458 research outputs found
Observations of the variability of shallow trade wind cumulus cloudiness and mass flux
Two years of ground-based remote sensing observations are used to study the vertical structure of marine cumulus near the island of Barbados, including their cloud fraction and mass flux profile. Daily radar derived cloud fraction profiles peak at different height levels depending on the depth of the cumuli and thus the extent to which they precipitate. Nonprecipitating cumuli have a peak cloud fraction of about 5% near mean cloud base (700m), whereas precipitating cumuli tend to have a peak of only 2% near cloud base. Nineteen percent of the precipitating cumuli are accompanied by large cloud fractions near the detrainment level of cumulus tops (similar to 1700m). Day-to-day variations in cloud fraction near cloud base are modest (similar to 3%). Nonprecipitating cumuli have their largest reflectivities near cloud top and an ascending core surrounded by a subsiding shell. Precipitating cumuli with enhanced elevated cloudiness (stratiform outflow) are deeper and contain larger vertical gradients in reflectivity and Doppler velocity than precipitating cumuli without such outflow. Bulk (3h) statistics reveal that nonprecipitating shallow cumuli are active and organized. They contain on average 79% in-cloud updrafts with 86% of them being organized in large coherent structures contributing to a maximum updraft mass flux of 8-36gm(-2)s(-1) just above cloud base. Alternatively, downdrafts contribute insignificantly to the mass flux and show little vertical and temporal variability (0-7gm(-2)s(-1)). Complementary Raman lidar information suggests that updraft mass flux profile slope is inversely related to environmental relative humidity
X-ray spectral variability of the Seyfert galaxy NGC 4051
We report on the X-ray spectral variability of the Seyfert 1 galaxy NGC 4051
observed with the Rossi X-ray Timing Explorer (RXTE) during a 1000 day period
between May 1996 and March 1999. The spectra were obtained as part of
monitoring observations and from two long observations using the RXTE
Proportional Counter Array (PCA). During the monitoring period the 2-10 keV
flux of NGC 4051 varied between 10E-12 and 7x 10E11 (cgs). We re-analysed RXTE
PCA observations from a distinct low state in May 1998 using the latest
background and detector response models. The RXTE and BeppoSAX observations of
NGC 4051 during the low state show a very hard spectrum with a strong
unresolved fluorescence line. This emission, probably due to reflection from a
molecular torus, is likely to be constant over long time-scales and is
therefore assumed as an underlying component at all flux states. By subtracting
the torus component we are able to determine the spectral variability of the
primary continuum. In the variable component we observe a strong
anti-correlation of X-ray flux and spectral hardness in the PCA energy band. We
show that the changes in hardness are caused by slope variability of the
primary power law spectrum rather than by changing reflection or variable
photoelectric absorption. The primary spectral index varies between Gamma=1.6
for the faintest states and Gamma=2.3 during the brightest states, at which
level the spectral index approaches an asympotic value. We find that the
response of the flux of the 6.4 keV iron fluorescence line to changes in the
continuum flux depends on the timescale of the observation. The profile of the
line is very broad and indicates an origin in the innermost regions of the
accretion disk.Comment: accepted for publication in MNRA
Symmetry and reciprocity constraints on diffraction by gratings of quasi-planar particles
Symmetry and reciprocity constraints on polarization state of the field
diffracted by gratings of quasi-planar particles are considered. It is shown
that the optical activity effects observed recently in arrays of quasi-planar
plasmonic particles on a dielectric substrate are due to the reflection of the
field at the air-dielectric slab interface and are proportional to this
reflection coefficient.Comment: 11 pages, 3 figures, 12 references; minor corrections for better
appearanc
Early generation hybrids may drive range expansion of two invasive fishes DataSet
Introgressive hybridization between two invasive species has the potential to contribute to their invasion success and provide genetic resiliency to rapidly adapt to new environments. Additionally, differences in the behaviour of hybrids may lead to deleterious ecosystem effects that compound any negative impacts of the invading parental species. Invasive silver carp (Hypophthalmichthys molitrix) and bighead carp (H. nobilis) exhibit introgressive hybridization which could influence their invasion ecology. In order to investigate the role hybrids may have in the invasion ecology of bigheaded carps, [CAA1] we examined the distribution, movements, and environmental cues for movement of two invasive fishes (bighead carp, silver carp) and their hybrids in the Illinois River (USA). Early generation hybrids (e.g., F1,F2, and first generation backcross individuals) composed a greater proportion of the population at the invasion front where abundances of bigheaded carp were low. A greater proportion of early hybrids passed through dams upstream towards the invasion front than did other hybrids and parental species. The movements and environmental cues for movement of late-generation backcrosses (more genetically similar to parental genotype) were not different from the parental species with which they shared the most alleles. Although the direction of the relationship between movement and environment was sometimes different for the parental species and associated advanced generation hybrids, these results indicate that management for parental species will also influence most hybrids. Although early generation hybrids are rare, our results indicate they may disperse towards low-density population zones (i.e., invasion fronts) or are produced at greater frequency in low density areas. These rare hybrids have the potential to produce a variety of unique genetic combinations which could result in more rapid adaptation of a non-native population to their invaded range potentially facilitating the establishment of invasive species
VHE Gamma Rays from PKS 2155-304
The close X-ray selected BL Lac PKS 2155-304 has been observed using the
University of Durham Mark 6 very high energy (VHE) gamma ray telescope during
1996 September/October/November and 1997 October/November. VHE gamma rays with
energy > 300 GeV were detected from this object with a time-averaged integral
flux of (4.2 +/- 0.7 (stat) +/- 2.0 (sys)) x 10^(-11) per cm2 per s. There is
evidence for VHE gamma ray emission during our observations in 1996 September
and 1997 October/November, with the strongest emission being detected in 1997
November, when the object was producing the largest flux ever recorded in
high-energy X-rays and was detected in > 100 MeV gamma-rays. The VHE and X-ray
fluxes show evidence of a correlation.Comment: 14 pages, 6 figures, accepted for publication in Ap.
