1,085 research outputs found
Modulational instability of solitary waves in non-degenerate three-wave mixing: The role of phase symmetries
We show how the analytical approach of Zakharov and Rubenchik [Sov. Phys.
JETP {\bf 38}, 494 (1974)] to modulational instability (MI) of solitary waves
in the nonlinear Schr\"oedinger equation (NLS) can be generalised for models
with two phase symmetries. MI of three-wave parametric spatial solitons due to
group velocity dispersion (GVD) is investigated as a typical example of such
models. We reveal a new branch of neck instability, which dominates the usual
snake type MI found for normal GVD. The resultant nonlinear evolution is
thereby qualitatively different from cases with only a single phase symmetry.Comment: 4 pages with figure
Broadband X-ray spectral analysis of the Seyfert 1 galaxy GRS 1734-292
We discuss the broadband X-ray spectrum of GRS 1734-292 obtained from
non-simultaneous XMM-Newton and NuSTAR observations, performed in 2009 and
2014, respectively. GRS1734-292 is a Seyfert 1 galaxy, located near the
Galactic plane at . The NuSTAR spectrum ( keV) is dominated by
a primary power-law continuum with and a high-energy
cutoff keV, one of the lowest measured by NuSTAR in a
Seyfert galaxy. Comptonization models show a temperature of the coronal plasma
of keV and an optical depth, assuming a slab
geometry, or a similar temperature and
assuming a spherical geometry. The 2009 XMM-Newton
spectrum is well described by a flatter intrinsic continuum
() and one absorption line due to Fe\textsc{XXV}
K produced by a warm absorber. Both data sets show a modest iron
K emission line at keV and the associated Compton reflection, due
to reprocessing from neutral circumnuclear material
Studying the accretion geometry of EXO 2030+375 at luminosities close to the propeller regime
The Be X-ray binary EXO 2030+375 was in an extended low luminosity state
during most of 2016. We observed this state with NuSTAR and Swift, supported by
INTEGRAL observations as well as optical spectroscopy with the NOT. We present
a comprehensive spectral and timing analysis of these data here to study the
accretion geometry and investigate a possible onset of the propeller effect.
The H-alpha data show that the circumstellar disk of the Be-star is still
present. We measure equivalent widths similar to values found during more
active phases in the past, indicating that the low-luminosity state is not
simply triggered by a smaller Be disk. The NuSTAR data, taken at a 3-78 keV
luminosity of ~6.8e35 erg/s (for a distance of 7.1 kpc), are well described by
standard accreting pulsar models, such as an absorbed power-law with a
high-energy cutoff. We find that pulsations are still clearly visible at these
luminosities, indicating that accretion is continuing despite the very low mass
transfer rate. In phase-resolved spectroscopy we find a peculiar variation of
the photon index from ~1.5 to ~2.5 over only about 3% of the rotational period.
This variation is similar to that observed with XMM-Newton at much higher
luminosities. It may be connected to the accretion column passing through our
line of sight. With Swift/XRT we observe luminosities as low as 1e34 erg/s
during which the data quality did not allow us to search for pulsations, but
the spectrum is much softer and well described by either a blackbody or soft
power-law continuum. This softer spectrum might be due to the fact that
accretion has been stopped by the propeller effect and we only observe the
neutron star surface cooling.Comment: 11 pages, 6 figures, accepted for publication in A&A (v2 including
language edits
Pirellulosomes: a new type of membrane-bounded cell compartment in planctomycete bacteria of the genus Pirellula
A distinct type of cellular organization was found in two species of the planctomycete genus Pirellula, Pirellula marina and Pirellula staleyi. Both species possess two distinct regions within the cell which are separated by a single membrane. The major region of the cell, the pirellulosome, contains the fibrillar condensed nucleoid. The other area, the polar cap region, forms a continuous layer surrounding the entire pirellulosome and displays a cap of asymmetrically distributed material at one cell pole. Immuno- and cytochemical-labelling of P. marina demonstrated that DNA is located exclusively within the pirellulosome; cell RNA is concentrated in the pirellulosome, with some RNA also located in the polar cap region
Statistics of selectively neutral genetic variation
Random models of evolution are instrumental in extracting rates of
microscopic evolutionary mechanisms from empirical observations on genetic
variation in genome sequences. In this context it is necessary to know the
statistical properties of empirical observables (such as the local homozygosity
for instance). Previous work relies on numerical results or assumes Gaussian
approximations for the corresponding distributions. In this paper we give an
analytical derivation of the statistical properties of the local homozygosity
and other empirical observables assuming selective neutrality. We find that
such distributions can be very non-Gaussian.Comment: 4 pages, 4 figure
NuSTAR and Suzaku X-ray Spectroscopy of NGC 4151: Evidence for Reflection from the Inner Accretion Disk
We present X-ray timing and spectral analyses of simultaneous 150 ks Nuclear
Spectroscopic Telescope Array (NuSTAR) and Suzaku X-ray observations of the
Seyfert 1.5 galaxy NGC 4151. We disentangle the continuum emission, absorption,
and reflection properties of the active galactic nucleus (AGN) by applying
inner accretion disk reflection and absorption-dominated models. With a
time-averaged spectral analysis, we find strong evidence for relativistic
reflection from the inner accretion disk. We find that relativistic emission
arises from a highly ionized inner accretion disk with a steep emissivity
profile, which suggests an intense, compact illuminating source. We find a
