Engineering topics

Katz, J. I.

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

At least 19 records

NASA NTRS1997

Possible Evidence for Relativistic Shocks in Gamma-Ray Bursts

Relativistic shock models of gamma-ray bursts may be tested by comparing 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 of 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 radiation observed with the predicted integrated spectrum has a spiky time dependence, consistent with the necessarily short radiation time.

Cohen, E.↗

NASA NTRS1997

Single Close Encounters do not make Eccentric Planetary Orbits

The recent discovery of a planet in an orbit with eccentricity e = 0.63 +/- 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.

Katz, J. I.↗

NASA NTRS1997

A Precessing Disk in OJ 287?

Sillanpaa et al. have demonstrated that the active galactic nucleus OJ 287 has intensity peaks that recur within a period of about 12 yr. I suggest that this is the result of the sweeping of a precessing relativistic beam across our line of sight. In analogy to Hercules X-1 and SS 433, precession is attributed to the torque exerted by a companion mass on an accretion disk. Secondary maxima observed 1.2 yr after two of these peaks may be evidence of nodding motion.

Katz, J. I.↗

NASA NTRS1996

The Long and the Short of Gamma-Ray Bursts

We report evidence from the Third BATSE Gamma-Ray Burst Catalog that long (T(sub 90) greater than 10 s) and short (T(sub 90) less than 10 s) gamma-ray bursts (GRBS) represent distinct source populations. Their spatial distributions are significantly different, with long bursts having = 0.282+/- 0.014 but short bursts having = 0.385 +/- 0.019, differing by 0.103 +/- 0.024, significant at the 4.3 or level. 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 milliseconds, consistent with the shortest observed timescales in GRBS. We briefly investigate the parameters of clusters in which neutron star collisions may occur, and discuss the nuclear evolution of expelled and accelerated matter.

Katz, J. I.↗

NASA NTRS1994

Yet another model of soft gamma repeaters

We develop a model of soft gamma repeaters (SGRs) in which a supernova leaves planets orbiting a neutron star in intersecting orbits. These planets will collide in approximately 10(exp 4) yr if their orbits are coplanar. Some fragments of debris lose their angular momentum in the collision and fall onto the neutron star, producing a SGR. The initial accretion of matter left by the collision with essntially no angular momentum may produce a superburst like that of 1979 March 5, while debris fragments which later lose their angular momentum produce an irregular but non-Poissonian pattern of smaller bursts resembling those observed in spectrum and duration.

Katz, J. I.↗

NASA NTRS1994

Low-frequency spectra of gamma-ray bursts

Particles with energies below the mean energy E(sub 0) in relativistic shocked plasmas should assume an equilibrium energy distribution. This leads to a synchrotron spectrum F(sub nu) proportional to nu(exp 1/3) up to approximately the critical frequency nu(sub 0) of an electron with the energy E(sub 0). Application to gamma-ray bursts (GRBs) implies that a burst with 10(exp -5) ergs/sq cm/s of soft gamma rays and h nu(sub 0) = 300 keV should simultaneously be about 18th magnitude in visible light and a few micro-J at 1 GHz (less if self-absorbed); the low-freqency intensities peak later at higher values.

Katz, J. I.↗

NASA NTRS1994

Delayed hard photons from gamma-ray bursts

The delayed hard (up to 25 GeV) photons observed more than an hour following a gamma-ray burst on 1994 February 17 may result from the collisions of relativistic nucleons with a dense cloud, producing pi(0). The required cloud density is approx. 2 x 10(exp 11)/cu cm. This cloud may be the remains of the disrupted envelope of a neutron star, and may survive as an excretion disk of approx. 10(exp 14) - 10 (exp 15) cm radius around the coalescing binary.

Katz, J. I.↗

NASA NTRS1994

Two populations and models of gamma-ray bursts

Gamma-ray burst statistics are best explained by a source population at cosmological distances, while spectroscopy and intensity histories of some individual bursts imply an origin on Galactic neutron stars. To resolve this inconsistency I suggest the presence of two populations, one at cosmological distances and the other Galactic. I build on ideas of Shemi & Piran (1990) and of Rees & Meszaros (1992) involving the interaction of fireball debris with surrounding clouds to explain the observed intensity histories in burst at cosmological distances. The distances to the Galactic population are undetermined because they are two few to affect the statistics of intensity and direction; I explain them as resulting from magnetic reconnection in neutron star magnetospheres. An appendix describes the late evolution of the debris as a relativistic blast wave.

Katz, J. I.↗

NASA NTRS1994

Radio and optical emission: Spectral shapes and breaks in GRB

Relativistic blast wave models of Gamma Ray Bursts (GRB) 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 determines a characteristic (break) frequency of synchrotron radiation. At much lower frequencies, a spectrum F(sub nu) varies as nu(sup 1/3) is predicted independently of the details of the emitting region. This is consistent with the observed soft x ray emission of GRB. It implies low visible and radio intensities, unless there are collective emission processes.

Katz, J. I.↗

NASA NTRS1994

Relativistic shock spectra: A prediction

I argue that particles heated by relativistic shocks should assume an equilibrium energy distribution. This leads to a synchrotron spectrum F(sub nu) varies as nu(sup 1/3) up to approximately the critical frequency nu(sub 0) of an electron with the mean electron energy. Application to gamma ray bursts (GRB's) implies that a burst with 10(exp -5) erg/(sq cm s) of soft gamma-rays and h(nu(sub 0)) = 300 KeV should be about 18th magnitude in visible light and a few micro-Jy at 1 GHz (less if self-absorbed).

