Characteristic Delay Time Research Articles

Probing Intergalactic Magnetic Fields in the GLAST Era through Pair Echo Emission from TeV Blazars

More than a dozen blazars are known to be emitters of multi-TeV gamma rays, often with strong and rapid flaring activity. By interacting with photons of the cosmic microwave and infrared backgrounds, these gamma rays inevitably produce electron-positron pairs, which in turn radiate secondary inverse Compton gamma rays in the GeV-TeV range with a characteristic time delay that depends on the properties of the intergalactic magnetic field (IGMF). For sufficiently weak IGMF, such "pair echo" emission may be detectable by the Gamma-ray Large Area Space Telescope (GLAST), providing valuable information on the IGMF. We perform detailed calculations of the time-dependent spectra of pair echos from flaring TeV blazars such as Mrk 501 and PKS 2155-304, taking proper account of the echo geometry and other crucial effects. In some cases, the presence of a weak but non-zero IGMF may enhance the detectability of echos. We discuss the quantitative constraints that can be imposed on the IGMF from GLAST observations, including the case of non-detections.

The ExtendedHSTSupernova Survey: The Rate of SNe Ia atz> 1.4 Remains Low

We use the HST ACS imaging of the two GOODS fields during Cycles 11, 12, and 13 to derive the Type Ia supernova rate in four redshift intervals in the range -->0.2 . The sample now consists of 56 Type Ia supernovae. The rates we derive are consistent with our results based on the Cycle 11 observations. In particular, the small number of supernovae detected at -->z > 1.4 supports our previous result that there is a drop in the Type Ia supernova rate at high redshift, suggesting a long time delay between the formation of the progenitor star and the explosion of the supernova. If described by a simple one-parameter model, we find a characteristic delay time of 2-3 Gyr. However, a number of recent results based on, e.g., low-redshift supernova samples and supernova host galaxy properties suggest that the supernova delay-time distribution is bimodal. In this model a major fraction of the Type Ia supernova rate are prompt and follow the star formation rate, while a smaller fraction of the rate have a long delay time, making this channel proportional to mass. While our results are fully consistent with the bimodal model at low redshifts, the low rate we find at -->z > 1.4 appears to contradict these results. Models that correct for star formation hidden by dust may explain at least part of the differences. Here we discuss this possibility together with other ways to reconcile data and models.

Polyphase (non-resonant) multipactor in rectangular waveguides

The polyphase multipactor, i.e. the non-resonant form of secondary electron emission rf discharges in vacuum, has been analysed and studied experimentally. The multipactor discharge was observed in an evacuated standard rectangular waveguide through which pulsed high-power microwave radiation in the decimeter wavelength range was transmitted. The power interval in which the two-sided (between the wide walls of the waveguide) multipactor appeared has been determined. It is found that there is a characteristic delay time for the onset of the multipactor breakdown as compared with the time at which the microwave power is applied. The dependence of this delay time on the microwave power has been established. The experimental results are compared with results of numerical simulations which make it possible to estimate the secondary emission properties of the waveguide walls. Reasons for some observed discrepancies between numerical results and experimental data are discussed as well as the nature of the observed multipactor delay.

The Double Nucleation Model for Sickle Cell Haemoglobin Polymerization: Full Integration and Comparison with Experimental Data

Sickle cell haemoglobin (HbS) polymerization reduces erythrocyte deformability, causing deleterous vaso-occlusions. The double-nucleation model states that polymers grow from HbS aggregates, the nuclei, (i) in solution (homogeneous nucleation), (ii) onto existing polymers (heterogeneous nucleation). When linearized at initial HbS concentration, this model predicts early polymerization and its characteristic delay-time (Ferrone et al. J Mol Biol 183(4):591-610, 611-631, 1985). Addressing its relevance for describing complete polymerization, we constructed the full, non-linearized model (Simulink), The MathWorks). Here, we compare the simulated outputs to experimental progress curves (n = 6-8 different [HbS], 3-6 mM range, from Ferrone's group). Within 10% from start, average root mean square (rms) deviation between simulated and experimental curves is 0.04 +/- 0.01 (25 degrees C, n = 8; mean +/- standard error). Conversely, for complete progress curves, averaged rms is 0.48 +/- 0.04. This figure is improved to 0.13 +/- 0.01 by adjusting heterogeneous pathway parameters (p < 0.01): the nucleus stability (sigma(2) micro( cc ): + 40%), and the fraction of polymer surface available for nucleation (phi), from 5e(-7), (3 mM) to 13 (6 mM). Similar results are obtained at 37 degrees C. We conclude that the physico-chemical description of heterogeneous nucleation warrants refinements in order to capture the whole HbS polymerization process.

