4,476 research outputs found
Response of VIRGO detectors to pre-big-bang gravitons
The sensitivity achievable by a pair of VIRGO detectors to stochastic and
isotropic gravitational wave backgrounds produced in pre-big-bang models is
discussed in view of the development of a second VIRGO interferometer. We
describe a semi-analytical technique allowing to compute the signal-to-noise
ratio for (monotonic or non-monotonic) logarithmic energy spectra of relic
gravitons of arbitrary slope. We apply our results to the case of two
correlated and coaligned VIRGO detectors and we compute their achievable
sensitivities. We perform our calculations both for the usual case of minimal
string cosmological scenario and in the case of a non-minimal scenario where a
long dilaton dominated phase is present prior to the onset of the ordinary
radiation dominated phase. In this framework, we investigate possible
improvements of the achievable sensitivities by selective reduction of the
thermal contributions (pendulum and pendulum's internal modes) to the noise
power spectra of the detectors. Since a reduction of the shot noise does not
increase significantly the expected sensitivity of a VIRGO pair (in spite of
the relative spatial location of the two detectors) our findings support the
experimental efforts directed towards a substantial reduction of thermal noise.Comment: 23 pages in Latex styl
Sensitivity of a VIRGO pair to stochastic GW backgrounds
The sensitivity of a pair of VIRGO interferometers to gravitational waves
backgrounds (GW) of cosmological origin is analyzed for the cases of maximal
and minimal overlap of the two detectors. The improvements in the detectability
prospects of scale-invariant and non-scale-invariant logarithmic energy spectra
of relic GW are discussed.Comment: 25 pages in RevTex style with 6 figure
An Extended Grid of Nova Models: II. The Parameter Space of Nova Outbursts
This paper is a sequel to an earlier paper devoted to multiple, multicycle
nova evolution models (Prialnik & Kovetz 1995, first paper of the series),
which showed that the different characteristics of nova outbursts can be
reproduced by varying the values of three basic and independent parameters: the
white dwarf mass-M_{WD}, the temperature of its isothermal core-T_{WD} and the
mass transfer rate-Mdot. Apart from being the largest computational classical
novae parameter-space survey, we show here that the parameter space is
constrained by several analytical considerations and find its limiting
surfaces. Consequently, we extend the grid of multicycle nova evolution models
presented in Paper I almost to its limits, adding multicycle nova outburst
calculations for a considerable number of new parameter combinations. In
particular, the extended parameter pace that produces nova eruptions includes
low mass transfer rates down to 5e-13 Msun/yr, and more models for low T_{WD}.
Resulting characteristics of these runs are added to the former parameter
combination results, to provide a full grid spanning the entire parameter space
for Carbon-Oxygen white dwarfs. The full grid covers the entire range of
observed nova characteristics, even those of peculiar objects, which have not
been numerically reproduced until now. Most remarkably, runs for very low Mdot
lead to very high values for some characteristics, such as outburst amplitude
A>~20, high super-Eddington luminosities at maximum, heavy element abundance of
the ejecta Z_{ej}~0.63 and high ejected masses m_{ej}~7e-4 Msun.Comment: Accepted for publication in ApJ - 04/2005. Complete grid results
(Tables 2+3) data available at: http://geophysics.tau.ac.il/personal/oferya/
under "Publications
Sensitivity of wide band detectors to quintessential gravitons
There are no reasons why the energy spectra of the relic gravitons, amplified
by the pumping action of the background geometry, should not increase at high
frequencies. A typical example of this behavior are quintessential inflationary
models where the slopes of the energy spectra can be either blue or mildly
violet. In comparing the predictions of scenarios leading to blue and violet
graviton spectra we face the problem of correctly deriving the sensitivities of
the interferometric detectors. Indeed, the expression of the signal-to-noise
ratio not only depends upon the noise power spectra of the detectors but also
upon the spectral form of the signal and, therefore, one can reasonably expect
that models with different spectral behaviors will produce different
signal-to-noise ratios. By assuming monotonic (blue) spectra of relic gravitons
we will give general expressions for the signal-to-noise ratio in this class of
models. As an example we studied the case of quintessential gravitons. The
minimum achievable sensitivity to of different pairs of
detectors is computed, and compared with the theoretical expectations.Comment: 10 pages in Revtex style, 3 figure
Production and detection of relic gravitons in quintessential inflationary models
A large class of quintessential inflationary models, recently proposed by
Peebles and Vilenkin, leads to post-inflationary phases whose effective
equation of state is stiffer than radiation. The expected gravitational waves
logarithmic energy spectra are tilted towards high frequencies and
characterized by two parameters: the inflationary curvature scale at which the
