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J. I. Katz

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

At least 109 records · Page 6Linked to original sources
arXiv1999

Radio Emission from Supernovae

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.

astro-ph↗
arXiv1998

Gamma Ray Bursts vs. Afterglows

When does a GRB stop and its afterglow begin? A GRB may be defined as emission by internal shocks and its afterglow as emission by an external shock, but it is necessary to distinguish them observationally. With these definitions irregularly varying emission (at any frequency) must be the GRB, but smoothly varying intensity is usually afterglow. The GRB itself and its afterglow may overlap in time and in frequency, and distinguishing them will, in general, require detailed modeling.

astro-ph↗
arXiv1998

A Cubic Micron of Equilibrium Pair Plasma?

Is it possible to create a small volume of equilibrium pair plasma in the laboratory with an intense laser pulse? No. The required power would exceed 10^{18} W. The number of seed particles required to absorb the laser energy would approach the number of pairs in the desired plasma, and their radiation in a visible laser field would occur at extremely high frequencies.

physics.plasm-ph↗
arXiv1997

Temperature Dependence of the Index of Refraction of Fused Silica

In order to explain the positive temperature coefficient of the visible index of refraction of fused silica I develop a simple model which assumes a single ultraviolet resonance. Assuming thermal broadening and using the Kramers- Kronig relations leads to a temperature coefficient in qualitative, but not quantitative, agreement with measurement.

cond-mat.mtrl-sci↗
arXiv1997

What Have We Learned From GRB Afterglows?

The discovery by BeppoSAX and coordinated ground-based observations of persistent X-ray, visible and radio counterparts to GRB has successfully concluded a search begin in 1973. The problem of explaining the mechanisms of GRB and their persistent counterparts remains. There are two classes of models: 1) GRB continue weakly for days at all frequencies; 2) GRB emission shifts to lower frequencies as relativistic debris sweeps up surrounding gas (in an ``external shock'') and slows. In 1) the visible afterglow is accompanied by continuing gamma-ray emission, as hinted by the high energy emission of GRB940217 and the ``Gang of Four'' bursts of October 27--29, 1996; the persistent emission may fluctuate. Behavior of this sort may be found in ``internal shock'' models. Models of class 2) have been the subject of several theoretical studies which disagree in assumptions and details but which predict that at each frequency the flux should rise and then decline, with the maximum coming later at lower frequencies. Some of this behavior has been observed, but data from GRB970508 show that its afterglow cannot be simply extrapolated from its gamma-ray emission. It is likely that both classes of processes occur in most GRB. Comparisons between GRB show that they are not all scaled versions of the same event. These results suggest that most gamma-ray emission is the result of ``internal shocks'' while most afterglow is the result of ``external shocks'', and hint at the presence of collimated outflows. Self-absorption in the radio spectrum of GRB970508 permitted the size of the radiating surface to be estimated, and in future GRB it may be possible to follow the expansion of the shell in detail and to construct an energy budget.

astro-ph↗
arXiv1997

Implications of the Visible and X-Ray Counterparts to GRB970228

The gamma-ray burst source GRB970228 has been observed after a delay of 8--12 hours in X-rays and after one day in visible and near infrared light. This marks the first detection of emission at lower frequencies following the gamma-ray observation of a GRB and the first detection of any visible counterpart to a GRB. We consider possible delayed visible and X-ray emission mechanisms, and conclude that the intrinsic gamma-ray activity continued at a much reduced intensity for at least a day. There are hints of such continued activity in other GRB, and future observations can decide if this is true of GRB in general. The observed multi-band spectrum of GRB970228 agrees with the predictions of relativistic shock theory when the flux is integrated over a time longer than that required for a radiating electron to lose its energy.

astro-ph↗
arXiv1997

Possible Evidence for Relativistic Shocks in Gamma-Ray Bursts

Relativistic shock models of gamma-ray bursts may be tested by comparison of their predicted low energy asymptotic spectral indices s to observations. Synchrotron radiation theory predicts that the instantaneous spectrum has s = - 1/3 and the spectrum integrated over the radiative decay of the electrons' energies has s = 1/2, with other cases lying between these limits. We examine the spectra of 11 bursts obtained by the Large Area Detectors on BATSE. One agrees with the predicted instantaneous spectrum, as does the initial portion of a second, and three are close to the predicted integrated spectrum. All the observed asymptotic spectral slopes lie in the predicted range. This evidence for relativistic shocks is independent of detailed models of bursts and of assumptions about their distances. Radiation observed with the predicted instantaneous spectrum has a comparatively smooth time dependence, consistent with the necessarily long radiation time, while that with the predicted integrated spectrum has a spiky time dependence, consistent with the necessarily short radiation time.

astro-ph↗
arXiv1997

Yet Another Model of Gamma-Ray Bursts

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.