Remnant evolution after a carbon-oxygen white dwarf merger

We systematically explore the evolution of the merger of two carbon-oxygen (CO) white dwarfs. The dynamical evolution of a 0.9 Msun + 0.6 Msun CO white dwarf merger is followed by a three-dimensional SPH simulation. We use an elaborate prescription in which artificial viscosity is essentially absent, unless a shock is detected, and a much larger number of SPH particles than earlier calculations. Based on this simulation, we suggest that the central region of the merger remnant can, once it has reached quasi-static equilibrium, be approximated as a differentially rotating CO star, which consists of a slowly rotating cold core and a rapidly rotating hot envelope surrounded by a centrifugally supported disc. We construct a model of the CO remnant that mimics the results of the SPH simulation using a one-dimensional hydrodynamic stellar evolution code and then follow its secular evolution. The stellar evolution models indicate that the growth of the cold core is controlled by neutrino cooling at the interface between the core and the hot envelope, and that carbon ignition in the envelope can be avoided despite high effective accretion rates. This result suggests that the assumption of forced accretion of cold matter that was adopted in previous studies of the evolution of double CO white dwarf merger remnants may not be appropriate. Our results imply that at least some products of double CO white dwarfs merger may be considered good candidates for the progenitors of Type Ia supernovae. In this case, the characteristic time delay between the initial dynamical merger and the eventual explosion would be ~10^5 yr. (Abridged).

Pulse thermometers based on synthetic single crystal boron-doped diamonds

A time delay of electrical resistance of boron-doped semiconductor diamond sensor at heating pulses was investigated. We used a symmetric scheme with 2 equivalent 2 × 2 × 1.5 mm 3 single crystal elements connected via thermal conductive electrical insulating glue. One of elements served as pulse heater and the other one as a sensor. About 40 ms characteristic time delay of diamond sensor was achieved. It is supposed, that using smaller crystals for sensors and by optimization of their electrical and heat conductivity properties, a regular value of time delay less than 1 ms will be attained. The potential application of diamond thermometers for temperature control in combustion engines is shown.

Chemistry and Star Formation in the Host Galaxies of Type Ia Supernovae

We study the effect of environment on the properties of Type Ia supernovae by analyzing the integrated spectra of 57 local Type Ia supernova host galaxies. We deduce from the spectra the metallicity, current star formation rate, and star formation history of the host and compare these to the supernova decline rates. Additionally, we compare the host properties to the difference between the derived supernova distance and the distance determined from the best-fit Hubble law. From this we investigate possible uncorrected systematic effects inherent in the calibration of Type Ia supernova luminosities using light-curve fitting techniques. Our results indicate a statistically insignificant correlation in the direction of higher metallicity spiral galaxies hosting fainter Type Ia supernovae. However, we present qualitative evidence suggesting that progenitor age is more likely to be the source of variability in supernova peak luminosities than is metallicity. We do not find a correlation between the supernova decline rate and host galaxy absolute B magnitude, nor do we find evidence of a significant relationship between decline rate and current host galaxy star formation rate. A tenuous correlation is observed between the supernova Hubble residuals and host galaxy metallicities. Further host galaxy observations will be needed to refine the significance of this result. Finally, we characterize the environmental property distributions for Type Ia supernova host galaxies through a comparison with two larger, more general galaxy distributions using Kolmogorov-Smirnov tests. The results show the host galaxy metallicity distribution to be similar to the metallicity distributions of the galaxies of the NFGS and SDSS. Significant differences are observed between the SN Ia distributions of absolute B magnitude and star formation histories and the corresponding distributions of galaxies in the NFGS and SDSS. Among these is an abrupt upper limit observed in the distribution of star formation histories of the host galaxy sample, suggesting a Type Ia supernovae characteristic delay time lower limit of approximately 2.0 Gyr. Other distribution discrepancies are investigated and the effects on the supernova properties are discussed.

