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Starting as highly relativistic collimated jets, gamma-ray burst outflows gradually slow down and become nonrelativistic spherical blast waves. Although detailed analytical solutions describing the afterglow emission received by an on-axis observer during both the early and late phases of the outflow evolution exist, a calculation of the received flux during the intermediate phase and for an off-axis observer requires either a more simplified analytical model or direct numerical simulations of the outflow dynamics. In this paper, we present light curves for off-axis observers covering the long-term evolution of the blast wave, calculated from a high-resolution two-dimensional relativistic hydrodynamics simulation using a synchrotron radiation model. We compare our results to earlier analytical work and calculate the consequence of the observer angle with respect to the jet axis both for the detection of orphan afterglows and for jet break fits to the observational data. We confirm earlier results in the literature finding that only a very small number of local type Ibc supernovae can harbor an orphan afterglow. For off-axis observers, the observable jet break can be delayed up to several weeks, potentially leading to overestimation of the beaming-corrected total energy. In addition we find that, when using our off-axis light curves to create synthetic Swift X-ray data, jet breaks are likely to remain hidden in the data.

A pulsar wind nebula (PWN) inside a supernova remnant provides a unique insight into the properties of the central neutron star, the relativistic wind powered by its loss of rotational energy, its progenitor supernova, and the surrounding environment. In this paper, we present a new semianalytic model for the evolution of such a PWN throughout its lifetime. This model couples the dynamical and radiative evolution of the PWNe, and predicts both the dynamical (e.g., radius and expansion velocity) and radiative (radio to TeV g-ray spectrum) properties of the PWN during this period. As a result, it is well suited for using the observed properties of a PWN to constrain the physical characteristics of the neutron star, pulsar wind, progenitor supernova, and surrounding environment. We also discuss the expected evolution for a particular set of these parameters, and show that it reproduces the large spectral break inferred from the radio and X-ray spectrum of many young PWNe, and the low break frequency, low radio luminosity, high TeV g-ray luminosity, and high magnetization observed for several older PWNe. The predicted spectrum of this PWN also contains spectral features which appear during different evolutionary phases detectable with new radio and g-ray observing facilities such as the Extended Very Large Array and the Fermi Gamma-ray Space Telescope. Finally, this model has implications for determining if PWNe can inject a sufficient number of energetic electrons and positrons into their surroundings to explain the recent measurements of the cosmic-ray positron fraction by PAMELA and the cosmic-ray lepton spectrum by ATIC and HESS.

Direct multidimensional numerical simulation is the most reliable approach for calculating the fluid dynamics and observational signatures of relativistic jets in gamma-ray bursts (GRBs). We present a two-dimensional relativistic hydrodynamic simulation of a GRB outflow during the afterglow phase, which uses the fifth-order weighted essentially nonoscillatory scheme and adaptive mesh refinement. Initially, the jet has a Lorentz factor of 20. We have followed its evolution up to 150 years. Using the hydrodynamic data, we calculate synchrotron radiation based upon standard afterglow models and compare our results with previous analytic work. We find that the sideways expansion of a relativistic GRB jet is a very slow process and previous analytic works have overestimated its rate. In our computed light curves, a very sharp jet break is seen and the postbreak light curves are steeper than analytic predictions. We find that the jet break in GRB afterglow light curves is mainly caused by the missing flux when the edge of the jet is observed. The outflow becomes nonrelativistic at the end of the Blandford-McKee phase. But it is still highly nonspherical, and it takes a rather long time for it to become a spherical Sedov-von Neumann-Taylor blast wave. We find that the late-time afterglows become increasingly flatter over time. But we disagree with the common notion that there is a sudden flattening in light curves due to the transition into the Sedov-von Neumann-Taylor solution. We have also found that there is a bump in light curves at very late times ([?]1000 days) due to radiation from the counter jet. We speculate that such a counter jet bump might have already been observed in GRB 980703.

The compact remnants of core-collapse supernovae--neutron stars and black holes--have properties that reflect both the structure of their stellar progenitors and the physics of the explosion. In particular, the masses of these remnants are sensitive to the density structure of the presupernova star and to the explosion energy. To a considerable extent, the final mass is determined by the "fallback," during the explosion, of matter that initially moves outward, yet ultimately fails to escape. We consider here the simulated explosion of a large number of massive stars (9-100 M) of Population I (solar metallicity) and III (zero metallicity) and find systematic differences in the remnant mass distributions. As pointed out by Chevalier, supernovae in more compact progenitor stars have stronger reverse shocks and experience more fallback. For Population III stars above about 25 M and explosion energies less than 1.5 x 1051 ergs, black holes are a common outcome, with masses that increase with increasing main-sequence mass up to a maximum hole mass, for very low explosion energy, of about 40 M. If such stars produce primary nitrogen, however, their black holes are systematically smaller. For modern supernovae with nearly solar metallicity, black hole production is much less frequent and the typical masses, which depend sensitively on explosion energy, are smaller. The maximum black hole mass is about 15 M. We explore the neutron star initial mass function for both populations and, for reasonable assumptions about the initial mass cut of the explosion, find good agreement with the average of observed masses of neutron stars in binaries. We also find evidence for a bimodal distribution of neutron star masses with a spike around 1.2 M (gravitational mass) and a broader distribution peaked around 1.4 M.

