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J. I. Katz

Publications and source records attributed to J. I. Katz.

At least 73 records · Page 4Linked to original sources
arXiv2017

AR Sco: A White Dwarf Synchronar

The emission of the white dwarf-M dwarf binary AR Sco is driven by the rapid synchronization of its white dwarf, rather than by accretion. This requires a comparatively large magnetic field $\sim 100$ gauss at the M dwarf and $\sim 10^8$ gauss on the white dwarf, larger than the fields of most intermediate polars but within the range of fields of known magnetic white dwarfs. The spindown power is dissipated in the atmosphere of the M dwarf by magnetic reconnection, accelerating particles that produce the observed synchrotron radiation. The displacement of the optical maximum from conjunction may be explained either by dissipation in a bow wave as the white dwarf's magnetic field sweeps past the M dwarf or by a misaligned white dwarf's rotation axis and oblique magnetic moment. In the latter case the rotation axis precesses with a period of decades, predicting a drift in the orbital phase of maximum. Binaries whose emission is powered by synchronization may be termed synchronars, in analogy to magnetars.

astro-ph.SR↗
arXiv2016

Are FRB Wandering Narrow Beams?

It is generally assumed that the sources of Fast Radio Bursts radiate roughly isotropically, so that the observed low duty cycle of any individual source indicates a similar low duty cycle of its radio-frequency emission. An alternative hypothesis is that the radiative duty cycle is ${\cal O}(1)$, but that the radiation is emitted in a beam with a solid angle comparable to the observed duty cycle, whose direction wanders or sweeps across the sky. This hypothesis relaxes the extreme power demands of isotropically radiating models of FRB at the price of multiplying the number of sources. The constraints on pulsar models are relaxed; rather than being unprecedentedly fast-spinning and highly-magnetized with short spin-down times, their parameters may be closer to those of typical radio pulsars. The nanoshots of Galactic pulsars pose analogous energetic problems that may be resolved if their emission also is beamed, suggesting a relation between these phenomena, despite their vastly different energy scales.

astro-ph.HE↗
arXiv2016

Dimensional Bounds on Vircator Emission

Vircators (Virtual Cathode Oscillators) are sources of short-pulsed, high power, microwave (GHz) radiation. An essentially dimensional argument relates their radiated power, pulse energy and oscillation frequency to their driving voltage and fundamental physical constants. For a diode of width and gap 10 cm and for voltages of a few hundred keV the peak radiated power cannot exceed ${\cal O} (\text{30 GW})$ and the broad-band single cycle radiated energy cannot exceed ${\cal O}(\text{3 J})$. If electrons can be accelerated to relativistic energies higher powers and radiated energies may be possible.

physics.plasm-ph↗
arXiv2016

How Soft Gamma Repeaters May Make Fast Radio Bursts

There are several phenomenological similarities between Soft Gamma Repeaters and Fast Radio Bursts, including duty factors, time scales and probable repetition. The sudden release of magnetic energy in a neutron star magnetosphere, as in popular models of SGR, can meet the energy requirements of FRB but requires both the presence of magnetospheric plasma in order that dissipation occur in a transparent region and a mechanism for releasing much of that energy quickly. FRB sources and SGR are distinguished by long-lived (up to thousands of years) current-carrying coronal arches remaining from formation of the young neutron star and their decay ends the phase of SGR/AXP/FRB activity even though "magnetar" fields may persist. Runaway increase in resistance when the current density exceeds a threshold releases magnetostatic energy in a sudden burst and produces high brightness GHz emission of FRB by a coherent process; SGR are produced when released energy thermalizes as an equilibrium pair plasma. Failures of some alternative FRB models and the non-detection of SGR~1806-20 at radio frequencies are discussed in appendices.

astro-ph.HE↗
arXiv2016

Fast Radio Bursts---A Brief Review: Some Questions, Fewer Answers

Fast Radio Bursts are millisecond bursts of radio radiation at frequencies of about 1 GHz, recently discovered in pulsar surveys. They have not yet been definitively identified with any other astronomical object or phenomenon. The bursts are strongly dispersed, indicating passage through a high column density of low density plasma. The most economical interpretation is that this is the interglactic medium, indicating that FRB are at "cosmological" distances with redshifts in the range 0.3--1.3. Their inferred brightness temperatures are as high as $10^{37\,\circ}$K, implying coherent emission by "bunched" charges, as in radio pulsars. I review the astronomical sites, objects and emission processes that have been proposed as the origin of FRB, with particular attention to Soft Gamma Repeaters and giant pulsar pulses.