New Constraints on Type Ia Supernova Progenitor Models

We use the StarTrack population synthesis code to discuss potential progenitor models of SNe Ia: the single-degenerate scenario, the semidetached double white dwarf binary, and the double-degenerate scenario. Using the most recent calculations of accretion onto white dwarfs, we consider SN Ia explosions of various white dwarfs in binary systems. We calculate the evolutionary delay times from the zero-age main sequence to the explosion and verify which scenarios can satisfy the constraints found by Strolger et al., i.e., a delay time of 2-4 Gyr. It is found that the semidetached double white dwarf binary model is inconsistent with observations. Only SN Ia progenitors in the double-degenerate scenario are compatible with observations in all tested evolutionary models, with a characteristic delay time of ~3 Gyr. If double-degenerate mergers are excluded as SN Ia progenitors, we find that the single-degenerate scenario may explain the observed delay times for models with low common-envelope efficiency. It is also noted that the delay time distributions within the various tested SN Ia scenarios vary significantly and can potentially be used as an additional constraint on progenitor models.

A model approach to evaluate the effect of temperature and food concentration on individual life-history and population dynamics of Daphnia

We present a model framework for the simulation of growth and reproduction of Daphnia at varying conditions of food concentration and temperature. The core of our framework consists of an individual level model that simulates allocation of assimilated carbon into somatic growth, maintenance costs, and reproduction on the basis of a closed carbon budget. A fixed percentage of assimilated carbon is allocated into somatic growth and maintenance costs. Special physiological adaptations in energy acquisition and usage allow realistic model performance even at very low food concentrations close to minimal food requirements. All model parameters are based on physiological measures taken from the literature. Model outputs were thoroughly validated on data from a life-table experiment with Daphnia galeata. For the first time, a successful model validation was performed at such low food concentrations. The escalator boxcar train (EBT) was used to integrate this individual level model into a stage-structured population model. In advance to previous applications of the EBT to Daphnia we included an additional clutch compartment into the model structure that accounts for the characteristic time delay between egg deposition and hatching in cladocerans. By linking two levels of biological organisation, this model approach represents a comprehensive framework for studying Daphnia both at laboratory conditions and in the field. We compared outputs of our stage-structured model with predictions by two other models having analogous parameterisation: (i) another individual level Daphnia model (Kooijman–Metz model) and (ii) a classical unstructured population model. In contrast to our Daphnia model, the Kooijman–Metz model lacks the structure to account for the optimisation of energy acquisition and maintenance requirements by individual daphnids. The unstructured population model showed different patterns of population dynamics that were not in concordance with typical patterns observed in the field. Thus, we conclude our model provides a comprehensive tool for the simulation of growth and reproduction of Daphnia and corresponding population dynamics.

The application of convolution-based statistical model on the electrical breakdown time delay distributions in neon

The convolution-based model of the electrical breakdown time delay distribution is applied for statistical analysis of experimental results obtained in neon-filled diode tube at 6.5 mbar. At first, the numerical breakdown time delay density distributions are obtained by stochastic modeling as the sum of two independent random variables, the electrical breakdown statistical time delay with exponential, and discharge formative time with Gaussian distribution. Then, the single characteristic breakdown time delay distribution is obtained as the convolution of these two random variables with previously determined parameters. These distributions show good correspondence with the experimental distributions, obtained on the basis of 1000 successive and independent measurements. The shape of distributions is investigated, and corresponding skewness and kurtosis are plotted, in order to follow the transition from Gaussian to exponential distribution.

