150 research outputs found
On the Structure of Quasi-Universal Jets for Gamma-Ray Bursts
The idea that GRBs originate from uniform jets has been used to explain
numerous observations of breaks in the GRB afterglow lightcurves. We explore
the possibility that GRBs instead originate from a structured jet that may be
quasi-universal, where the variation in the observed properties of GRBs is due
to the variation in the observer viewing angle. We test how various models
reproduce the jet data of Bloom, Frail, & Kulkarni (2003), which show a
negative correlation between the isotropic energy output and the inferred jet
opening angle (in a uniform jet configuration). We find, consistent with
previous studies, that a power-law structure for the jet energy as a function
of angle gives a good description. However, a Gaussian jet structure can also
reproduce the data well, particularly if the parameters of the Gaussian are
allowed some scatter. We place limits on the scatter of the parameters in both
the Gaussian and power-law models needed to reproduce the data, and discuss how
future observations will better distinguish between these models for the GRB
jet structure. In particular, the Gaussian model predicts a turnover at small
opening angles and in some cases a sharp cutoff at large angles, the former of
which may already have been observed. We also discuss the predictions each
model makes for the observed luminosity function of GRBs and compare these
predictions with the existing data.Comment: 13 pages, including 10 figures; To appear in Ap
A Hybrid Simulated Annealing Heuristic for Multistage Heterogeneous Fleet Scheduling with Fleet Sizing Decisions
This paper deals with multistage heterogeneous fleet scheduling with fleet sizing decisions (MHFS-FSD). This MHFS-FSD attempts to integrate vehicles allocation and fleet sizing decisions considering the vehicle routing of multiple vehicle types. The problem is formulated as mixed integer programming model. The matrix formulation denoting vehicle allocation scheme is explored according to the characteristic of this problem. Generating vehicle allocation scheme with greedy heuristic procedure (VA-GHP) as initial solution of problem is presented. The USP-IVA method to update the initial solution generated by VA-GHP approach is developed. And then, incorporating VA-GHP and USP-IVA into simulated annealing algorithm, a novel heuristic called HSAH-GHP&IVA is proposed. Finally, some experiments are designed to test the proposed heuristic and the results show that the heuristic can generate reasonably good solutions in short CPU times.
Document type: Articl
Early afterglow, magnetized central engine, and a quasi-universal jet configuration for long GRBs
Two separate topics are discussed. (1) We describe the classifications of the
long GRB early afterglow lightcurves within the framework of the fireball shock
model, focusing on the interplay between the reverse and forward shock emission
components. We will also provide evidence that the central engine of at least
two bursts are entrained with strong magnetic fields, and discuss the
implications of this result for our understanding of the GRB phenomenon; (2) We
argue that the current gamma-ray burst (GRB) and X-ray flash (XRF) data are
consistent with a picture that all GRB-XRF jets are structured and
quasi-universal, with a typical Gaussian-like jet structure.Comment: 5 pages, 8 figures, to appear in the Proceedings of Santa Fe GRB
Conference (talk by Zhang at the meeting
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