Optical Flash of GRB 990123: Constraints on the Physical Parameters of the Reverse Shock

Abstract

The optical flash accompanying GRB 990123 is believed to be powered by the reverse shock of a thin shell. With the best fitted physical parameters for GRB 990123 (Panaitescu & Kumar 2001) and the assumption that the parameters in the optical flash are the same as those in the afterglow, we show that: 1) the shell is thick but not thin, and we have provided the light curve for the thick shell case which coincides with the observation; 2) the theoretical peak flux of the optical flash accounts for only 3 × 10 −4 of the observed. In order to compensate this divergency, the physical parameters the electron energy ration and the magnetic ratio ǫ e , ǫ B should be 0.61, 0.39 respectively, which are much different from those in the late afterglow.

Open questions in GRB physics

“…PIC simulations [206] suggest that for a relativistic (Γ 5) magnetized (σ > 0.03) shock, particle acceleration is possible only within a narrow range of magnetic inclination angles ( 34 o /Γ). On the other hand, the reverse shock model to interpret early optical flashes require that the reverse shock is more magnetized than the forward shock (with σ close to 0.1) [166,165,167]. A ∼ 10% linear polarization degree was measured in the early optical afterglow for GRB 090102 [169], which is consistent with emission from a magnetized reverse shock.…”

“…The bright optical flashes seen in several GRBs (e.g. GRBs 990123, 021211, 061126) require that the RS region is much more magnetized than the FS region, suggesting that the engine is carrying a strong magnetic field [165,166,167,168], with a σ close to (but does not exceed) unity. An early optical polarimetry observation of GRB 090102 revealed a 10 ± 1% polarization degree of emission during the early steep decay phase believed to be of the RS origin [169].…”

The role of kink instability in Poynting-flux dominated jets

“…6 is that, for σ 0 > ∼ 100, the flow remains Poynting-flux dominated even at large distances away from the source where deceleration of the outflow because of its interaction with the interstellar medium or the stellar wind is expected, which means that the instability is not fast enough to convert most of the magnetic energy into bulk motion of matter. Afterglow observations can in principle probe to the magnetic content of the ejecta through early observations of the reverse shock emission (Fan et al 2002;Zhang et al 2003;Kumar & Panaitescu 2003). Modeling of the forward and reverse shock emission in cases where quick follow ups were possible suggests the existence of frozen-in magnetic fields in the ejecta (Kumar & Panaitescu 2003) that are dynamically important, with σ > ∼ 0.1 (Zhang & Kobayashi 2005).…”

Evaluating the Bulk Lorentz Factors of Outflow Material: Lessons Learned from the Extremely Energetic Outburst GRB 160625B

“…More efforts are needed to identify the origin of this flash (one possibility is that such a flash was triggered by the main GRB outflow ejected at t ∼ 186 s catching up with the decelerated outflow material ejected at t ∼ 0 s). If it was originated from the reverse shock of the outflow, the Lorentz factor can be measured in another way for GRB 160625B (Sari & Piran 1999;Wang et al 2000;Soderberg & Ramirez-Ruiz 2002;Fan et al 2002;Zhang et al 2003). The detailed modeling of such a component is beyond the scope of this work.…”