New two- and three-dimensional calculations are presented of relativistic jet propagation and break out in massive Wolf-Rayet stars. Such jets are thought to be responsible for gamma-ray bursts. As it erupts, the highly relativistic jet core (3deg to 5deg; G [?] 100) is surrounded by a cocoon of less energetic, but still moderately relativistic ejecta (G ~ 15) that expands and becomes visible at larger polar angles (~10deg). These less energetic ejecta may be the origin of X-ray flashes and other high-energy transients, which will be visible to a larger fraction of the sky, albeit to a shorter distance than common gamma-ray bursts. Jet stability is also examined in three-dimensional calculations. If the jet changes angle by more than 3deg in several seconds, it will dissipate, producing a broad beam with inadequate Lorentz factor to make a common gamma-ray burst. This may be an alternate way to make X-ray flashes.

We examine the propagation of two-dimensional relativistic jets through the stellar progenitor in the collapsar model for gamma-ray bursts (GRBs). Each jet is parameterized by a radius where it is introduced and by its initial Lorentz factor, opening angle, power, and internal energy. In agreement with previous studies, we find that relativistic jets are collimated by their passage through the stellar mantle. Starting with an initial half-angle of up to 20deg, they emerge with half-angles that, though variable with time, are around 5deg. Interaction of these jets with the star and their own cocoons also causes mixing that sporadically decelerates the flow. We speculate that this mixing instability is chiefly responsible for the variable Lorentz factor needed in the internal shock model and for the complex light curves seen in many GRBs. In all cases studied, the jet is shocked deep inside the star following a brief period of adiabatic expansion. This shock converts most of the jet's kinetic energy into internal energy, so even initially "cold" jets become hot after going a short distance. The jet that finally emerges from the star thus has a moderate Lorentz factor, modulated by mixing, and a very large internal energy. In a second series of calculations, we follow the escape of that sort of jet. Conversion of the remaining internal energy gives terminal Lorentz factors along the axis of approximately 150 for the initial conditions chosen. Because of the large ratio of internal to kinetic energy in both the jet ([?]80%) and its cocoon, the opening angle of the final jet is significantly greater than at breakout. A small amount of material emerges at large angles, but with a Lorentz factor still sufficiently large to make a weak GRB. This leads us to propose a "unified model" in which a variety of high-energy transients, ranging from X-ray flashes to "classic" GRBs, may be seen depending on the angle at which a standard collapsar is observed. We also speculate that the breakout of a relativistic jet and its collision with the stellar wind will produce a brief transient with properties similar to the class of "short-hard" GRBs. Implications of our calculations for GRB light curves, the luminosity-variability relation, and the GRB-supernova association are also discussed.

There is growing observational evidence that gamma-ray bursts (GRBs) are powered by black holes accreting rapidly through a disk. The supernova-like outburst that accompanies some gamma-ray bursts suggest that some long-duration GRBs may be driven by the accretion of a rotating stellar core onto a central black hole. Such a system can be produced when a compact remnant spirals into the core of its binary companion. During the inspiral, orbital angular momentum is injected into the helium core. By the time the compact remnant reaches the center of the helium core, it too has gained angular momentum as well as mass, producing a rapidly accreting black hole (or neutron star) at the center of a rotating stellar core. Whether or not such a merger (termed He-merger) can produce a GRB depends upon the initial mass and spin of the central black hole, as well as the angular momentum in the stellar core. In this paper, we use a three-dimensional smooth particle hydrodynamics code to follow the He-merger process and make quantitative estimates of the initial mass and spin of the central compact remnant, as well as the angular momentum in the accreting helium core. During the inspiral, a 2 M compact remnant gains ~0.5-3.5 M depending on the mass of the helium core (more massive cores provide more accretion). The accretion rates on the central remnant are initially very high, 105 M yr-1 up to 106 M yr-1 (the accretion rate increases with increasing helium star mass), and the central remnant quickly becomes a black hole if it was not one already. From these accretion rates, we estimate GRB explosion energies. In all mergers, magnetically driven jets are expected to produce GRB explosions with energies above 1051 ergs. For neutrino-annihilation-driven explosions, the GRB energy increases dramatically with helium star mass: the merger of a 2 M compact remnant with a 4 M helium star only produces a 1047 ergs explosion in ~500 s, whereas the merger of a 2 M compact remnant with a 16 M helium star produces a greater than 1052 ergs explosion in ~65 s.