High Frequency Cutoff Research Articles

Relic gravitational waves from cosmic strings: Updated constraints and opportunities for detection.

We examine the spectrum of gravitational radiation emitted by a network of cosmic strings, with emphasis on the observational constraints and the opportunities for detection. The analysis improves over past work, as we use a phenomenological model for the radiation spectrum emitted by a cosmic string loop. This model attempts to include the effect of the gravitational back reaction on the radiation emission by an individual loop with a high frequency cutoff in the spectrum. Comparison of the total spectrum due to a network of strings with the recently improved bound on the amplitude of a stochastic gravitational wave background, due to measurements of noise in pulsar signal arrival times, allows us to exclude a range of values of $\ensuremath{\mu}$, the cosmic string linear mass density, for certain values of cosmic string and cosmological parameters. We find the conservative bound $\frac{G\ensuremath{\mu}}{{c}^{2}}l5.4(\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ which is consistent with all other limits. We consider variations of the standard cosmological scenario, finding that an underdense, ${\ensuremath{\Omega}}_{0}l1$ universe has little effect on the spectrum, whereas the portion of the spectrum probed by gravitational wave detectors is strongly sensitive to the thermal history of the cosmological fluid. We discuss the opportunity for the observation of this stochastic background by resonant mass and laser interferometer gravitational wave detectors.

Exact calculation of the energy density of cosmological gravitational waves.

In this paper we calculate the Bogoliubov coefficients and the energy density of the stochastic gravitational wave background for a universe that undergoes inflation followed by radiation domination and matter domination, using a formalism that gives the Bogoliubov coefficients as continous functions of time. By making a reasonable assumption for the equation of state during reheating, we obtain in a natural way the expected high frequency cutoff in the spectral energy density.

Parameters of EEG dimensional complexity in Alzheimer's disease

The dimensional complexity or fractal dimension of the EEG recorded at 17 locations from the 10–20 system was studied using a very fast new algorithm. The paper contains 2 parts. First, the influence of several parameters (embedding dimension, delay time, high and low frequency cutoff, ADC precision and sweep length) on the estimation of dimensional complexity is investigated. Then it is shown that patients with Alzheimer's disease (n = 50) have significantly lower dimensional complexity than age-approximated non-demented controls (n = 42).

Individual differences in contrast sensitivity functions: Longitudinal study of 4-, 6- and 8-month-old human infants

Contrast sensitivity functions (CSFs) of 25 infants were measured longitudinally at 4, 6 and 8 months of age using a preferential-looking method and the method of constant stimuli. Sine-wave gratings varied from 0.27 to 4.32 c/deg, contained eight unattenuated cycles (with edges tapered to uniform gray), and rose to the desired contrast in 2 sec. (1) The average CSF was described on log-log coordinates by a band-pass function. With development it increased in overall sensitivity to contrast, shifted its peak toward slightly higher spatial frequencies, and increased its high frequency cutoff. (2) Log sensitivity at the CSF peak was slightly higher for female than male infants at 6 months, consistent with the hypothesis that vernier acuity (which also may differ between the sexes at this age) is partly mediated by analyzers tuned to low frequencies. (3) Within age groups the individual differences were such that log sensitivities for neighboring spatial frequencies generally correlated more highly than distant frequencies. With development the correlations among distant frequencies below 1.0 c/deg increased. Monte Carlo simulations of a model that shifts spatial analyzers to higher frequencies with age reproduced these results but simulations of adultlike, unshifting analyzers did not. (4) Measures taken 2 months apart tended to correlate more highly than those taken 4 months apart, though some individual differences in the CSF peak remained stable over 4 months.

Propagation and damping of broadband upstream whistlers

Previous studies indicated that damping rates of upstream whistlers strongly depend on the details of the electron distribution function. Moreover, detailed analysis of Doppler-shift and whistler dispersion relation indicated that upstream whistlers propagate obliquely in a broad band. In this paper we present results of a kinetic calculation of damping lengths of wide-band whistlers using the sum of 7-drifting bi-Maxwellian electron distributions as a best fit to the ISEE 1 electron data. For 2 cases, when upstream whistlers are observed, convective damping lengths derived from ISEE magnetic field and ephemeris data are compared with theoretical results. We find that the calculated convective damping lengths are consistent with the data and that upstream whistlers remain marginally stable. We also show that the slope of plasma frame spectra of upstraem whistlers, obtained by direct fitting of the observed spectra is between 5 and 7 with a sharp lower frequency cutoff corresponding to a wavelength of about one ion inertial length. When the solar wind velocity is directed largely along the wave normal of the upstream whistlers the polarization of the right hand waves becomes reversed and low frequencies are switched to high resulting in a peaked spectrum with a strong high frequency cutoff. The overall spectral, wave and particle characteristics, proximity to the shock as well as propagation and damping properties indicate that these waves cannot be generated locally. Instead the observed upstream whistlers arise in the shock ramp most likely by a variety of cross-field drift and/or anisotropy driven instabilities.

