arXiv subjects

J. I. Katz

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

120 records · Page 7Linked to original sources

arXiv1995

Long and Short GRB

We report evidence from the 3B Catalogue that short (T_90 < 10 s) and long (T_90 > 10 s) GRB represent different populations and processes: Their spectral behavior is qualitatively different, with short bursts harder in the BATSE range, but chiefly long bursts detected at higher photon energies; \langle V/V_max \rangle = 0.385 \pm 0.019 for short GRB but \langle V/V_max \rangle = 0.282 \pm 0.014 for long GRB, differing by 0.103 \pm 0.024. Long GRB may be the consequence of accretion-induced collapse, but this mechanism fails for short GRB, for which we suggest colliding neutron stars.

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arXiv1994

The Eddington Limit and Soft Gamma Repeaters

Observed intensities and inferred distances of soft gamma repeaters imply luminosities in excess of the nominal (electron-scattering opacity) Eddington limit by four to six orders of magnitude. I review the physical basis of this limit. Accretional luminosities may exceed it if energy is hydrodynamically coupled from accreting matter to closed field lines where it forms a pair gas. This magnetically confined pair gas radiates roughly a black body spectrum with $k_B T_e \approx 23$ KeV, consistent with observations, at a luminosity up to $\sim 3 \times 10^{44}$ erg s$^{-1}$ for a surface field of $10^{13}$ gauss. Magnetic transparency is not required. I discuss the minutes-long continuing emission of March 5, 1979, steady counterparts to SGR, their spin periods and the recoil problem.

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arXiv1994

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 February 17, 1994 may result from the collisions of relativistic nucleons with a dense cloud, producing $π^0$. The required cloud density is $\sim 2 \times 10^{11}$ cm$^{-3}$. This cloud may be the remains of the disrupted envelope of a neutron star, and may survive as an excretion disc of $\sim 10^{14}$--$10^{15}$ cm radius around the coalescing binary.

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arXiv1994

Yet Another Model of Soft Gamma Repeaters

We develop a model of SGR in which a supernova leaves planets orbiting a neutron star in intersecting eccentric orbits. These planets will collide in $\sim 10^4$ years 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 essentially no angular momentum may produce a superburst like that of March 5, 1979, while debris fragments which later lose their angular momentum produce an irregular pattern of smaller bursts.

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arXiv1994

Low Frequency Spectra of Gamma-Ray Bursts

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 $F_ν\propto ν^{1/3}$ up to approximately the critical frequency $ν_0$ 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 $h ν_0 = 300$ KeV should be about 18th magnitude in visible light and a few $μ$Jy at 1 GHz (less if self-absorbed).

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arXiv1993

Radio and Optical Emission, Spectral Shapes and Breaks in GRB

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 $F_nu \propto ν^{1/3}$ 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.

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arXiv1992

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 Mészáros and Rees (1993) 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.

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arXiv1992

A Burst of Speculation

Self-consistent models of gamma-ray burst source regions at 100 Kpc distance are possible if the radiating plasma is confined to very thin sheets, and I estimate parameters. Energy sources might be elastic (by starquakes) or magnetic (by reconnection), but mechanisms remain obscure. I discuss a very speculative model involving collisions between comets in a hypothetical inner Oort cloud.

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arXiv1992

Particle Acceleration in (by) Accretion Discs

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 $π^0$ and by bremsstrahlung and Compton processes following the decay of $π^\pm$ to $e^\pm$.

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arXiv1992

The Ptolemaic Gamma-Ray Burst Universe

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.

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arXiv1992

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 pair annihilation. The radiation field 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 ($E > 1$ 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 March 5, 1979 event are briefly discussed. We discuss more generally the problem of very luminous bursts, and implications of Galactic halo distances for flare models.

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arXiv1992

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 100Å 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.

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