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Register a new userRelativistic Bullets Ejection from Supernovae and Generation of Gamma Ray Bursts
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Hideyuki Umeda
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It is generally believed that cosmological Gamma Ray Bursts (GRBs) are produced by the deceleration of relativistic objects with Lorentz factor (Gamma) >~ 100. We study the possibility that some GRBs are produced along with relativistic matter ejection from supernovae. In this model, it is quite likely that the matter has to travel through the progenitors thick envelope before generating GRBs. Under the assumption that the ejected matter is described as a single collective matter, we obtain constraints on the matter to have Gamma >~ 100 at the breakout of the progenitor. One advantage of considering this type of model is that the expected GRB energy is sufficiently large, in contrast to the GRB generation model by the shock breakout in the energetic supernova explosion. We find that in general the cross section of the matter has to be very small compared with the progenitors radius and thus the matter has to be bullet (or jet)-like rather than shell-like.
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The connection between long GRBs and supernovae is now well established. I briefly review the evidence in favor of this connection and summarise where we are observationally. I also use a few events to exemplify what should be done and what type of data are needed. I also look at what we can learn from looking at SNe not associated with GRBs and see how GRBs fit into the broad picture of stellar explosions.
Einsteins general relativity predicts that pressure, in general stresses, play a similar role to energy density in generating gravity. The source of gravitational field, the active gravitational mass density, sometimes referred to as Whittakers mass density, is not conserved, hence its changes can propagate as monopole gravitational waves. Such waves can be generated only by astrophysical sources with varying gravitational mass. Here we show that relativistic fireballs, considered in modelling gamma-ray burst phenomena, are likely to radiate monopole gravitational waves from high-pressure plasma with varying Whittakers mass. Also, ejection of a significant amount of initial mass-energy of the progenitor contributes to the monopole gravitational radiation. We identify monopole waves with h^11+h^22 waves of Eddingtons classification which propagate (in the z-direction) together with the energy carried by massless fields. We show that the monopole waves satisfy Einsteins equations, with a common stress-energy tensor for massless fields. The polarization mode of monopole waves is Phi_22, i.e. these are perpendicular waves which induce changes of the radius of a circle of test particles only (breathing mode). The astrophysical importance of monopole gravitational waves is discussed.
The gravitational wave (GW) memory from a radiating and decelerating point mass is studied in detail. It is found that for isotropic photon emission the memory generated from the photons is essentially the same with the memory from the point mass that radiated the photons so that it is anti-beamed. On the other hand, for anisotropic emission the memory from the photons may have a non-vanishing amplitude even if it is seen with small viewing angles. In the decelerating phases of gamma-ray burst (GRB) jets the kinetic energy of the jet is converted into the energy of gamma-ray photons. Then it would be possible to observe a variation in the GW memory associated with GRB jets on the timescale of the gamma-ray emission if the emission is partially anisotropic. Such an anisotropy in the gamma-ray emission has been suggested by the polarizations detected in recent observations of GRBs. The GW memory from GRB jets would provide clues to clarifying the geometry of the jets and the emission mechanism in GRBs. Thus it will be an interesting target for the next generation detectors of the GWs.
Using a two-dimensional hydrodynamics code (PROMETHEUS), we study the continued evolution of rotating massive helium stars whose iron core collapse does not produce a successful outgoing shock, but instead forms a black hole. We study the formation of a disk, the associated flow patterns, and the accretion rate for disk viscosity parameter, alpha ~ 0.001 and 0.1. For the standard 14 solar mass model the average accretion rate for 15 s is 0.07 solar masses per second and the total energy deposited along the rotational axes by neutrino annihilation is (1 - 14) x 10**51 erg, depending upon the evolution of the Kerr parameter and uncertain neutrino efficiencies. Simulated deposition of this energy in the polar regions results in strong relativistic outflow - jets beamed to about 1.5% of the sky. The jets remain highly focused, and are capable of penetrating the star in 5 - 10 s. After the jet breaks through the surface of the star, highly relativistic flow can commence. Because of the sensitivity of the mass ejection and jets to accretion rate, angular momentum, and disk viscosity, and the variation of observational consequences with viewing angle, a large range of outcomes is possible ranging from bright GRBs like GRB 971214 to faint GRB-supernovae like SN 1998bw. X-ray precursors are also possible as the jet first breaks out of the star. While only a small fraction of supernovae make GRBs, we predict that all GRBs longer than a few seconds will make supernovae similar to SN 1998bw. However, hard, energetic GRBs shorter than a few seconds will be difficult to make in this model.
We present time-dependent axisymmetric magnetohydrodynamic simulations of the interaction of a relativistic magnetized wind produced by a proto-magnetar with a surrounding stellar envelope, in the first $sim 10$ seconds after core collapse. We inject a super-magnetosonic wind with $dot E = 10^{51}$ ergs s$^{-1}$ into a cavity created by an outgoing supernova shock. A strong toroidal magnetic field builds up in the bubble of plasma and magnetic field that is at first inertially confined by the progenitor star. This drives a jet out along the polar axis of the star, even though the star and the magnetar wind are each spherically symmetric. The jet has the properties needed to produce a long-duration gamma-ray burst (GRB). At $sim 5$ s after core bounce, the jet has escaped the host star and the Lorentz factor of the material in the jet at large radii $sim 10^{11}$ cm is similar to that in the magnetar wind near the source. Most of the spindown power of the central magnetar escapes via the relativistic jet. There are fluctuations in the Lorentz factor and energy flux in the jet on $sim 0.01-0.1$ second timescale. These may contribute to variability in GRB emission (e.g., via internal shocks).
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