Shapes of cat auditory nerve fiber tuning curves

Tuning curves of auditory nerve fibers in normal-hearing cats were fitted by a computational model comprising four processes. One process accounts for sensitivity in tuning curve tails and consists of an approximation to bandpass filtering by extracochlear structures. The second and third processes describe passive and active components of basilar membrane (BM) mechanics, respectively. The former consists of a lowpass filter function, which provides baseline threshold sensitivity and filtering above characteristic frequency (CF), and the latter consists of a Gaussian that accounts for sharp tuning and high sensitivity around CF. A fourth process, modeled as a high-pass filter, was needed in many fits to account for breaks and plateaus in threshold sensitivity at frequencies above CF. The latter three processes operated on cochlear spatial coordinates rather than stimulus frequency. The four-process description closely accounted for shapes of most tuning curves. Tuning curve tails possessed minima at 40–80 dB SPL, and minima increased with fiber CF. High-frequency cutoffs of tail filters tended to increase with CF, but low-frequency cutoffs were generally constant across CF. Functions describing tails varied from ear to ear but behaved in a similar manner for fibers from a single ear. Passive components of BM resonances possessed baselines with sensitivities that decreased with CF and cutoff slopes that increased with CF. The magnitude of the active component increased smoothly with CF over an 80 + dB range, and its spatial extent was essentially constant at 1.5 mm or 6% of cochlear length regardless of gain magnitude, fiber CF, or threshold sensitivity. Tuning curves from fibers with high and medium spontaneous rates (SRs) and similar CFs had nearly identical shapes, with the sole difference being essentially constant differences in sensitivity across the entire excitatory frequency range. Tuning curve shapes from fibers with low SRs were more variable. These could either resemble those obtained from similarly-tuned fibers with higher SRs, or they could exhibit lower tip-to-tail ratios and reduced active component magnitudes. The latter were typically associated with low maximum discharge rates.

Transfer characteristics of lateral geniculate nucleus X-neurons in the cat: effects of temporal frequency.

The dependency of intrageniculate signal transfer on stimulus temporal frequency was investigated by comparing responses of individual X-relay cells with their direct retinal inputs in anesthetized and paralyzed cats. Temporal frequency response functions of lateral geniculate nucleus (LGN) X-cells were more narrowly tuned than those of their retinal inputs. The efficiency of signal transfer was consistently highest at or around the geniculate cells' optimal temporal frequency, and the degree of signal transfer, which was more closely related to the LGN cells' firing rate than to the firing rate of their retinal input, decreased for both lower and higher temporal frequencies. The high temporal frequency cut-offs were significantly lower in geniculate cell responses than those of their direct retinal inputs. This reduction in temporal resolution was exaggerated for relatively low stimulus spatial frequencies. The present results provide clear evidence for the notion that LGN cells function as nonlinear temporal filters and that this stimulus-dependent signal transmission appears to be regulated by complex local mechanisms.

Casimir Force on a Dielectric Cylinder

The Casimir surface force density on a compact material cylinder of radius a is calculated, at zero temperature. A Green function approach is followed. The general theory is formulated so as to hold for arbitrary permittivities ϵ(ω) and permeabilities μ(ω), whereas when it comes to explicit calculations the condition ϵ(ω) μ(ω) = 1 is assumed to hold. A simple dispersion relation is chosen, implying a high frequency cutoff ω 0. The theory yet diverges, at high angular momenta. Divergences of this sort usually appear whenever there are curved boundaries present. On physical grounds an angular momentum cutoff m 0 can be introduced, being of order ω 0 a. A semi-quantitative calculation of the force thereby becomes possible. The calculated force is attractive.