astro-ph.HE↗
arXiv2016

A Study of the Occurrence of Supercooling of Water

Supercooling of water can be easily studied with a simple apparatus suitable for the student laboratory. We describe such an apparatus and its capabilities. The parameters influencing supercooling include the initial temperature of the water and the temperature and the type of chilling medium. We correlate the occurrence of supercooling with the ability of the chilling medium to promptly nucleate ice; if it nucleates promptly, the layer of ice crystals formed on the boundary will initiate freezing of the bulk water without supercooling. If the chilling medium is unable to nucleate ice promptly, ice nucleation is delayed and the water supercools. Students can study and compare supercooling of distilled and natural water. Even quite dirty river water may be supercooled by as much as 5 C.

physics.chem-ph↗
arXiv2016

Inferences from the Distributions of Fast Radio Burst Pulse Widths, Dispersion Measures and Fluences

The widths, dispersion measures, dispersion indices and fluences of Fast Radio Bursts (FRB) impose coupled constraints that all models must satisfy. Observation of dispersion indices close to their low density limit of $-2$ sets a model-independent upper bound on the electron density and a lower bound on the size of any dispersive plasma cloud. The non-monotonic dependence of burst widths (after deconvolution of instrumental effects) on dispersion measure excludes the intergalactic medium as the location of scattering that broadens the FRB in time. Temporal broadening far greater than that of pulsars at similar high Galactic latitudes implies that scattering occurs close to the sources, where high densities and strong turbulence are plausible. FRB energetics are consistent with supergiant pulses from young, fast, high-field pulsars at cosmological distances. The distribution of FRB dispersion measures is inconsistent with expanding clouds (such as SNR). It excludes space-limited distributions (such as the local supercluster), but agrees with a homogeneous cosmological distribution with intergalactic dispersion. The FRB $\log{N}$--$\log{S}$ relation also indicates a cosmological distribution, aside from the anomalously bright Lorimer burst.

astro-ph.HE↗
arXiv2015

Comment on "Stress induction in the bacteria Shewanella oneidensis and Deinococcus radiodurans in response to below-background ionizing radiation", Castillo, et al. Int. J. Rad. Biol., 2015; Early Online DOI:10.3109/09553002.2015.1062571

Castillo, et al. report hormesis by background levels of radiation, at which there is $< 10^{-3}$ ionization per bacterium in a replication time. This suggests radiation products accumulate in the growth medium over much longer times. Experiments are proposed to test this hypothesis.

physics.bio-ph↗
arXiv2015

Implications of Fast Radio Burst Pulse Widths

The pulse widths, dispersion measures and dispersion indices of Fast Radio Bursts (FRB) impose coupled constraints that all models must satisfy. We show that if the dispersion measures resulted from propagation through the intergalactic medium at cosmological distances and the pulse widths were a consequence of scattering by single thin screens, then the screens' electron densities were $\gtrsim 20$/cm$^3$, $10^8$ times the mean intergalactic density. This problem is resolved if the radiation scattered close to its source, where high densities are possible. Observation of dispersion indices close to their low density limit of $-2$ sets a model-independent upper bound on the electron density and a lower bound on the size of the dispersive plasma cloud, excluding terrestrial or Solar System origin. The scattering and much of the dispersion measures may be attributed to scattering regions about 1 AU from the sources, with electron densities $\sim 3 \times 10^8$/cm$^3$. The inferred parameters are only marginally consistent; re-examination of the assumed relation between dispersion measure and distance is warranted. Origin in an ionized starburst or protogalaxy is suggested, but statistical arguments exclude compact young SNR in the Galactic neighborhood. An appendix applies these arguments to PSR J1745-2900 at the Galactic Center. We suggest that its pulse width and angular broadening may be reconciled if we are near a caustic or focal point produced by refraction, rather than by the classic thin sheet scattering model.

astro-ph.HE↗
arXiv2014

Quantifying "Cliffs" in Design Space

This paper studies the regions of parameter space of engineering design in which performance is sensitive to design parameters. Some of these parameters (for example, the dimensions and compositions of components) constitute the design, but others are intrinsic properties of materials or Nature. The paper is concerned with narrow regions of parameter space, "cliffs", in which performance (some measure of the final state of a system, such as ignition or non-ignition of a flammable gas, or failure or non-failure of a ductile material subject to tension) is a sensitive function of the parameters. In these regions performance is also sensitive to uncertainties in the parameters. This is particularly important for intrinsically indeterminate systems, those whose performance is not predictable from measured initial conditions and is not reproducible.