Flashover of a vacuum-insulator interface: A statistical model

We have developed a statistical model for the flashover of a $45\ifmmode^\circ\else\textdegree\fi{}$ vacuum-insulator interface (such as would be found in an accelerator) subject to a pulsed electric field. The model assumes that the initiation of a flashover plasma is a stochastic process, that the characteristic statistical component of the flashover delay time is much greater than the plasma formative time, and that the average rate at which flashovers occur is a power-law function of the instantaneous value of the electric field. Under these conditions, we find that the flashover probability is given by $1\ensuremath{-}\mathrm{exp}(\ensuremath{-}{E}_{p}^{\ensuremath{\beta}}{t}_{\mathrm{eff}}C/{k}^{\ensuremath{\beta}})$, where ${E}_{p}$ is the peak value in time of the spatially averaged electric field $E(t)$, ${t}_{\mathrm{eff}}\ensuremath{\equiv}\ensuremath{\int}[E(t)/{E}_{p}{]}^{\ensuremath{\beta}}dt$ is the effective pulse width, $C$ is the insulator circumference, $k\ensuremath{\propto}\mathrm{exp}(\ensuremath{\lambda}/d)$, and $\ensuremath{\beta}$ and $\ensuremath{\lambda}$ are constants. We define $E(t)$ as $V(t)/d$, where $V(t)$ is the voltage across the insulator and $d$ is the insulator thickness. Since the model assumes that flashovers occur at random azimuthal locations along the insulator, it does not apply to systems that have a significant defect, i.e., a location contaminated with debris or compromised by an imperfection at which flashovers repeatedly take place, and which prevents a random spatial distribution. The model is consistent with flashover measurements to within 7% for pulse widths between 0.5 ns and $10\text{ }\ensuremath{\mu}\mathrm{s}$, and to within a factor of 2 between 0.5 ns and 90 s (a span of over 11 orders of magnitude). For these measurements, ${E}_{p}$ ranges from 64 to $651\text{ }\text{ }\mathrm{kV}/\mathrm{cm}$, $d$ from 0.50 to 4.32 cm, and $C$ from 4.96 to 95.74 cm. The model is significantly more accurate, and is valid over a wider range of parameters, than the J. C. Martin flashover relation that has been in use since 1971 [J. C. Martin on Pulsed Power, edited by T. H. Martin, A. H. Guenther, and M. Kristiansen (Plenum, New York, 1996)]. We have generalized the statistical model to estimate the total-flashover probability of an insulator stack (i.e., an assembly of insulator-electrode systems connected in series). The expression obtained is consistent with the measured flashover performance of a stack of five 5.72-cm-thick, 1003-cm-circumference insulators operated at 100 and $158\text{ }\text{ }\mathrm{kV}/\mathrm{cm}$. The expression predicts that the total-flashover probability is a strong function of the ratio ${E}_{p}/k$, and that under certain conditions, the performance improves as the capacitance between the stack grading rings is increased. In addition, the expression suggests that given a fixed stack height, there exists an optimum number of insulator rings that maximizes the voltage at which the stack can be operated. The results presented can be applied to any system (or any set of systems connected in series) subject to random failures, when the characteristic statistical delay time of a failure is much greater than its formative time.

Small-angle static light scattering of concentrated silica suspensions during in situ destabilization

