GRB 080319B: Prompt Emission from Internal Forward-Reverse ShocksYun-Wei Yu1,2, X. Y. Wang1, & Z. G. Dai1

（俞云伟，王祥玉，戴子高）1 Department of Astronomy, Nanjing University2 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:0806.2010):

Racusin et al. (2008)

1, Temporal coincidence

2, Optical excessGRB 080319B

Forward shocks

Reverse

shocks

1 Dynamics and electron distribution

Unshocked shell 4

Unshocked shell 123

Reverse shock Forward shockContact discontinuity (CD) surface

The structure of the internal forward-reverse shocks

shell 1shell 4

Shocked regions

14 3 2

i =1, 4 Lk,1~ Lk,4

Lk,

1

1

Lk,

4

4

For GRB 080319B

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 080319B , 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∝-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 080319B

IC emission SSC

EIC

Conclusion The temporal coincidence implies both emissions could o

riginate 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 080319B.

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.