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11 - Jets and gamma-ray burst unification schemes

Published online by Cambridge University Press:  05 December 2012

Jonathan Granot
Affiliation:
31 Rupin Street, Rehovot 76345, Israel
Enrico Ramirez-Ruiz
Affiliation:
Astronomy and Astrophysics Department, University of California, Santa Cruz, CA 95064, USA
Chryssa Kouveliotou
Affiliation:
NASA-Marshall Space Flight Center, Huntsville
Ralph A. M. J. Wijers
Affiliation:
Universiteit van Amsterdam
Stan Woosley
Affiliation:
University of California, Santa Cruz
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Summary

Evidence for bulk relativistic motion in gamma-ray bursts

The first line of evidence for ultrarelativistic bulk motion of the outflows that produce GRBs arises from the compactness argument. It relies on the observed short and intense pulses of gamma rays and their non-thermal energy spectrum that often extends up to high photon energies. Together, these facts imply that the emitting region must be moving relativistically. In order to understand this better, let us first consider a source that is either at rest or moves at a Newtonian velocity, β≡ v/c ≪1, corresponding to a bulk Lorentz factor Γ (1 – β2)−1/2 ≈ 1. For such a source the observed variability timescale (e.g., the width of the observed pulses) ∆t, implies a typical source size or radius R <c∆t, due to light time travel effects (for simplicity we ignore here cosmological effects, such as redshift or time of dilation). GRBs often show significant variability down to millisecond timescales, implying R<3 × 107(∆t/ 1 ms) cm. At cosmological distances their isotropic equivalent luminosity, L, is typically in the range of 1050–1053 erg s−1. In addition, the (observed part of the) εFε GRB spectrum typically peaks around a dimensionless photon energy of ε≡ Eph/mec2 ˜ 1, so that (for a Newtonian source) a good fraction of the total radiated energy is carried by photons that can pair produce with other photons of similar energy. (F is the radiative flux and Fε ≡ dF/dε.)

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