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
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.,
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.