Will GRB 990123 Perform an Encore?

The recent gamma ray burst, GRB 990123, has an absorption redshift z_s=1.60, implying an apparent energy $E \ge 3 \times 10^{54} erg$, and a peak luminosity $L_{max} \ge 6 \times10^{53}erg/s$, assuming isotropic emission. This energy is ten times larger than hitherto measured and in excess of the rest mass of the sun. Optical observations have revealed an associated galaxy displaced from the line of sight by $\sim 0.6''$. This raises the possibility that the burst is enhanced by gravitational lensing. We argue that existing observations probably only allow magnifications $\mu>400$ if the galaxy is at z_d=1.60 and the burst originates at much higher redshift. It should be possible to exclude this possibility by examining the burst time structure. If, as we anticipate, multiple imaging can be excluded, GRB 990123 remains the most intrinsically luminous event yet observed in its entirety.Comment: Accepted by MNRAS: 4 pages, latex, no figure

Making Clean Energy with a Kerr Black Hole: a Tokamak Model for Gamma-Ray Bursts

In this paper we present a model for making clean energy with a Kerr black hole. Consider a Kerr black hole with a dense plasma torus spinning around it. A toroidal electric current flows on the surface of the torus, which generates a poloidal magnetic field outside the torus. On the surface of the tours the magnetic field is parallel to the surface. The closed magnetic field lines winding around the torus compress and confine the plasma in the torus, as in the case of tokamaks. Though it is unclear if such a model is stable, we look into the consequences if the model is stable. If the magnetic field is strong enough, the baryonic contamination from the plasma in the torus is greatly suppressed by the magnetic confinement and a clean magnetosphere of electron-positron pairs is built up around the black hole. Since there are no open magnetic field lines threading the torus and no accretion, the power of the torus is zero. If some magnetic field lines threading the black hole are open and connect with loads, clean energy can be extracted from the Kerr black hole by the Blandford-Znajek mechanism. The model may be relevant to gamma-ray bursts. The energy in the Poynting flux produced by the Blandford-Znajek mechanism is converted into the kinetic energy of the electron-positron pairs in the magnetosphere around the black hole, which generates two oppositely directed jets of electron-positron pairs with super-high bulk Lorentz factors. The jets collide and interact with the interstellar medium, which may produce gamma-ray bursts and the afterglows.Comment: 14 pages, 1 figure, accepted by Ap

To the Lighthouse

The extreme hypothesis that essentially all types of ultrarelativistic outflow -- specifically AGN jets, pulsar wind nebulae and GRB --are electromagnetic, rather than gas dynamical, phenomena is considered. Electromagnetic flows are naturally anisotropic and self-collimating so as to produce jet-like features. The relativistic force-free description of these flows, which is simpler than the relativistic MHD description, is explained. It is suggested that the magnetic field associated with AGN jets and GRB is quite extensively distributed in latitude, without necessarily increasing by much the total power. It is also proposed that the observed emission from these sources traces out regions of high current density where global instabilities drive a turbulence spectrum that is ultimately responsible for the particle acceleration and the synchrotron, inverse Compton and synchro-Compton emission. The direct extraction of spin energy from a black hole is re-examined and an electromagnetic model of GRB explosions is developed. It is also suggested that some GRB ``lighthouses'' be identified with accretion-induced collapse of a neutron star to form a black hole in a binary system.Comment: 25 pages, 2 figures. To appear in "Lighthouses of the Universe" Proc. Symposium held in Garching, Germany (Aug 6-9 2001) ed. M. Gilfanov, R. Sunyaev et al. Berlin:Springer. Revised version, correcting minor error

Current Issues

Cosmic explosions are observed in many astrophysical environments. They range in scale from hydromagnetic instabilities in the terrestrial magnetotail and solar ``nanoflares'' to cosmological gamma ray bursts, supernovae and the protracted intervals of nuclear activity that produce the giant quasars and radio galaxies. There are many parallels in the analyses of the explosion sites that are highlighted at this workshop, specifically stellar coronae, accretion disks, supernovae and compact objects. In this introductory talk, some general issues are discussed and some more specific questions relating to the individual sites are raised.Comment: To appear in Cosmic Explosions: Proc. 10th Maryland Conference on Astrophysics. Ed. S. Holt and W. Zhang AI

Gravitational arcs as a perturbation of the perfect ring

The image of a point situated at the center of a circularly symmetric potential is a perfect circle. The perturbative effect of non-symmetrical potential terms is to displace and break the perfect circle. These 2 effects, displacement and breaking are directly related to the Taylor expansion of the perturbation at first order on the circle. The numerical accuracy of this perturbative approach is tested in the case of an elliptical potential with a core radius. The contour of the images and the caustics lines are well re-produced by the perturbative approach. These results suggests that the modeling of arcs, and in particular of tangential arcs may be simplified by using a general perturbative representation on the circle. An interesting feature of the perturbative approach, is that the equation of the caustic line depends only on the values on the circle of the lens displacement field along the $\theta$ direction.Comment: 9 pages, 2 figure

Black Holes and Relativistic Jets

There is strong observational evidence that AGN, Galactic X-ray transients and (probably) $\gamma$-ray bursts are associated with black holes, and that these sources are able to form collimated, ultrarelativistic outflows. There is much interest in trying to understand how these prime movers are able to release energy from accreting gas and their own spin energy. Electromagnetic field plays a large role in many of the mechanisms under active consideration. In this article, several of the many possible ``metabolic pathways'' through which mass, angular momentum and energy can flow around and away from black hole magnetospheres are discussed. Particular importance is attached to the interactions between the inflowing disk, the outflowing wind, the black hole and the jet. Some important unresolved questions are identified and it is argued that large scale numerical computation will almost certainly be necessary to address them.Comment: 21 pages Progress of Theoretical Physics Supplement in pres