The aggregation of concentrated aqueous silica suspensions is characterized by means of static light scattering. We use an in situ destabilization mechanism based on the enzyme-catalyzed hydrolysis of urea. This method enables us to continuously and homogeneously change the interparticle potential from repulsive to attractive without disturbing the aggregation process. Moreover, our electrostatically stabilized suspensions can be destabilized by two different methods. In the first method, the pH is shifted toward the isoelectric point of the particles (ΔpH method), thereby leading to a decrease of their surface charge. In the second method, the ionic strength is continuously increased at constant pH (Δ I method), leading to a compression of the electrical double layer around the charged particles. A laboratory-built flat-cell light-scattering instrument is used, which allows fast data acquisition and an adjustment of the sample cell thickness. To circumvent multiple scattering effects, we use a very small sample thickness (≈13 μm). In addition, the refractive index difference between the aqueous phase and the particles is reduced by adding sucrose to the liquid phase of our suspensions. We are able to characterize the structural changes at the very early stages of the destabilization process, where no significant effects are yet detected in macroscopic rheological measurements. While during the ΔpH destabilization, the scattering curve shows significant changes only after some characteristic delay time, it changes continuously during the Δ I destabilization. The latter is attributed to the formation of a weak pre-gel structure in the suspensions, as a shallow secondary minimum appears in the interparticle potential. Data are evaluated by using a HMSA square-well structure factor model. Results are in good agreement with those predicted from DLVO theory.

Unintended filtering in a typical photodiode detection system for optical tweezers

We characterize the frequency-dependent response of a photo detection system based on a Si-PIN photodiode and a laser with wavelength 1064 nm, a system commonly used with optical tweezers. We chopped the laser beam with chopper frequencies from 200 Hz to 14 kHz, and found an exponentially delayed response of the detection system with a characteristic delay time of ∼20 μs. The physical mechanism causing this time delay is silicon’s transparency to 1064 nm light: Photons are absorbed and create charge carriers not only in the diode’s depletion layer, where they are detected within nano-seconds, but predominantly in the n-layer, where they remain undetected till transported out by thermal diffusion. The diode’s response is dominated by this delay which can be characterized as a first-order low-pass filter with a 3dB-frequency of 8–9 kHz, depending on laser intensity. Measurements exploiting frequencies near or above this 3dB-frequency must be corrected for this unintended filter effect. We describe how to do this, and how to diagnose other systems which may or may not have the same problem. Explanations are intended for users of photo detection systems, and present the little semi-conductor physics needed to make sense.

Impurity-induced photoconductivity in gallium-doped Pb 1− xGe xTe alloys

Photoelectric properties of Pb 1− x Ge x Te〈Ga〉 (0.04⩽ x⩽0.08) alloys were investigated under controlled infrared illumination. At least two types of relaxation processes of the photoconductivity after removing the illumination at T< T c≈60 K and an anomalous peak of negative photoconductivity followed by characteristic delay time on the front of kinetic curves were observed. Results were explained assuming the photoexcitation of the electrons from gallium-induced defect states to the conduction band and existing energy barrier separating localized and band states in configuration space. Appearance of a negative photoconductivity peak was attributed to the phase transition of the alloys from the rhombohedral phase to the cubic one due to the reconstruction of defect centers surroundings induced by illumination.

Dielectric breakdown of one-dimensional Mott insulatorsSr2CuO3andSrCuO2

Dielectric breakdown phenomena accompanied by characteristic time delay and subsequent negative differential resistance effect have been observed for typical one-dimensional (1D) Mott insulators, ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ and ${\mathrm{SrCuO}}_{2}.$ In the both compounds, the threshold electric-field for the breakdown, defined as a field above which the differential resistance becomes negative, has been found to increase exponentially with decreasing temperature. This result is similar to the case of charge-density-wave depinning, and indicates the collective nature of charge dynamics in these 1D systems. The electric-field dependence of the delay time is analyzed in terms of a phenomenological model.

Effect of Negative Photoconductivity in Pb1?xGexTe Alloys Doped with Gallium

The photoconductivity kinetics of the n-Pb1—xGexTe〈Ga〉(0.04 ≤ x ≤ 0.08) single crystals was investigated under controlled illumination emitted from the infrared (IR) source. A high IR photosensitivity and at least two types of the relaxation processes with drastically different characteristic times at T < Tc = 50 to 60 K were observed. The front part of the kinetic curves revealed an anomalous peak of negative photoconductivity following a characteristic delay time. The results are explained assuming the Jahn-Teller nature of the Ga-induced defect centers. A model was proposed of a transition of the alloys from the rhombohedral phase to the cubic phase due to the reconstruction of defect center surroundings induced by illumination.

