In the field of GRBs, evidence of PFD jets has been collected independently in several directions. First, a prominent thermal emission component as expected in the fireball-internal-shock model (e.g., Mészáros & Rees 2000) has been seen only in a small fraction of GRBs (e.g., GRB 090902B, Ryde et al. 2010; Zhang et al. 2011). The majority of GRBs either show no evidence of a thermal component or a weak, sub-dominant thermal component (e.g., Abdo et al. 2009; Guiriec et al. 2011; Axelsson et al. 2012). These GRBs require that the GRB central engine is highly magnetized, and jet is still PFD at the emission site (Zhang & Pe’er 2009; Gao & Zhang 2015). Next, strong linear polarization was discovered during the prompt gamma-ray emission phase for some GRBs (Yonetoku et al. 2011, 2012), and during the reverse-shock-dominated early optical afterglow emission phase for some others (Steele et al. 2009; Mundell et al. 2013), which hint at the existence of globally ordered magnetic fields in the jet. Furthermore, strong PeV neutrino emission as predicted by the MFD models has not been observed from GRBs so far (Abbasi et al. 2012), which is consistent with the expectation of the PFD models (Zhang & Kumar 2013). Finally, the MFD internal shock (IS) model for GRBs also suffers some criticisms, such as low energy dissipation efficiency (Kumar 1999; Panaitescu et al. 1999), electron fast cooling (Ghisellini et al. 2000), the electron number excess (Bykov & Mészáros 1996; Daigne & Mochkovitch 1998; Shen & Zhang 2009), and inconsistency with some empirical (Amati/Yonetoku) relations (Zhang & Mészáros 2002; Liang et al. 2010). Zhang & Yan (2011) proposed a novel PFD outflow model named as “the Internal-Collision-induced MAgnetic Reconnection and Turbulence (ICMART),” which can potentially keep the merits of the IS model but alleviate the criticisms faced by the IS model mentioned above. The main idea of the ICMART model is that the GRB jets are PFD. The Poynting flux is catastrophically discharged at a relatively large distance (e.g., 1015 cm) from the central engine through collision-induced magnetic reconnection. The magnetic energy is converted to particle energy and radiation efficiently, leading to a very high radiation efficiency as demanded by the GRB data (Panaitescu & Kumar 2002; Zhang et al. 2007). A PFD jet has less leptons than the MFD model so that the electron excess problem is avoided. A large emission radius favors a moderately fast cooling, which can account for the right low-energy spectral index observed in GRBs (Uhm & Zhang 2014). It also gives a natural explanation of the seconds-duration of “slow variability component” observed in GRBs (Gao et al. 2012). The rapid “fast variability component” can be interpreted within this scenario as mini jets due to locally Lorentz boosted regions (see also Lyutikov & Blandford 2003; Narayan & Kumar 2009)3

3
Lyutikov & Blandford (2003) and Narayan & Kumar (2009) proposed that GRB variability is a consequence of mini jets due to relativistic outflow from reconnection or relativitic turbulence. There is no simple explanation to the observed slow variability component in these models. Zhang & Yan (2011) attributed the two variability components (slow and fast) as due to central engine activity and mini jets, respectively. Monte Carlo simulations by Zhang & Zhang (2014) showed that the ICMART model can indeed reproduce the observed GRB light curves.
. It is speculated that turbulent reconnection in a moderately high-σ flow can give rise to relativistic motion of mini jets within the bulk relativistic motion of the jets.
Citation: Liang et al. 2010
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