physics.soc-ph↗
arXiv2014

Plasma Temperature Inference from DT/DD Neutron Discrimination

DD and DT reaction rates may be compared to determine plasma temperatures in the 10--200 eV range. Distinguishing neutrons from these two reactions is difficult when yields are low or unpredictable. Time of flight methods fail if the source is extended in time. These neutrons may be distinguished because inelastic scattering of more energetic neutrons by carbon produces a 4.44 Mev gamma-ray, and because hydrogenous material preferentially attenuates lower energy neutrons. We describe a detector system that can discriminate between lower and higher energy neutrons for fluences as low as $\mathcal{O}(10^2)$ neutrons/sterad even when time of flight methods fail, define a figure of merit and calculate its performance over a broad range of parameters.

physics.ins-det↗
arXiv2014

Coherent Emission in Fast Radio Bursts

The fast (ms) radio bursts reported by Thornton, {\it et al.} have extremely high brightness temperatures if at the inferred cosmological distances. This implies coherent emission by "bunches" of charges. We model the emission region as a screen of dipole radiators resonant at the frequency of observation and calculate the necessary charge bunching. From this we infer the minimum electron energy required to overcome electrostatic repulsion. The FRB, like the giant pulses of the Crab pulsar, display banded spectra that may be harmonics of plasma frequency emission by plasma turbulence. If FRB are the counterparts of comparatively frequent Galactic events, these may be detected by radio telescopes in their far sidelobes or by small arrays of dipoles.

astro-ph.HE↗
arXiv2014

What Perytons Aren't, and Might Be

Chirped radio pulses known as perytons are consistent with plasma propagation, but an astronomical origin has been excluded. TE and TM waveguide modes have dispersion relations formally similar to those of plasmas, and emission at the plasma frequency of a plasma recombining by two-particle processes also produces a chirp $δt \propto ω^{-2}$, but these hypotheses fail quantitatively. The chirp might be approximately fitted to three-body recombination, cyclotron radiation in exponentially decaying fields, or to the resonance of a distributed LC circuit whose component conductors move, but no specific source has been identified.

astro-ph.HE↗
arXiv2014

A Mean Field Model of Layering Instability in Shearing Suspensions

Concentrated suspensions may shear-thin when the suspended particles form planar sheets that slide over one another with less friction than if the particles are randomly distributed. In a naïve model the suspension is described by a mean effective viscosity, and particles that collide with each other redistribute the mean density in the shearing direction. This leads to a diffusion equation for the particle density. If the viscosity in the unthinned state is a steeply increasing function of particle density the effective diffusion coefficient is negative and the diffusion equation, meaningful only on scales larger than the particle separation, is ill-posed. This singularity corresponds to the formation of planar sheets of particles and defines a critical particle density for the onset of shear thinning.

cond-mat.soft↗
arXiv2014

Historic Trends in U. S. Drought Forcing in a Warming Climate

The mean North American and world climates have warmed significantly since the beginning of climatologically significant anthropogenic emission of greenhouse gases in the 19th Century. It has been suggested that warming may increase the frequency or severity of droughts. We define and study the statistics of an aridity index that describes the precipitation forcing function of a drought, considering drought to be a season with low enough precipitation to be significant for agriculture. Our aridity index is a reciprocal function of the seasonal precipitation, which is more significant for agriculture than mean precipitation. Using NOAA data from sites in 13 diverse climate regimes in the 48 contiguous United States with time series running over the period 1940--1999 but including two data series from 1900 or 1910, and computing their decadal averages, we search for linear trends in their aridity indices. We find no linear trends significant at the $2σ$ level. At five sites $3σ$ upper bounds on any systematic trends are in the range 1.0--2.8%/decade, while at two sites $3σ$ lower bounds are -0.5%/decade and -2.2%/decade; at other sites the bounds are less restrictive.

physics.ao-ph↗
arXiv2013

Scanning of Vehicles for Nuclear Materials

Might a nuclear-armed terrorist group or state use ordinary commerce to deliver a nuclear weapon by smuggling it in a cargo container or vehicle? This delivery method would be the only one available to a sub-state actor, and it might enable a state to make an unattributed attack. Detection of a weapon or fissile material smuggled in this manner is difficult because of the large volume and mass available for shielding. Here I review methods for screening cargo containers to detect the possible presence of nuclear threats. Because of the large volume of innocent international commerce, and the cost and disruption of secondary screening by opening and inspection, it is essential that the method be rapid and have a low false-positive rate. Shielding can prevent the detection of neutrons emitted spontaneously or by induced fission. The two promising methods are muon tomography and high energy X-radiography. If they do not detect a shielded threat object they can detect the shield itself.

physics.soc-ph↗