Studies on the emission from the receding jet of GRB Xin Wang, Y. F. Huang, and S. W. Kong Department of Astronomy, Nanjing University, China A&A submitted (arXiv: v2)

Contents 1.Introduction 2.Model 3.Numerical Results 4.Comparisons 5.Conclusion

 The standard fireball-shock model  Beaming effect: achromatic break in afterglow LCs, polarization, energy crisis, orphan afterglow Introduction XOXO ~10 13 cm >10 16 cm M é sz á ros 03 Detailed numerical investigation on the counter-jet emission is still missing

Model  We use the convenient generic dynamical model advanced by Huang et al (2000).  The physical picture is that the homogeneous double-sided jet expands into a homogeneous interstellar medium (ISM).  Dynamics:  Radiation: synchrotron radiation & self-absorption

Numerical Results 1.Dynamic evolution of the receding jet 2.Total equal arrival time surface (EATS) 3.The overall light curves (LCs) considering the contribution from the receding jet component 4.The effects of various parameters on the receding jet component 5.Different characteristics for the twin jets Firstly, we assume the twin jets have the same characteristics. The parameters of the “standard” condition are defined as follows:

Dynamic evolution of the receding jet jet break time is determined by in a rather long observer’s time (t ～ 100 d), γ of the receding jet remains almost constant the emission from the receding jet will be very weak in this period, since it is highly beamed backwardly.

EATS for the receding jet branch has much smaller typical radius much flatter curvature much smaller area as compared with those for the forward jet branch at the same observer time. Total equal arrival time surface (EATS)  Due to that speed of light is not infinite, photons received at an observer time t obs are emitted from a distorted ellipsoid not simultaneously, which is determined by

(c)(d) The overall light curves LCs of the double-sided jet with two parameters altered compared with the “standard” condition, i.e. n=1000/cm 3 and z=0.1 (d L =454.8 Mpc).

The effects of various parameters All 8.46 GHz LCs

Different characteristics for twin jets 8.46GHz LCs. In each panel, the solid line is plotted under the “standard” condition, i.e., the parameters are completely the same for the twin jets (but we have evaluated as 0.01 and as here). For other LCs, one or two parameters are changed for the receding jet only.

Comparisons  with theoretical derivations  with other numerical results Zhang & MacFadyen (2009) Our results are consistent with other colleagues’. Li & Song (2004) Granot & Loeb (2003)

Conclusion 1.It is found that the contribution from the receding jet is quite weak and only manifests as a plateau (~0.1 for 8.46 GHz) typically, which is hard to detect. 2.At lower frequency, the relative intensity of the receding jet component becomes stronger, as compared with the peak flux of the forward jet component. 3.Generally, our result is consistent with Zhang & MacFadyen’s and Li & Song’s. However the subtle difference between our numerical results and Li & Song’s analytical derivations is ascribed to the EATS effect and the deceleration of the external shock. 4.Contribution from the receding jet can be greatly enhanced if the circum-burst environment is very dense and/or the micro-physics parameters of receding jet is different and/or the burst has a low redshift. Thus we suggest that nearby GRBs (z ≤ 0.1) be good candidates for observations.

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