On the interpretation of the correlated optical and radio outbursts in AO0235+164

A detailed analysis of the spectral properties of the correlated optical and radio outbursts occurred in the BL Lac object AO02235+164 is presented, taking the outburst in Oct.–Nov. 1975 as an example. The observed features can be interpreted reasonably in the frame of relativistic shock-jet models; (1) The background jet spectrum is determined and its structure is analysed on the basis of Königl's jet model. The distributions of electron density and magnetic field along the jet and other parameters are discussed. (2) With the background jet spectrum subtracted, the spectrum of the outburst is determined. It is shown that it can be represented by a uniform synchrotron radiation source with the optically thin spectral index α ∼ 0.25, the break frequency v b 3 × 10 12Hz and the high frequency cutoff v ̃ h ∼ 2 × 10 14 Hz . (3) The adiabatic acceleration of particles and amplification of magnetic field, and the parameters of the emitting regions (i.e. the post-shock regions) are discussed. (4) The simultaneity of the optical and radio outbursts is considered to be due to the optical thinness of the emitting region. The optically thin spectral index in radio band ( α ∼ 0.25) is smaller than that of the background jet spectrum ( α s2 ≅ 0.5) at similar frequencies (∼ 10 12 Hz >), and this can be used to explain the much larger amplitude of variations in optical band. (5) It is suggested that the difference between the spike profile of optical outburst and the broad profile of radio outburst (as an example, for the correlated optical radio outburst in 1979) could be explained by shock propagating through irregularities of energetic electrons and magnetic field along the jet, that is, the scale of the optical emission region along the jet might be much smaller than that of the radio emission region. In summary, the results given here show that the correlated optical-radio outbursts in AO0235+164 may be intrinsic to the source, not due to gravitational micro-lensing effects.

On Nature of the High Frequency Cutoff in Pulsar Radio Spectra

AbstractUsing values of the frequencies vc for the high-frequency cutoffs of 31 pulsars obtained by other authors, we confirmed the relationship (obtained by us earlier) between vc and the period of pulsar P. The plot vc(χ) was drawn where χ is the relative angular distance between the line of sight and the center of the radiation cone. It is shown that pulsars with long periods cluster in the lower left corner of this diagram. Some geometrical and physical reasons causing high-frequency cutoffs are analyzed. The results can be explained by a model of thin plasma sheets. The interaction of these sheets causes generation of Langmuir waves at distances ~108 cm from the center of the neutron star. Pulsars without high-frequency cutoffs can be described by a model in which vc is very near the frequency of maximum in the spectra of these pulsars.

The Connection of Spectra with Other Pulsar Parameters

AbstractAn analysis has been made of the connection of characteristic frequencies and spectral indices with the following parameters of pulsars: P, Ṗ, T, L400, B, and χ. It is shown that a number of pulsar spectra apparently demonstrate the presence of two emission regions at a short distance between levels in the magnetosphere of pulsars, where the effects of the low-frequency turnover (LFT) and the high frequency cutoff (HFC) take place.

Fibers innervating different parts of the lateral line system of an Antarctic notothenioid, Trematomus bernacchii, have similar frequency responses, despite large variation in the peripheral morphology.

Regional differences in the architecture and size of lateral line canals and neuromasts were measured in an Antarctic fish, Trematomus bernacchii, and the data were used in models of canal and cupular mechanics to predict the frequency response of these two peripheral structures. These modeled predictions were then compared to frequency response functions measured with single unit recording techniques from anterior and posterior lateral line fibers innervating different canals on the head and trunk of fish of various sizes. Despite large variations in the peripheral morphology of head and trunk canals in fish of different sizes, lateral line fibers were relatively homogeneous in their frequency response properties. In response to stimuli of equal pk-pk acceleration levels, all canal neuromast fibers responded with equal and maximum responsiveness in the 10-45 Hz range, after which responsiveness fell off at about 18 dB/octave. Whereas the biomechanical models of cupular and canal responsiveness predicted the region of equal and maximum responsiveness in the 10-45 Hz range, they did not predict the high frequency cutoff nor the slope. Rather, these models predicted responsiveness out to at least 540 Hz, and a high frequency slope of 12 dB/octave. In terms of the frequency response of peripheral fibers, we conclude that (1) there can be considerable morphological variability, with little consequence for function, as long as some minimum standards for maintaining constant acceleration responsiveness in the 10-45 Hz range are met, and (2) there must be additional filters between the cupula and primary afferent fibers.

Some comments on the correlation dimension of 1/f α noise

It has recently been observed by Osborne and Provenzale that a stochastic (infinite degree of freedom) time series with a 1/f α power spectrum can exhibit a finite correlation dimension, even for arbitrarily large data sets. I will discuss the relevance of this observation to the practical estimation of dimension from a time series, and in particular I will argue that a good dimension algorithm need not be trapped by this anomalous fractal scaling. Further, I will analytically treat the case of Gausian 1/f α noise, with explicit high and low frequency cutoffs, and derive the scaling of the correlation integral C( N, r) in various regimes of the ( N, r) plane.