astro-ph↗
arXiv1996

After HETE

The outstanding task of gamma-ray burst astronomy is to test the hypothesis that they are at cosmological distances. This can be done by determining the coordinates of at least a few bursts to ~ 15'' or better. If they are in distant galaxies, the next task is to determine from which component of the galaxies the bursts originate; sub-arcsecond positions would answer this question. I outline ground-based systems which can accomplish these tasks, given BATSE/BACODINE coordinates and conservative assumptions about visible counterparts; these systems would be a ground-based upgrade to GRO which would accomplish the chief objectives of HETE.

astro-ph↗
arXiv1996

Single Close Encounters Do Not Make Eccentric Planetary Orbits

The recent discovery of a planet in an orbit with eccentricity $e = 0.63 \pm 0.08$ around the Solar-type star 16 Cyg B, together with earlier discoveries of other planets in orbits of significant eccentricity, raises the question of the origin of these orbits, so unlike the nearly circular orbits of our Solar system. In this paper I consider close encounters between two planets, each initially in a nearly circular orbit (but with sufficient eccentricity to permit the encounter). Such encounters are described by a two-body approximation, in which the effect of the attracting star is neglected, and by the approximation that their separation vector follows a nearly parabolic path. A single encounter cannot produce the present state of these systems, in which one planet is in an eccentric orbit and the other has apparently been lost. Even if the requirement that the second planet be lost is dropped, nearly circular orbits cannot scatter into eccentric ones.

astro-ph↗
arXiv1996

Jets from Collapsing Bubbles

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.

physics.flu-dyn↗
arXiv1996

Sub-Suns and Low Reynolds Number Flow

The phenomenon called the ``sub-Sun'' is the specular reflection of sunlight by horizontally oriented plates of ice. Although well-known in meteorological optics, the hydrodynamics of the orientation is not quantitatively understood. I review the theory of torques on objects at low Reynolds numbers, define coefficients $C_o$, $C_p$, and $C_ψ$ which describe the orienting torques on discs, rods, and hexagonal prisms, and report here the results of experiments to measure $C_o$ and $C_p$.

ao-sci↗
arXiv1996

Statistics and Microphysics of the Fracture of Glass

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.

cond-mat↗
arXiv1996

Electrostriction of Polar Glasses

We develop a finite temperature theory for the susceptibility and electrostriction of isotropic substances in which permanent electric dipoles are restrained from free rotation by elastic forces. All parameters are constrained by the measured susceptibility and elastic constants. When applied to polyurethane, the predicted electrostriction is approximately consistent wiht some of the wide range of data. The saturation of the electrostriction at high field may be explained qualitatively if the dipoles consist of several amide groups locked together by crystallization of the hard segments of the polymer.

cond-mat↗
arXiv1996

A Precessing Disc in OJ287?

Sillanpää, {\it et al.} (1996) have demonstrated that the AGN OJ287 has intensity peaks which recur with a period of about 12 years. I suggest that this is the result of the sweeping of a precessing relativistic beam across our line of sight. In analogy to Her~X-1 and SS433, precession is attributed to the torque exerted by a companion mass on an accretion disc. Secondary maxima observed 1.2 years after two of these peaks may be evidence of nodding motion.

astro-ph↗
arXiv1995

The Long and the Short of Gamma-Ray Bursts

We report evidence from the 3B Catalogue that long ($T_{90} > 10$ s) and short ($T_{90} < 10$ s) gamma-ray bursts represent distinct source populations. Their spatial distributions are significantly different, with long bursts having $\langle V/V_{max} \rangle = 0.282 \pm 0.014$ but short bursts having $\langle V/V_{max} \rangle = 0.385 \pm 0.019$, differing by $0.103 \pm 0.024$, significant at the $4.3 σ$ level. Long and short bursts also differ qualitatively in their spectral behavior, with short bursts harder in the BATSE (50--300 KeV) band, but long bursts more likely to be detected at photon energies > 1 MeV. This implies different spatial origin and physical processes for long and short bursts. Long bursts may be explained by accretion-induced collapse. Short bursts require another mechanism, for which we suggest neutron star collisions. These are capable of producing neutrino bursts as short as a few ms, consistent with the shortest observed time scales in GRB. We briefly investigate the parameters of clusters in which neutron star collisons may occur, and discuss the nuclear evolution of expelled and accelerated matter.

astro-ph↗
arXiv1995

Two Classes of Gamma-Ray Bursts

Data from the 3B Catalogue suggest that short and long GRB are the results of different classes of events, rather than different parameter values within a single class: Short bursts have harder spectra in the BATSE bands, but chiefly long bursts are detected at photon energies over 1 MeV, implying that their hard photons are radiated by a process not found in short bursts. The values of \langle V/V_{max} \rangle for short and long bursts differ by 4.3 σ, implying different spatial distributions. Only the soft gamma-ray radiation mechanisms are the same in both classes.

astro-ph↗