced with the version accepted to MNRA
Surface roughness over the northern half of the Greenland Ice Sheet from airborne laser altimetry
This is the published version, also available here: http://dx.doi.org/10.1029/2008JF001067.Surface roughness, defined as the standard deviation of small-scale elevation fluctuations from the linear trend over 0.5 km, can be estimated from high-resolution airborne laser altimetry. Here we present results for the northern half of the Greenland Ice Sheet using laser data collected in May 1995. Roughness is smallest in the central region straddling the ice divide, increases in amplitude toward the coast, and appears to be correlated with slope of the ice surface. For most of the study region surface roughness is 8 cm or less (<2.5 cm water equivalent). In smaller regions associated with fast flow, larger values are found. Comparison of the size of small-scale topographic disturbances with the spatial noise estimated from five closely spaced ice cores drilled in northwest Greenland shows good agreement. Similar correspondence was found earlier using nine ice cores from the Summit region. These results indicate that the airborne laser altimeter provides an efficient platform for characterizing the statistical nature of the snow surface over large areas of the polar ice sheets
Fatal Dengue in Patients with Sickle Cell Disease or Sickle Cell Anemia in Curaçao: Two Case Reports
<p>Fatal Dengue in Patients with Sickle Cell Disease or Sickle Cell Anemia in Curaçao: Two Case Reports</p
Monitoring international migration flows in Europe. Towards a statistical data base combining data from different sources
The paper reviews techniques developed in demography, geography and statistics that are useful for bridging the gap between available data on international migration flows and the information required for policy making and research. The basic idea of the paper is as follows: to establish a coherent and consistent data base that contains sufficiently detailed, up-to-date and accurate information, data from several sources should be combined. That raises issues of definition and measurement, and of how to combine data from different origins properly. The issues may be tackled more easily if the statistics that are being compiled are viewed as different outcomes or manifestations of underlying stochastic processes governing migration. The link between the processes and their outcomes is described by models, the parameters of which must be estimated from the available data. That may be done within the context of socio-demographic accounting. The paper discusses the experience of the U.S. Bureau of the Census in combining migration data from several sources. It also summarizes the many efforts in Europe to establish a coherent and consistent data base on international migration.
The paper was written at IIASA. It is part of the Migration Estimation Study, which is a collaborative IIASA-University of Groningen project, funded by the Netherlands Organization for Scientific Research (NWO). The project aims at developing techniques to obtain improved estimates of international migration flows by country of origin and country of destination
Factors affecting phage D29 infection: a tool to investigate different growth states of mycobacteria
Bacteriophages D29 and TM4 are able to infect a wide range of mycobacteria, including pathogenic and non pathogenic species. Successful phage infection of both fast- and slow-growing mycobacteria can be rapidly detected using the phage amplification assay. Using this method, the effect of oxygen limitation during culture of mycobacteria on the success of phage infection was studied. Both D29 and TM4 were able to infect cultures of M. smegmatis and Mycobacterium avium subspecies paratuberculosis (MAP) grown in liquid with aeration. However when cultures were grown under oxygen limiting conditions, only TM4 could productively infect the cells. Cell attachment assays showed that D29 could bind to the cells surface but did not complete the lytic cycle. The ability of D29 to productively infect the cells was rapidly recovered (within 1 day) when the cultures were returned to an aerobic environment and this recovery required de novo RNA synthesis. These results indicated that under oxygen limiting conditions the cells are entering a growth state which inhibits phage D29 replication, and this change in host cell biology which can be detected by using both phage D29 and TM4 in the phage amplification assay
Magnetoluminescence
Pulsar Wind Nebulae, Blazars, Gamma Ray Bursts and Magnetars all contain
regions where the electromagnetic energy density greatly exceeds the plasma
energy density. These sources exhibit dramatic flaring activity where the
electromagnetic energy distributed over large volumes, appears to be converted
efficiently into high energy particles and gamma-rays. We call this general
process magnetoluminescence. Global requirements on the underlying, extreme
particle acceleration processes are described and the likely importance of
relativistic beaming in enhancing the observed radiation from a flare is
emphasized. Recent research on fluid descriptions of unstable electromagnetic
configurations are summarized and progress on the associated kinetic
simulations that are needed to account for the acceleration and radiation is
discussed. Future observational, simulation and experimental opportunities are
briefly summarized.Comment: To appear in "Jets and Winds in Pulsar Wind Nebulae, Gamma-ray Bursts
and Blazars: Physics of Extreme Energy Release" of the Space Science Reviews
serie
Magnetic fields in cosmic particle acceleration sources
We review here some magnetic phenomena in astrophysical particle accelerators
associated with collisionless shocks in supernova remnants, radio galaxies and
clusters of galaxies. A specific feature is that the accelerated particles can
play an important role in magnetic field evolution in the objects. We discuss a
number of CR-driven, magnetic field amplification processes that are likely to
operate when diffusive shock acceleration (DSA) becomes efficient and
nonlinear. The turbulent magnetic fields produced by these processes determine
the maximum energies of accelerated particles and result in specific features
in the observed photon radiation of the sources. Equally important, magnetic
field amplification by the CR currents and pressure anisotropies may affect the
shocked gas temperatures and compression, both in the shock precursor and in
the downstream flow, if the shock is an efficient CR accelerator. Strong
fluctuations of the magnetic field on scales above the radiation formation
length in the shock vicinity result in intermittent structures observable in
synchrotron emission images. Resonant and non-resonant CR streaming
instabilities in the shock precursor can generate mesoscale magnetic fields
with scale-sizes comparable to supernova remnants and even superbubbles. This
opens the possibility that magnetic fields in the earliest galaxies were
produced by the first generation Population III supernova remnants and by
clustered supernovae in star forming regions.Comment: 30 pages, Space Science Review
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