Katz, J. I.↗

NASA NTRS1993

Two populations and models of gamma ray bursts

Gamma-ray burst statistics are best explained by a source population at cosmological distances, while spectroscopy and intensity histories of some individual bursts imply an origin on Galactic neutron stars. To resolve this inconsistency I suggest the presence of two populations, one at cosmological distances and the other Galactic. I build on ideas of Shemi and Piran (1990) and of Rees and Mesozaros (1992) involving the interaction of fireball debris with surrounding clouds to explain the observed intensity histories in bursts at cosmological distances. The distances to the Galactic population are undetermined because they are too few to affect the statistics of intensity and direction; I explain them as resulting from magnetic reconnection in neutron star magnetospheres. An appendix describes the late evolution of the debris as a relativistic blast wave.

Katz, J. I.↗

NASA NTRS1993

Nova dust nucleation - Kinetics and photodissociation

Dust is observed to form in nova ejecta. The grain temperature is determined by the diluted nova radiation field rather than the gas kinetic temperature, making classical nucleation theory inapplicable. We used kinetic equations to calculate the growth of carbon nuclei in these ejecta. For expected values of the parameters too many clusters grew, despite the small sticking probability of atoms to small clusters, and the clusters only reached radii of about 100 A when the carbon vapor was depleted. We then included the effects of cluster photodissociation by ultraviolet radiation from the nova. This suppresses nucleation, but too well, and no grains form at all. Finally we suggest that a few growing carbon nuclei may be protected from photodissociation by a sacrificial surface layer of hydrogen.

Johnson, D. J.↗

NASA NTRS1992

Radiation transfer in gamma-ray bursts

We have calculated gamma-ray radiative transport in regions of high-energy density, such as gamma-ray burst source regions, using a discrete ordinate, discrete energy group method. The calculations include two-photon pair production and annihilation, as well as three-photon annihilation. The radiation fields itself acts as an absorbing medium, and the optical depth depends on its intensity, so the problem is intrinsically nonlinear. Spherical divergence produces effective collimation of the flux. At high optical depth the high energy (E is greater than 1 MeV) portion of the emergent spectrum assumes a nearly universal form. An approximate limit is derived for the high-energy flux from a gamma-ray burst source region of given size, and the implications of this limit for the distance to the 1979 March 5 event are briefly discussed. We discuss more generally the problem of very luminous bursts, and implications of Galactic halo distances for flare models.

Carrigan, B. J.↗

NASA NTRS1992

The Ptolemaic gamma-ray burst universe

The BATSE experiment on GRO has demonstrated the isotropic arrival directions and flat log N 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 disk. 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.

Katz, J. I.↗

NASA NTRS1992

Proton acceleration in neutron star magnetospheres

To explain the emission of TeV and PeV gamma rays from accreting X-ray binary sources, protons must be accelerated to several times the gamma-ray energy. It is shown here that at certain times, the plasma in the accretion column of the neutron star may form a deep enough pool that the top portion becomes unstable to convective motions in spite of the strong magnetic field. The resulting turbulence produces fluctuations in the strength of the magnetic field that travel up the accretion column, taking energy out to the region of the energetic protons. The protons resonantly absorb this energy and are accelerated to high energies. Including the synchrotron radiation losses of the protons, it is shown that they can be accelerated to energies that are high enough to explain the gamma-ray observations.

Smith, I. A.↗

NASA NTRS1989

Flash-photoionized nebulae

Under conditions of high radiation intensity and low gas density, recombination may be neglected in determining the ionization state of a photoionized gas. Calculations of the ionization structure of nebulae in this 'flash-photoionized' regime are reported. Very hard spectra of ionizing ultraviolet radiation may be produced by filtration of the ionizing flux through a neutral hydrogen layer which preferentially absorbs photons just above the hydrogen photoionization threshold. Fluxes with these hard spectra produce gas layer in which helium is largely doubly ionized while hydrogen is largely neutral. Such a layer leads to anomalously high ratios of He II to H I recombination line strengths. These results are applied to the problem of the spectrum of the arc in the cluster of galaxies A370. It is found that the spectrum may possibly be reconciled with the light echo model.

Katz, J. I.↗

NASA NTRS1988

Predictions for arc polarization

Maps for the prediction of the polarization of light echo arcs are presented. It is noted that the results can be compared with observations in order to test the light-echo model of Katz (1987) and Milgrom (1987) and to determine its parameters. As long as the model assumptions of single scattering and planar geometry remain valid, the polarization is shown to be independent of the geometrical thickness and the optical depth of the dust layer. Cases such as the light source being in the plane of scatterers and arcs consisting of nearly forward scattered light are considered.

Katz, J. I.↗

NASA NTRS1988

Particle acceleration in accreting magnetospheres

The possibility that energetic protons are accelerated within the closed magnetosphere of accelerating neutron stars is considered. The accelerating mechanism is suggested to be plasma turbulence excited by the accretion flow. Rough estimates show that this mechanism may be capable of accelerating protons to the energies of about 10 to the 16th eV required to explain observations of about 10 to the 15th eV gamma rays from some thermal X-ray sources. Proton synchrotron radiation may be observable at energies ranging from the infrared to about GeV gamma rays.

Katz, J. I.↗