Outflow Residual Collisions and Optical Flashes Zhuo Li （黎卓） Weizmann Inst, Israel moving to Peking Univ, Beijing Li & Waxman 2008, ApJL

Internal shock model Residual collisions in internal shock model Compared with observations (naked eye GRB b)

GRBs Multiple peaks; 10 ’ s s Rapid variability: down to ~1 ms Non-thermal (broken PL) spectra Energy peak ~ MeV Extend to >10 GeV in some EGRET bursts Compact objects Ultra relativistic expansions

Internal shocks: Compact source; relativistic, fluctuating outflow ~10 13 cm X O Meszaros 03

If MeV gamma-ray emission dominates energy, MeV gamma-rays originated from synchrotron mechanism In internal shock model, the magnetic field is in equipartition value –Characteristic frequency The outflow is optically thick to optical photons –Absorption frequency No prompt optical emission expected –in contrast with recent observational results (Waxman 03) (Li & Waxman 08) (Derishev 02)

GRB Galama et al (1999)

GRB a Vestrand et al (2005)

GRB a Vestrand et al (2006)

Optical flashes inconsistent with internal shock model? Large radius production? Simultaneity? What after gamma-ray production at small radii?

Residual collisions: direct results of internal shock model Velocity fluctuation, shell number, decreasing with time First generation collisions: gamma rays What about later residual collisions?

Simple case: merging-shell model A sequence of N>>1 equal-mass shells; separated by c X t var ; random distribution of LFs, variance<mean LF. Variance of (cm frame) velocities and (obs frame) LFs of merged groups are decreasing as: Fluctuation energy: Typical radiation frequency: Considering IC scattering MeV photons and syn absorption:

MC simulation: dropping sticking assumption Evolution of an outflow composed of equal-mass shells N=10 3, initially separated by c x 1 ms, with random Lorentz factors 300 x 3 y, where y is normally distributed with zero mean and unit standarddeviation. In each collision, 1/3 of the internal energy generated is radiated.

Observations spectral indices between the optical and gamma-ray bands are in the range of 0 – 0.5 (Yost et al 07) corresponding to a ratio of optical to gamma ray flux 1– optical energy is a tiny fraction of gamma ray energy –10 -3 Theory is in consistence

Naked-eye GRB b The energy ratio of optical to gamma-ray is –10 -3, consistent with other GRBs in Yost et al (07) The optical decay time scale, a few seconds, much longer than that in gamma-ray band, consistent with different sizes of emission regions Racusin et al (2008)

Compared to other authors One-zone model predicts gamma-optical correlation –Inconsistent with observation (Kumar & Panaitescu 08; Yu et al 08) SSC model predicts strong GeV emission by IC scattering MeV gamma rays (Kumar & Panaitescu 08) –Suggest huge radiation energy (~1E55erg!) or efficiency.

Remarks Optical flashes of GRBs are consistent with internal (residual) shocks. Inverse Compton emission is not dominant, but “smear” the pair production spectral cutoff. GRB b inconsistent with magnetic field dominated outflow model.