preliminary, near-maximal black hole spin a>0.9 accounting for statistical and
systematic modeling errors. We find a relatively moderate reflection fraction
with respect to predictions for the lamp post geometry, in which the
illuminating corona is modeled as a point source. Through a time-resolved
spectral analysis, we find that modest coronal and inner disk reflection flux
variation drives the spectral variability during the observations. We discuss
various physical scenarios for the inner disk reflection model, and we find
that a compact corona is consistent with the observed features.Comment: 20 pages, 12 figures, accepted for publication in Ap
Impact of sensory differences on consumer acceptability of yoghurt and yoghurt-like products
8 pages, 5 tables, 6 figures.-- Available online 7 October 2010.The aim of this work was to obtain information about how perceptible sensory differences affect consumer acceptability for yoghurt and a yoghurt-like product. Descriptive sensory profiles of six commercial samples, three of plain yoghurt and three of plain fermented milk, were determined using a trained panel (n = 10). Sample acceptance was determined by a group of consumers (n = 120). Initially, two groups of consumers were identified using Cluster analyses. For one group 46 (38%) of the consumers, variability in sensory attributes did not affect sample acceptability. For the second group, of 74 (62%) of the consumers, variability in sensory attributes had a significant effect and three consumer subgroups with different preference criteria were detected. Partial least squares regression was used to determine the sensory factors driving liking/disliking for each consumer subgroup. The information obtained can be important in predicting or explaining the market response to these types of products.MICINN of Spain for financial support (Project AGL 2007-63444) and Fondo Social Europeo for financing the contract of author S. Bayarri in the program I3P from CSIC.Peer reviewe
NuSTAR observations of the powerful radio-galaxy Cygnus A
We present NuSTAR observations of the powerful radio galaxy Cygnus A,
focusing on the central absorbed active galactic nucleus (AGN). Cygnus A is
embedded in a cool-core galaxy cluster, and hence we also examine archival
XMM-Newton data to facilitate the decomposition of the spectrum into the AGN
and intracluster medium (ICM) components. NuSTAR gives a source-dominated
spectrum of the AGN out to >70keV. In gross terms, the NuSTAR spectrum of the
AGN has the form of a power law (Gamma~1.6-1.7) absorbed by a neutral column
density of N_H~1.6x10^23 cm^-2. However, we also detect curvature in the hard
(>10keV) spectrum resulting from reflection by Compton-thick matter out of our
line-of-sight to the X-ray source. Compton reflection, possibly from the outer
accretion disk or obscuring torus, is required even permitting a high-energy
cutoff in the continuum source; the limit on the cutoff energy is E_cut>111keV
(90% confidence). Interestingly, the absorbed power-law plus reflection model
leaves residuals suggesting the absorption/emission from a fast
(15,000-26,000km/s), high column-density (N_W>3x10^23 cm^-2), highly ionized
(xi~2,500 erg cm/s) wind. A second, even faster ionized wind component is also
suggested by these data. We show that the ionized wind likely carries a
significant mass and momentum flux, and may carry sufficient kinetic energy to
exercise feedback on the host galaxy. If confirmed, the simultaneous presence
of a strong wind and powerful jets in Cygnus A demonstrates that feedback from
radio-jets and sub-relativistic winds are not mutually exclusive phases of AGN
activity but can occur simultaneously.Comment: 13 pages; accepted for publication in The Astrophysical Journa
Modulational instability in periodic quadratic nonlinear materials
We investigate the modulational instability of plane waves in quadratic
nonlinear materials with linear and nonlinear quasi-phase-matching gratings.
Exact Floquet calculations, confirmed by numerical simulations, show that the
periodicity can drastically alter the gain spectrum but never completely
removes the instability. The low-frequency part of the gain spectrum is
accurately predicted by an averaged theory and disappears for certain gratings.
The high-frequency part is related to the inherent gain of the homogeneous
non-phase-matched material and is a consistent spectral feature.Comment: 4 pages, 7 figures corrected minor misprint
Geographic distribution and genetic diversity of the Ehrlichia sp. from Panola Mountain in Amblyomma americanum
Background:
A novel Ehrlichia, closely related to Ehrlichia ruminantium, was recently discovered from Panola Mountain State Park, GA, USA. We conducted a study to determine if this agent was recently introduced into the United States.
Methods:
We developed a sensitive PCR assay based on the conserved gltA (citrate synthase) gene and tested DNA samples extracted from 1964 field-collected and 1835 human-biting Amblyomma americanum from 23 eastern states of the USA.
Results:
The novel agent was detected in 36 ticks collected from 10 states between 1998 and 2006. Infected ticks were collected both from vegetation (n = 14, 0.7%) and from humans (n = 22, 1.2%). Fragments of the conserved gltA gene and the variable map1 gene were sequenced from positive samples. Two distinct clades, with 10.5% nucleic acid divergence over the 730 bp map1 sequence, were identified.
Conclusion:
These data suggest that the Panola Mountain Ehrlichia was not recently introduced to the United States; this agent has an extensive distribution throughout the range of its tick vector, has been present in some locations for several years, and displays genetic variability. Furthermore, people in several states were exposed to this agent through the bite of infected ticks, underscoring the potential public health risk of this emerging ehrlichiosis
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