Complex X-ray spectral behaviour of NGC 4051 in the low flux state
The Narrow Line Seyfert 1 galaxy NGC 4051 was observed in one of its
prolonged low-lux states by XMM-Newton in November 2002. Here we present the
results of an analysis of EPIC-pn data obtained during the observation. Within
the low state, the source shows complex spectral variability which cannot
easily be explained by any simple model. However, by making a `flux-flux' plot
which combines the low state data with data obtained during a normal flux
state, we demonstrate that the extremely hard spectrum observed above 2 keV
results from a continuation of the spectral variability seen in the normal
state, which is caused by spectral pivoting of the power-law continuum. The
pivoting power-law appears to be attached to a Comptonised thermal component of
variable flux (blackbody temperature kT~0.1 keV, consistent with the small
black hole mass in NGC 4051) which dominates the soft X-ray band in the low
state, and is probably the source of seed photons for Comptonisation.
Additional constant thermal and reflection components, together with absorption
by ionised gas, seem to be required to complete the picture and explain the
complex X-ray spectral variability seen in the low state of NGC 4051.Comment: Accepted for publication in MNRAS. 13 pages, 10 figs. A higher
resolution eps version of Fig. 8 is included in the source file
Evidence for Intergalactic Absorption in the TeV Gamma-Ray Spectrum of Mkn 501
The recent HEGRA observations of the blazar Mkn 501 show strong curvature in
the very high energy gamma-ray spectrum. Applying the gamma-ray opacity derived
from an empirically based model of the intergalactic infrared background
radiation field (IIRF), to these observations, we find that the intrinsic
spectrum of this source is consistent with a power-law: dN/dE~ E^-alpha with
alpha=2.00 +/- 0.03 over the range 500 GeV - 20 TeV. Within current synchrotron
self-Compton scenarios, the fact that the TeV spectral energy distribution of
Mkn 501 does not vary with luminosity, combined with the correlated, spectrally
variable emission in X-rays, as observed by the BeppoSAX and RXTE instruments,
also independently implies that the intrinsic spectrum must be close to
alpha=2. Thus, the observed curvature in the spectrum is most easily understood
as resulting from intergalactic absorption.Comment: 7 pages, 1 figure, accepted in ApJ Letters 1999 April
Relation between millimeter wavelengths emission and high-energy emission for active galactic nuclei
After comparing the flux densities of a sample of active galactic nuclei
detected by energetic gamma-ray experiment telescope at 90 and 230 GHz with the
-ray emissions detected by Compton Gamma Ray Observatory and x-ray
emission, a strong correlation between the emissions at the millimeter
wavelength and the -ray emission is found. The average flux density of
x-ray is almost proportional to the average flux density at the millimeter
wavelength for quasars detected by energetic gamma-ray experiment telescope,
which strongly supports the previous idea that the x-ray emissions of this kind
sources are mainly produced by Synchrotron Self-Compton process.Comment: 6 pages, Chinese Physics Letters in pres
BATSE Observations of the Piccinotti Sample of AGN
BATSE Earth occultation data have been used to search for emission in the
20-100 keV band from all sources in the Piccinotti sample, which represents to
date the only complete 2-10 keV survey of the extragalactic sky down to a
limiting flux of 3.1 x 10^(-11) erg cm^(-2)$ s^(-1). Nearly four years of
observations have been analyzed to reach a 5sigma sensitivity level of about
7.8x 10^(-11) erg cm^(-2) s^(-1) in the band considered. Of the 36 sources in
the sample, 14 have been detected above 5sigma confidence level while marginal
detection (3<sigma<5) can be claimed for 13 sources; for 9 objects 2sigma upper
limits are reported. Comparison of BATSE results with data at higher energies
is used to estimate the robustness of our data analysis: while the detection
level of each source is reliable, the flux measurement maybe overestimated in
some sources by as much as 35%, probably due to incomplete data cleaning.
Comparison of BATSE fluxes with X-ray fluxes, obtained in the 2-10 keV range
and averaged over years, indicates that a canonical power law of photon index
1.7 gives a good description of the broad band spectra of bright AGNs and that
spectral breaks preferentially occur above 100 keV.Comment: 18 pages, 1 figure. Accepted for publication on Apj
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