A computational model of spatiochromatic image coding in early vision

We applied decorrelation and redundancy reduction methods to image signals in space and color to derive an image coding operation that simultaneously reduces correlation in both these dimensions. This method, which we apply to the “red” and “green” planes of a color image, is based on an eigenvector (Karhunen-Loeve) transformation in color in conjunction with linear predictive coding in space. The resulting image transformation has a distinct major component associated with an achromatic response, and a second component associated with a chromatic response. The achromatic component has a high spatial frequency bandpass form and contains a large portion of the signal energy. The chromatic component is spatially lowpass and contains chromatic differences which represent only a small portion of the signal energy. A color image is thereby transformed into two distinct components that contain primarily spatial information and primarily color information, respectively. These image components correlate with a high spatial frequency cutoff luminance mechanism and low spatial frequency cutoff chromatic mechanism in the visual system and can be implemented by a simple combination of retinal color-opponent receptive fields.

Binocularity in the Little Owl, <i>Athene noctua</i>

Visually evoked potentials (VEPs) have been recorded from the Wulst surface of the little owl, Athene noctua, in response to counterphase-reversal of sinusoidal gratings with different contrast, spatial frequency and mean luminance, presented either monocularly or binocularly. Monocular full-field stimuli presented to either eye evoked VEPs of similar amplitude, waveform and latency. Under binocular viewing, VEPs approximately doubled in amplitude without waveform changes. VEPs with similar characteristics could be obtained in response to stimulation of the contralateral, but not ipsilateral, hemifield. These results suggest that a 50% recrossing occurs in thalamic efferents and that different ipsilateral and contralateral regions converge onto the same Wulst sites. The VEP amplitude progressively decreased with increase of the spatial frequency beyond 2 cycles/degree, and the high spatial frequency cut-off (VEP acuity) was under binocular viewing (8 cycles/degree) higher than under monocular (5 cycles/degree) viewing (200 cd/m2, 45% contrast). The VEP acuity increased with increase in the contrast and decreased with reduction of the mean luminance. The binocular gain in both VEP amplitude and VEP acuity was largest at the lowest luminance levels. Binocular VEP summation occurred in the medium-high contrast range. With decreased contrast, both monocular and binocular VEPs progressively decreased in amplitude and tended to the same contrast threshold. The VEP contrast threshold depended on the spatial frequency (0.6-1.8% in the range 0.12-2 cycles/degree). Binocular VEPs often showed facilitatory interaction (binocular/monocular amplitude ratio greater than 2), but the binocular VEP amplitude did not change either by changing the stimulus orientation (horizontal vs. vertical gratings) or by inducing different retinal disparities.(ABSTRACT TRUNCATED AT 250 WORDS)

Atomic motions and high frequency cutoff in biological macromolecules

We point out that the structure of a class of biological macromolecules permits a reduction of the complete atomic dynamical description into motions of subunits consisting of mutually rigidly bound atomic subgroups with relatively weak, flexible joints connecting them. As an example performing an analysis of the DNA normal mode vibrations, we find a distinct bifurcation of the phonon frequency spectrum into two regions. Below a sharp cutoff at about 380 cm −1 the vibrations consist nearly entirely of simple translations and rotations of the subgroups, and are thus potentially capable of metabolic involvement. Above the cutoff, a subunit as a whole is essentially stationary, showing internal vibration only.

An explanation of abrupt infrared cutoffs of extragalactic radio sources by shock accelerated particles including self-generated turbulence

view Abstract Citations (12) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An Explanation of Abrupt Infrared Cutoffs of Extragalactic Radio Sources by Shock-accelerated Particles Including Self-generated Turbulence Fritz, Klaus-Dieter Abstract High-frequency cutoffs in the infrared synchrotron spectra of several extragalactic radio sources (including hot spots) require relativistic particle distributions with power-law behavior for low momenta and a nearly ideal sharp cutoff at a certain high momentum. The recent photometry of some hotspots provides an ideal opportunity to test theoretical models. So far, model spectra obtained by applying a first-order Fermi model and using a constant or variable spatial diffusion coefficient failed to match the steep cutoffs although they reproduce other features of the observed spectra. However, taking into account the self-generated turbulence by the relativistic particles on the upstream side of the shock completely changes the spectral behavior near the cutoff and leads to extraordinarily good fits to observed data. It turns out that the upstream side plays the major role in deteriming the shape of the cutoff. Applying a simple plane and parallel geometry of magnetic field and plasma velocity, the transport equation is solved numerically and the results are presented depending on one parameter, the background level of turbulence. Some important physical parameters can be derived and expressed in terms of observable quantities. Publication: The Astrophysical Journal Pub Date: December 1989 DOI: 10.1086/168161 Bibcode: 1989ApJ...347..692F Keywords: Extragalactic Radio Sources; Infrared Spectra; Relativistic Particles; Shock Waves; Synchrotron Radiation; Turbulence Effects; Astronomical Models; Diffusion Coefficient; Particle Acceleration; Transport Theory; Wave Generation; Astrophysics; COSMIC RAYS: GENERAL; GALAXIES: NUCLEI; RADIATION MECHANISMS; TURBULENCE full text sources ADS | data products SIMBAD (2) NED (2)

