We describe and discuss the global properties of 45 gamma-ray bursts (GRBs) observed by HETE-2 during the first three years of its mission, focusing on the properties of X-Ray Flashes (XRFs) and X-ray-rich GRBs (XRRs). We find that the numbers of XRFs, XRRs, and GRBs are comparable. We find that the

Gamma-ray bursts (GRBs) fall into two classes: short-hard and long-soft bursts. The latter are now known to have X-ray and optical afterglows, to occur at cosmological distances in star-forming galaxies, and to be associated with the explosion of massive stars. In contrast, the distance scale, the energy scale and the progenitors of the short bursts have remained a mystery. Here we report the discovery of a short-hard burst whose accurate localization has led to follow-up observations that have identified the X-ray afterglow and (for the first time) the optical afterglow of a short-hard burst; this in turn led to the identification of the host galaxy of the burst as a late-type galaxy at z = 0.16 (ref. 10). These results show that at least some short-hard bursts occur at cosmological distances in the outskirts of galaxies, and are likely to be caused by the merging of compact binaries.

HETE-2 has provided strong evidence that the properties of X-Ray Flashes (XRFs), X-ray-rich GRBs, and GRBs form a continuum, and therefore that these three kinds of bursts are the same phenomenon. A key feature found by HETE-2 is that the density of bursts is roughly constant per logarithmic interval in burst fluence S E and observed spectral peak energy E obs peak , and in isotropic-equivalent energy E iso and spectral peak energy E peak in the rest frame of the burst. In this paper, we explore a unified jet model of all three kinds of bursts, using population synthesis simulations of the bursts and detailed modeling of the instruments that detect them. We show that both a variable jet opening-angle model in which the emissivity is a constant independent of the angle relative to the jet axis and a universal jet model in which the emissivity is a power-law function of the angle relative to the jet axis can explain the observed properties of GRBs reasonably well. However, if one tries to account for the properties of XRFs, X-ray-rich GRBs, and GRBs in a unified picture, the extra degree of freedom available in the variable jet opening-angle model enables it to explain the observations reasonably well while the power-law universal jet model cannot. The variable jet opening-angle model of XRFs, X-ray-rich GRBs, and GRBs implies that the energy E γ radiated in gamma rays is ∼ 100 times less than has been thought. The model also implies that most GRBs have very small jet opening angles (∼ half a degree). This suggests that magnetic fields may play an important role in GRB jets. It also implies that there are ∼ 10 4 − 10 5 more bursts with very small jet opening angles for every burst that is observable. If this is the case, the rate of GRBs could be comparable to the rate of Type Ic core collapse supernovae. These results show that XRFs may provide unique information about the structure of GRB jets, the rate of GRBs, and the nature of Type Ic supernovae.Subject headings: gamma rays: bursts -supernovae: general -ISM: jets and outflows -shock waves 1 We define "X-ray-rich" GRBs and XRFs as those events for which log[S X (2−30 kev)/S γ (30−400 kev)] > −0.5 and 0.0, respectively.2. We assume that, for most GRBs, E iso and E peak obey the relation (Lloyd-Ronning, Petrosian & Mallozzi 2000;Amati et al. 2002; Lamb et al. 2004):with a modest scatter, and that this relation holds for XRFs and X-ray-rich GRBs, as well as for GRBs.3. We assume that the observed ranges of ∼ 10 5 in E iso and L iso are due either to differences in the jet opening angle θ jet (in the variable jet opening-angle model) or to differences in the viewing angle θ view of the observer with respect to the axis of the jet (in the universal jet model). Simulations of Observed Gamma-Ray Bursts Overview of the SimulationsWe begin by giving an overview of our population synthesis simulations of observed GRBs before describing the simulations in mathematical detail. Our overall approach is to simulate the GRBs that are observed by different instruments by (1) mo...

Magnetic fields are ubiquitous in the Universe. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter. The standard theoretical model for the origin of these strong magnetic fields is through the amplification of tiny seed fields via turbulent dynamo to the level consistent with current observations. However, experimental demonstration of the turbulent dynamo mechanism has remained elusive, since it requires plasma conditions that are extremely hard to re-create in terrestrial laboratories. Here we demonstrate, using laser-produced colliding plasma flows, that turbulence is indeed capable of rapidly amplifying seed fields to near equipartition with the turbulent fluid motions. These results support the notion that turbulent dynamo is a viable mechanism responsible for the observed present-day magnetization.

We report the results of a series of three-dimensional (3D) simulations of the deflagration phase of the gravitationally confined detonation mechanism for Type Ia supernovae. In this mechanism, ignition occurs at one or several off-center points, resulting in a burning bubble of hot ash that rises rapidly, breaks through the surface of the star, and collides at a point opposite the breakout on the stellar surface. We find that detonation conditions are robustly reached in our 3D simulations for a range of initial conditions and resolutions. Detonation conditions are achieved as the result of an inwardly directed jet that is produced by the compression of unburnt surface material when the surface flow collides with itself. A high-velocity outwardly directed jet is also produced. The initial conditions explored in this paper lead to conditions at detonation that can be expected to produce large amounts of 56 Ni and small amounts of intermediate-mass elements. These particular simulations are therefore relevant only to high-luminosity Type Ia supernovae. Recent observations of Type Ia supernovae imply a compositional structure that is qualitatively consistent with that expected from these simulations.