ar X iv : a st ro - p h / 99 08 27 3 v 1 2 5 A ug 1 99 9 Can the Jet steepen the light curves of GRB afterglows ?
نویسنده
چکیده
Received ; accepted – 2 – ABSTRACT Beaming of relativistic ejecta in GRBs has been postulated by many authors in order to reduce the total GRB energy, thus it is very important to look for the observational evidence of beaming. Rhoads (1999) has pointed out that the dynamics of the blast wave, which is formed when the beamed ejecta sweeping the external medium, will be significantly modified by the sideways expansion due to the increased swept up matter. He claimed that shortly after the bulk Lorentz factor (Γ) of the blast wave drops below the inverse of the initial opening angle (θ 0) of the beamed ejecta, there will be a sharp break in the afterglow light curves. However, some other authors have performed numerical calculations and shown that the break of the light curve is weaker and much smoother than the one analytically predicted. In this paper we reanalyse the dynamical evolution of the jet blast wave, calculate the jet emission analytically, and we find that the sharp break predicted by Rhoads is usually not exist unless the beaming angle is extremely small. A prominent break may occur when Γ −1 ≃ θ 0 if the sideways expansion is not important. We also discuss the situation that the observer is located off the jet axis, and find that when Γ −1 < θ (θ is the angle between line of sight and the jet axis) the observed flux will increase rapidly with time (F ∝ T −3(α−1)/4+3)), and when γ −1 > θ the observed flux will decay with time as usual.
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#NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035-1000 ∗Department of Astronomy, University of California, Berkeley, CA 94720 †Hubble Fellow, California Institute of Technology, MS 105-24, Pasadena, CA 91125 ‡Dominion Astrophysical Observatory, 5071 W. Saanich Rd., Victoria, B.C., Canada V8X 4M6 §Department of Astronomy, P.O. Box 208101, Yale University, New Haven, CT 06520 ‖Observa...
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تاریخ انتشار 1999