Evoked responses to sinusoidal gratings in the pigeon optic tectum

Tectal evoked potentials (TEPs) in response to sinusoidal gratings of different contrast, spatial and temporal frequency have been recorded from the tectal surface of the pigeon. Responses have been digitally filtered in order to isolate transient oscillatory (fast) potentials (50-150 Hz), transient slow potentials (1-50 Hz), and the steady-state second-harmonic component (16.6 Hz). Transient slow potentials, as well as the steady-state second-harmonic component, are band-pass spatially tuned with a maximum at 0.5 cycles/deg and attenuation at higher and lower spatial frequencies. The high spatial frequency cutoff is 4-5 cycles/deg. Both transient slow potentials and the steady-state second-harmonic component increase in amplitude as a function of log contrast and saturate at about 20% contrast. The contrast sensitivity, as determined by extrapolating TEP amplitude to 0 V is 0.1-0.2%. These characteristics of spatial-frequency selectivity and contrast sensitivity are similar to those reported for single tectal cells. Unlike slow potentials, oscillatory potentials are not band-pass spatially tuned. In addition, their contrast response function does not saturate at moderate contrast. These differences suggest that tectal evoked slow and fast potentials reflect the activity of different neuronal mechanisms.

Representation of edges of variable blur by neuronal responses in the lateral geniculate body and the visual cortex of cats: Limits of linear prediction

We have measured the responses of cells in the cats lateral geniculate body and the visual cortex to edges which were blurred to various degrees (cosinusoidal blur). For the same cells also the responses were determined to sinusoidal gratings of various fundamental frequency and to slits of various blur and width. All stimuli were moved across the receptive fields at various speeds. The responses of most cells increased with increasing edge sharpness, but usually reached a maximum at a blur corresponding to a high frequency cutoff at 0.6–1.2 c/deg. The responses to the sharpest edges were usually smaller than those to a blurred edge (up to −50% in individual cells and −15% in the average). After normalization, the responses predicted from the spatial frequency tuning curves and the Fourier transform of the edge stimuli corresponded well to the measured blur functions up to the maximum of the edge response which varied considerably between cells, however. At edge sharpness beyond that maximum, the predicted curves rose up to edge sharpness with high frequency cutoff 1.6–1.8 times above that which produced the experimental neuronal response maximum. On the other hand, responses could increase with edge sharpening in spatial frequency regions, in which no or only small responses were seen with sinusoidal gratings (e.g. at lower spatial frequencies in “band pass neurons”). Geniculate X- and cortical simple cells as well as those geniculate Y-cells which showed phase locked grating responses behaved similarly in all respects. We concluded that edge sharpness is not represented by response amplitude of individual neurons but by the spatial distribution of excitatory peaks across the representation of the retinotopic cortical map. Our findings further indicate that spatial models of receptive fields assuming linear signal summation have only a limited value for predicting edge sharpness.

THE 2Δ0/(kBTc) AND THE PARAMETERS OF THE PHONON SPECTRUM FOR AMORPHOUS SUPERCONDUCTORS

In this paper, we propose a formula (2△0)/(kBTc)=4.95[1-T01/2/A(1/(λω0)+1/(20λ)+1/(20))], it can describe well the relations between 2△0/(kBTc) and the parameters of the phonon spectrum of disordered and amorphous superconductors for the non-transition metals and their alloys. Here T0 =0.66K, A =1/5K1/2, λ is electron-phonon coupling constant, is the average phonon frequency and ω0 the high frequency cutoff of the effective phonon spectrum a2F(ω). The values of 2△0/(kBTc) of disordered and amorphous superconductors which have the known parameters of the phonon spectrum have been calculated. Agreement with experiment is within a few percent. We show that the amorphous superconductor of non-transition metals and alloys is either a strong coupling superconductor which has a 2△0/(kBTc) valve much larger than the BCS theory value or a weak coupling superconductor which has a considerable smaller one than the BCS theory value.