401 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
The nature and origin of Seyfert warm absorbers
We collate the results of recent high resolution X-ray spectroscopic
observations of 23 AGN, and use the resulting information to try to provide
answers to some of the main open questions about warm absorbers: where do they
originate, what effect do they have on their host galaxies, and what is their
importance within the energetics and dynamics of the AGN system as a whole? We
find that the warm absorbers of nearby Seyferts and certain QSOs are most
likely to originate in outflows from the dusty torus, and that the kinetic
luminosity of these outflows accounts for well under 1% of the bolometric
luminosities of the AGN. Our analysis supports, however, the view that the
relativistic outflows recently observed in two PG quasars have their origin in
accretion disc winds, although the energetic importance of these outflows is
similar to that of the Seyfert warm absorbers. We find that the observed soft
X-ray absorbing ionisation phases fill less than 10% of the available volume.
Finally, we show that the amount of matter processed through an AGN outflow
system, over the lifetime of the AGN, is probably large enough to have a
significant influence on the evolution of the host galaxy and of the AGN
itself.Comment: v2: 15 pages, 9 figures, accepted for publication in A&A, minor typos
correcte
A Hard X-Ray Study of Ultraluminous X-ray Source NGC 5204 X-1 with NuSTAR and XMM-Newton
We present the results from coordinated X-ray observations of the
ultraluminous X-ray source NGC 5204 X-1 performed by NuSTAR and XMM-Newton in
early 2013. These observations provide the first detection of NGC 5204 X-1
above 10 keV, extending the broadband coverage to 0.3-20 keV. The observations
were carried out in two epochs separated by approximately 10 days, and showed
little spectral variation, with an observed luminosity of Lx = (4.95+/-0.11)e39
erg/s. The broadband spectrum confirms the presence of a clear spectral
downturn above 10 keV seen in some previous observations. This cutoff is
inconsistent with the standard low/hard state seen in Galactic black hole
binaries, as would be expected from an intermediate mass black hole accreting
at significantly sub-Eddington rates given the observed luminosity. The
continuum is apparently dominated by two optically thick thermal-like
components, potentially accompanied by a faint high energy tail. The broadband
spectrum is likely associated with an accretion disk that differs from a
standard Shakura & Sunyaev thin disk.Comment: 7 pages, 5 figures. Accepted for publication in Ap
The complex accretion geometry of GX 339-4 as seen by NuSTAR and Swift
We present spectral analysis of five NuSTAR and Swift observations of GX
339-4 taken during a failed outburst in summer 2013. These observations cover
Eddington luminosity fractions in the range ~0.9-6%. Throughout this outburst,
GX 339-4 stayed in the hard state, and all five observations show similar X-ray
spectra with a hard power-law with a photon index near 1.6 and significant
contribution from reflection. Using simple reflection models we find
unrealistically high iron abundances. Allowing for different photon indices for
the continuum incident on the reflector relative to the underlying observed
continuum results in a statistically better fit and reduced iron abundances.
With a photon index around 1.3, the input power-law on the reflector is
significantly harder than that which is directly observed. We study the
influence of different emissivity profiles and geometries and consistently find
an improvement when using separate photon indices. The inferred inner accretion
disk radius is strongly model dependent, but we do not find evidence for a
truncation radius larger than 100 r_g in any model. The data do not allow
independent spin constraints but the results are consistent with the literature
(i.e., a>0). Our best-fit models indicate an inclination angle in the range
40-60 degrees, consistent with limits on the orbital inclination but higher
than reported in the literature using standard reflection models. The iron line
around 6.4 keV is clearly broadened, and we detect a superimposed narrow core
as well. This core originates from a fluorescence region outside the influence
of the strong gravity of the black hole and we discuss possible geometries.Comment: 11 pages, 6 figures, 6 tables, plus 9 tables in the appendix.
Submitted to Ap
NuSTAR and Suzaku observations of the hard state in Cygnus X-1: locating the inner accretion disk
We present simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR ) and
Suzaku observations of the X-ray binary Cygnus X-1 in the hard state. This is
the first time this state has been observed in Cyg X-1 with NuSTAR, which
enables us to study the reflection and broad-band spectra in unprecedented
detail. We confirm that the iron line cannot be fit with a combination of
narrow lines and absorption features, and instead requires a relativistically
blurred profile in combination with a narrow line and absorption from the
companion wind. We use the reflection models of Garcia et al. (2014) to
simultaneously measure the black hole spin, disk inner radius, and coronal
height in a self-consistent manner. Detailed fits to the iron line profile
indicate a high level of relativistic blurring, indicative of reflection from
the inner accretion disk. We find a high spin, a small inner disk radius, and a
low source height, and rule out truncation to greater than three gravitational
radii at the 3{\sigma} confidence level. In addition, we find that the line
profile has not changed greatly in the switch from soft to hard states, and
that the differences are consistent with changes in the underlying reflection
spectrum rather than the relativistic blurring. We find that the blurring
parameters are consistent when fitting either just the iron line or the entire
broad-band spectrum, which is well modelled with a Comptonized continuum plus
reflection model.Comment: 12 pages, 7 figures, accepted for publication in Ap
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
Frequency and distribution of rare electrophoretic mobility variants in a population of human newborns in Ann Arbor, Michigan
We have summarized the frequency and distribution of the rare variants encountered during the screening of 258 815 allele products, the products of 51 different loci, in 3242 predominantly Caucasian (88 %) newborns. Seventy-nine different rare variants, representing 187 occurrences, were identified. Almost 60 % (46 of 79) of the rare variants occurred as singletons while another 20 % were seen in two unrelated individuals. No rare variants were detected at 18 loci while no variants, either rare or polymorphic, were detected at 14 loci. More rare variants were identified at loci that were classified as polymorphic and also at loci where the gene products exist as a monomer. A positive relationship was observed between variant frequency, either classes or copies, and subunit molecular mass.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65173/1/j.1469-1809.1987.tb01065.x.pd
Radio Frequency Spectra of 388 Bright 74 MHz Sources
As a service to the community, we have compiled radio frequency spectra from
the literature for all sources within the VLA Low Frequency Sky Survey (VLSS)
that are brighter than 15 Jy at 74 MHz. Over 160 references were used to
maximize the amount of spectral data used in the compilation of the spectra,
while also taking care to determine the corrections needed to put the flux
densities from all reference on the same absolute flux density scale. With the
new VLSS data, we are able to vastly improve upon previous efforts to compile
spectra of bright radio sources to frequencies below 100 MHz because (1) the
VLSS flux densities are more reliable than those from some previous low
frequency surveys and (2) the VLSS covers a much larger area of the sky
(declination >-30 deg.) than many other low frequency surveys (e.g., the 8C
survey). In this paper, we discuss how the spectra were constructed and how
parameters quantifying the shapes of the spectra were derived. Both the spectra
and the shape parameters are made available here to assist in the calibration
of observations made with current and future low frequency radio facilities.Comment: Accepted to ApJ
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
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