*2.2. Light Curve Properties*

Prompt GRB light curves show erratic behavior, and so far no common model has been accepted that would fully describe the observed behavior. The duration of emission is associated with the timescale on which the inner engine producing a GRB operates, while the temporal variability reflects its variations in time [46] (though other sources of the observed variability have been proposed, e.g., local relativistic turbulence [47], see below). Broadly, we distinguish two classes of events, short and long GRBs with the dividing line at T90∼2 s [48], where T<sup>90</sup> refers to the time in which 5% to 95% of the counts in the 50–300 keV band is accumulated.

It has been recognized that the dividing line of T90∼2 s depends on the specific gammaray detector used, thus additional information must be used to determine if a GRB is "long" or "short" (e.g., [49,50]). This has a theoretical implication: there is strong evidence that "long" GRBs are associated with the collapse of a massive star (the so-called "collapsar" model [51,52]). This evidence is based on the association of long GRBs with core-collapse supernova and thus massive star progenitors [53,54]. Short GRBs, on the other hand, are believed to be associated with the merger of two compact objects [55]. This idea has been proved by the association of the gravitational wave event GW170817 with a GRB (although this GRB may be atypical [1–3]).

In a small number of short GRBs, there is evidence of an extended emission lasting tens of seconds after the short initial spike [56–58], whose origin is still debated. Extended emission from short GRBs was also observed by the *Fermi* LAT at energies >100 MeV, e.g., in GRB 090510 or GRB 170127C [44].

The observed intrinsic variability during the prompt GRB emission can be rather short, down to ∼tens of millisecond timescale or lower [59,60]. It poses a major constraint on prompt emission models, as the short timescale on which the observed signal can vary in the simplest models is given by *δT*∼*R*/(*c*Γ 2 ) [61] (*R* is a typical radius of the emitting region and the Γ is the jet bulk Lorentz factor). For GRBs with LAT detection, short timescale variability during the prompt phase can be found in a handful of bursts, e.g., GRB 131108A [44] and GRB 170214A [62].

## *2.3. Polarization*

The leading models of the non-thermal emission, namely synchrotron emission and Compton scattering, both produce highly polarized emission [63]. However, in order to observe such a polarized signal, one has to break the spherical symmetry, which seems easier during the later time afterglow phase, due to lateral expansion of the slowing-down jet. Indeed, the first claimed detection of polarization signal was during the afterglow phase [64,65]. For a recent comprehensive study of polarization during the prompt phase for different scenarios see, e.g., [66].

High degree of linear polarization was claimed for several bursts, detected by different instruments: RHESSI, BATSE, and Integral [67–72]. Significant linear polarization was detected by the GAP instrument on *board IKAROS satellite* [73,74] for several GRBs: 100826A (<sup>Π</sup> <sup>=</sup> <sup>27</sup> <sup>±</sup> 11%), 110301A (<sup>Π</sup> <sup>=</sup> <sup>70</sup> <sup>±</sup> 22%), and GRB100826A (<sup>Π</sup> <sup>=</sup> <sup>84</sup>+<sup>16</sup> <sup>−</sup>28%), in all cases with more than 2.9 *σ* confidence.

In recent years, there have been dedicated missions to study GRB polarization, such as the Indian-led *ASTROSAT*, which reported several highly polarized signals detected by the CZTI instrument [75–77]. A second dedicated instrument is the POLAR detector [78]. The key result is that, while in many GRBs the time-integrated polarized signal is very low, there are rapid changes in the polarized signal, indicating the need for a time-resolved analysis, in which the signal is much more pronounced.