transition to the stiff phase occurs and the number of (non conformally
coupled) scalar degrees of freedom whose decay into fermions triggers the onset
of a gravitational reheating of the Universe. Depending upon the parameters of
the model and upon the different inflationary dynamics (prior to the onset of
the stiff evolution) the relic gravitons energy density can be much more
sizeable than in standard inflationary models, for frequencies larger than 1
Hz. We estimate the required sensitivity for detection of the predicted
spectral amplitude and show that the allowed region of our parameter space
leads to a signal smaller (by one 1.5 orders of magnitude) than the advanced
LIGO sensitivity at a frequency of 0.1 KHz. The maximal signal, in our context,
is expected in the GHz region where the energy density of relic gravitons in
critical units (i.e. ) is of the order of , roughly
eight orders of magnitude larger than in ordinary inflationary models. Smaller
detectors (not necessarily interferometers) can be relevant for detection
purposes in the GHz frequency window. We suggest/speculate that future
measurements through microwave cavities can offer interesting perspectives.Comment: 24 pages in Revtex style, 7 figure
An Overlapping Protein-Coding Region in Influenza A Virus Segment 3 Modulates the Host Response
Influenza A virus (IAV) infection leads to variable and imperfectly understood pathogenicity. We report that segment 3 of the virus contains a second open reading frame (âX-ORFâ), accessed via ribosomal frameshifting. The frameshift product, termed PA-X, comprises the endonuclease domain of the viral PA protein with a C-terminal domain encoded by the X-ORF and functions to repress cellular gene expression. PA-X also modulates IAV virulence in a mouse infection model, acting to decrease pathogenicity. Loss of PA-X expression leads to changes in the kinetics of the global host response, notably including increases in inflammatory, apoptotic and T-lymphocyte signaling pathways. Thus, we have identified a previously unknown IAV protein that modulates the host response to infection, a finding with important implications for understanding IAV pathogenesis
Magnetic Knots as The origin of Spikes in the Gravitational Waves Backgrounds
The dynamical symmetries of hot and electrically neutral plasmas in a highly
conducting medium suggest that, after the epoch of the electron-positron
annihilation, magnetohydrodynamical configurations carrying a net magnetic
helicity can be present. The simultaneous conservation of the magnetic flux and
helicity implies that the (divergence free) field lines will possess
inhomogeneous knot structures acting as source seeds in the evolution equations
of the scalar, vector and tensor fluctuations of the background geometry. We
give explicit examples of magnetic knot configurations with finite energy and
we compute the induced metric fluctuations. Since magnetic knots are
(conformally) coupled to gravity via the vertex dictated by the equivalence
principle, they can imprint spikes in the gravitational wave spectrum for
frequencies compatible with the typical scale of the knot corresponding, in our
examples, to a present frequency range of -- Hertz. At
lower frequencies the spectrum is power-suppressed and well below the COBE
limit. For smaller length scales (i.e. for larger frequencies) the spectrum is
exponentially suppressed and then irrelevant for the pulsar bounds. Depending
upon the number of knots of the configuration, the typical amplitude of the
gravitational wave logarithmic energy spectrum (in critical units) can be even
four orders of magnitude larger than the usual flat (inflationary) energy
spectrum generated thanks to the parametric amplification of the vacuum
fluctuations.Comment: Accepted for publication in Physical Review D, 20 pages in RevTex
style, 4 Encapsulated figure
Frictional drag between quantum wells mediated by phonon exchange
We use the Kubo formalism to evaluate the contribution of acoustic phonon
exchange to the frictional drag between nearby two-dimensional electron
systems. In the case of free phonons, we find a divergent drag rate
(). However, becomes finite when phonon
scattering from either lattice imperfections or electronic excitations is
accounted for. In the case of GaAs quantum wells, we find that for a phonon
mean free path smaller than a critical value, imperfection
scattering dominates and the drag rate varies as over many
orders of magnitude of the layer separation . When exceeds the
critical value, the drag rate is dominated by coupling through an
electron-phonon collective mode localized in the vicinity of the electron
layers. We argue that the coupled electron-phonon mode may be observable for
realistic parameters. Our theory is in good agreement with experimental results
for the temperature, density, and -dependence of the drag rate.Comment: 45 pages, LaTeX, 8 postscript file figure
Description of the Scenario Machine
We present here an updated description of the "Scenario Machine" code. This
tool is used to carry out a population synthesis of binary stars. Previous
version of the description can be found at
http://xray.sai.msu.ru/~mystery//articles/review/contents.htmlComment: 32 pages, 3 figures. Corrected typo
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