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Search Results: 1 - 10 of 363625 matches for " J. I. Katz "
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Circulation in Blowdown Flows
J. I. Katz
Physics , 2008,
Abstract: The blowdown of high pressure gas in a pressure vessel produces rapid adiabatic cooling of the gas remaining in the vessel. The gas near the wall is warmed by conduction from the wall, producing radial temperature and density gradients that affect the flow, the mass efflux rate and the thermodynamic states of both the outflowing and the contained gas. The resulting buoyancy-driven flow circulates gas through the vessel and reduces, but does not eliminate, these gradients. The purpose of this note is to estimate when blowdown cooling is rapid enough that the gas in the pressure vessel is neither isothermal nor isopycnic, though it remains isobaric. I define a dimensionless number, the buoyancy circulation number BC, that parametrizes these effects.
The Ptolemaic Gamma-Ray Burst Universe
J. I. Katz
Physics , 1992, DOI: 10.1007/BF00645080
Abstract: The BATSE experiment on GRO has demonstrated the isotropic arrival directions and flat $\log N$ {\it vs.} $\log S$ of cosmic gamma-ray bursts. These data are best explained if the burst sources are distributed throughout an extended spherical Galactic halo, as previously suggested by Jennings. The halo's radius is at least 40 Kpc, and probably is more than 100 Kpc. I consider possible origins of this halo, including primordial formation and neutron stars recoiling from their birthplaces in the Galactic disc. A simple geometrical model leads to a predicted relation between the dipole and quadrupole anisotropy. I suggest that neutron stars born with low recoil become millisecond pulsars, while those born with high recoil become the sources of gamma-ray bursts; these populations are nearly disjoint. Quiescent counterparts of gamma-ray bursts are predicted to be undetectably faint.
Particle Acceleration in (by) Accretion Discs
J. I. Katz
Physics , 1992, DOI: 10.1063/1.42698
Abstract: I present a model for acceleration of protons by the second-order Fermi process acting on randomly scrambled magnetic flux arches above an accretion disc. The accelerated protons collide with thermal protons in the disc, producing degraded energetic protons, charged and neutral pions, and neutrons. The pions produce gamma-rays by spontaneous decay of $\pi^0$ and by bremsstrahlung and Compton processes following the decay of $\pi^\pm$ to $e^\pm$.
Radio and Optical Emission, Spectral Shapes and Breaks in GRB
J. I. Katz
Physics , 1993, DOI: 10.1063/1.45862
Abstract: Relativistic blast wave models predict the spectrum of the emitted synchrotron radiation. The electrons in the shocked region are heated to a Wien distribution whose ``temperature'' is $1/3$ of the mean electron energy. This energy scale determines a characteristic (break) frequency of synchrotron radiation. At much lower frequencies a spectrum $F_nu \propto \nu^{1/3}$ is predicted independently of details of the emitting region. This is consistent with the observed soft X-ray spectra of GRB. It implies low visible and radio intensities, unless there are collective emission processes.
Low Frequency Spectra of Gamma-Ray Bursts
J. I. Katz
Physics , 1993, DOI: 10.1086/187523
Abstract: Particles with energies below the mean energy $E_0$ in relativistic shocked plasmas should assume an equilibrium energy distribution. This leads to a synchrotron spectrum $F_\nu \propto \nu^{1/3}$ up to approximately the critical frequency $\nu_0$ of an electron with the energy $E_0$. Application to GRBs implies that a burst with $10^{-5}$ erg/cm$^2$s of soft gamma-rays and $h \nu_0 = 300$ KeV should be about 18th magnitude in visible light and a few $\mu$Jy at 1 GHz (less if self-absorbed).
Statistics and Microphysics of the Fracture of Glass
J. I. Katz
Physics , 1996,
Abstract: The tensile strength of fused silica fibers is believed to approach its intrinsic value at low temperature, and modern experiments indicate very small, perhaps unmeasured, intrinsic dispersion in this strength. I consider the application of classical ``weakest link'' models to this problem in an attempt to determine the number and therefore the nature of the failure sites. If the skewness as well as the dispersion (Weibull modulus) of failure strengths are measured it may be possible to determine both the number of sites and the distribution of their strengths. Extant data are not sufficient, but I present calculated skewnesses for comparison with future data.
Yet Another Model of Gamma-Ray Bursts
J. I. Katz
Physics , 1997, DOI: 10.1086/304896
Abstract: Sari and Piran have demonstrated that the time structure of gamma-ray bursts must reflect the time structure of their energy release. A model which satisfies this condition uses the electrodynamic emission of energy by the magnetized rotating ring of dense matter left by neutron star coalescence; GRB are essentially fast, high field, differentially rotating pulsars. The energy densities are large enough that the power appears as an outflowing equilibrium pair plasma, which produces the burst by baryon entrainment and subsequent internal shocks. I estimate the magnetic field and characteristic time scale for its rearrangement, which determines the observed time structure of the burst. There may be quasi-periodic oscillations at the rotational frequencies, which are predicted to range up to 5770 Hz (in a local frame). This model is one of a general class of electrodynamic accretion models which includes the Blandford and Lovelace model of AGN, and which can also be applied to black hole X-ray sources of stellar mass. The apparent efficiency of nonthermal particle acceleration is predicted to be 10--50%, but higher values are possible if the underlying accretion flow is super-Eddington. Applications to high energy gamma-ray observations of AGN are briefly discussed.
Jets from Collapsing Bubbles
J. I. Katz
Physics , 1996,
Abstract: When an asymmetric bubble collapses it generally produces a well defined high velocity jet. This is remarkable because one might expect such a collapse to produce a complex or chaotic flow rather than an ordered one. I present a dimensional argument for the ubiquity of jets from collapsing bubbles, and model the aspherical collapse of a bubble with pieces of Rayleigh's solution for spherical collapse and its cylindrical analogue. This model explains the ubiquity of jet formation in aspherical collapse, and predicts the shape and velocity profile of the resulting jet. These predictions may be tested in the laboratory or by numerical calculation. An application to solid spall is suggested.
A Pedagogical "Toy" Climate Model
J. I. Katz
Physics , 2010,
Abstract: A "toy" model, simple and elementary enough for an undergraduate class, of the temperature dependence of the greenhouse (mid-IR) absorption by atmospheric water vapor implies a bistable climate system. The stable states are glaciation and warm interglacials, while intermediate states are unstable. This is in qualitative accord with the paleoclimatic data. The present climate may be unstable, with or without anthropogenic interventions such as CO$_2$ emission, unless there is additional stabilizing feedback such as "geoengineering".
Radio Emission from Supernovae
J. I. Katz
Physics , 1999, DOI: 10.1086/321514
Abstract: I consider radio emission from the remarkable SN1998bw. Ni-56 and Co-56 decays produce a gamma-ray flux whose Compton-scattered electrons naturally explain the observed mildly relativistic exapnsion of the radio source and its double- peaked history. Such models require a surrounding plasma, perhaps produced by the supernova progenitor, whose interaction with the nonrelativistic debris may account for the observed X-ray source. The radio spectrum appears to be self- absorbed. This interpretation determines the brightness temperature, and hence the energy of the radiating electrons, implying a surprisingly large magnetic field. Attempts to avoid this conclusion by interpreting the spectrum as the result of inverse bremsstrahlung absorption do not lead to significantly lower fields. The large inferred field may have several explanations: radiation from a central pulsar, a turbulent hydrodynamic dynamo or an aspherical Compton current, but a frozen-in field from the supernova progenitor is not adequate. The electron-ion and particle-field equipartition problems are discussed. Compton electrons also explain the inferred expansion speed of SN1987A's spots.
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