Dielectric Breakdown of the Insulating Charge-Ordered State inLa2−xSrxNiO4

Dielectic breakdown associated with characteristic delay time and subsequent negative differential resistance (NDR) have been observed in the charge-stripe phase of the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{4+\ensuremath{\delta}}$ crystal, whose hole doping level ${n}_{h}$ ( $\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}x+2\ensuremath{\delta}$) is around 1/3. Threshold voltage ( ${E}_{\mathrm{th}}$), above which the NDR emerges, varies with ${n}_{h}$ and reaches a maximum at the commensurate doping of ${n}_{h}\ensuremath{\sim}1/3$. The ${E}_{\mathrm{th}}$ increases exponentially with decreasing $T$, but tends to saturate at low temperatures perhaps due to the quantum fluctuation of the stripes. The results indicate that the dielectric breakdown is related to depinning and collective transport of the ordered holes on the stripe.

Loss of lag synchronization in coupled chaotic systems.

Lag synchronization denotes a particular form of synchronization in which the amplitudes of two interacting, nonidentical chaotic oscillators are correlated but there is a characteristic time delay between them. We study transitions to and between different forms of synchronization for the attractors defined as "in-phase" and "out-of-phase" and investigate the processes by which lag synchronization is lost in two coupled Rössler systems. With a small frequency mismatch between the two systems, these processes are related to the occurrence of a peculiar form of basin structure as more and more periodic orbits embedded into the synchronized chaotic state become unstable in a transverse direction.

Mathematical characterization of insect cell (Sf-9) death in suspended culture

The effect of baculovirus infection on cell death in suspended cultures was characterized based on work by Wu et al. (1993) Biotech. Bioeng. 41: 104–110 and Wu et al. (1994) Biotechnol. Prog. 10: 55–59. The post infection time can be separated into a constant viability phase characterized by a time delay, td, and a rapid death phase, which is characterized by a specific death rate constant, k. Results indicated that the characteristic time delay decreased with increased multiplicity on infection (MOI). Further, there was only a weak correlation between specific death rate and MOI, for the range of MOI tested. Cell infection and death rates were consistent with a more evenly distributed infection process likely found in suspension cultures.

The relativistic electron response at geosynchronous orbit during the January 1997 magnetic storm

The first geomagnetic storm of 1997 began on January 10. It is of particular interest because it was exceptionally well observed by the full complement of International Solar Terrestrial Physics (ISTP) satellites and because of its possible association with the catastrophic failure of the Telstar 401 telecommunications satellite. Here we report on the energetic electron environment observed by five geosynchronous satellites. In part one of this paper we examine the magnetospheric response to the magnetic cloud. The interval of southward IMF drove strong substorm activity while the interval of northward IMF and high solar wind density strongly compressed the magnetosphere. At energies above a few hundred keV, two distinct electron enhancements were observed at geosynchronous orbit. The first enhancement began and ended suddenly, lasted for approximately 1 day, and is associated with the strong compression of the magnetosphere. The second enhancement showed a more characteristic time delay, peaking on January 15. Both enhancements may be due to transport of electrons from the same initial acceleration event at a location inside geosynchronous orbit but the first enhancement was due to a temporary, quasi‐adiabatic transport associated with the compression of the magnetosphere while the second enhancement was due to slower diffusive processes. In the second part of the paper we compare the relativistic electron fluxes measured simultaneously at different local times. We find that the &gt;2‐MeV electron fluxes increased first at noon followed by dusk and then dawn and that there can be difference of two orders of magnitude in the fluxes observed at different local times. Finally, we discuss the development of data‐driven models of the relativistic electron belts for space weather applications. By interpolating fluxes between satellites we produced a model that gives the &gt;2‐MeV electron fluxes at all local times as a function of universal time. In a first application of this model we show that, at least in this case, magnetopause shadowing does not contribute noticeably to relativistic electron dropouts.