GRB B: Prompt Emission from Internal Forward-Reverse Shocks Yun-Wei Yu 1,2, X. Y. Wang 1, & Z. G. Dai 1 （俞云伟，王祥玉，戴子高） 1 Department of Astronomy, Nanjing University 2 Institute of Astrophysics, Huazhong Normal University

 The synchrotron and synchrotron self-Compton scenario may satisfy these tow requirements (Kumar & Panaitascu 2008).  In the popular internal shock model, paired forward and reverse shocks are generated by collisions of relativistic shells simultaneously.  Alternatively, the two-component synchrotron emission produced by these two types of shocks may account for the prompt optical and gamma-ray emissions of GRB 80319B. Yu, Wang & Dai (2008, arXiv: ): Racusin et al. (2008) 1, Temporal coincidence 2, Optical excess GRB B Forward shocks Reverse shocks

1 Dynamics and electron distribution Unshocked shell 4 Unshocked shell Reverse shockForward shock Contact discontinuity (CD) surface The structure of the internal forward-reverse shocks shell 1shell 4 Shocked regions

1432 i =1, 4 L k,1 ~ L k,4 L k,1 g 1 L k,4 g 4

For GRB B the reverse shock is relativistic, while the forward shock is possibly Newtonian. Writing, can get

The characteristic energy of the reverse-shocked electrons is much higher than the one of the forward-shocked electrons. The resulting synchrotron photons would peak at two different energy bands. For GRB B, the reverse shock is responsible for the prompt gamma-ray emission, while the forward shock contributes to the optical component.

If we assume the magnetic field maintains a steady value throughout the shocked region, we would get a synchrotron spectrum with a spectral slope F n ∝ n -1/2 below 100 keV, which is in contradiction to the much harder spectra observed (Ghisellini et al. 2000). To overcome this problem, Ghisellini et al. (2000) and Pe’er & Zhang (2006) suggested that the magnetic field created by a shock could decay on a length scale much shorter than the comoving width of the shocked region, i.e.,

2 Emission The reverse shock and prompt gamma-ray emission

The forward shock and prompt optical emission (Vestrand et al. 2008; Kumar & Panaitascu 2008) <1

Application to GRB B

IC emission SSC EIC

Conclusion The temporal coincidence implies both emissions could originate from the same dynamical process, but the significant excess of the optical flux requires two different emission origins. The MeV gamma-ray emission results from relativistic reverse shocks while the optical emission from non- relativistic forward shocks. Highly relativistic reverse shocks are required for GRB B. Within the observed optical and MeV gamma-ray bands, the synchrotron emission is the dominant component. A high energy (sub-GeV or GeV) emission (EIC) component is predicted, the flux of which is lower than or at most comparable to that of the synchrotron MeV emission.