*2.3. Ultra-Long GRBs and X-ray Flashes*

Furthermore, the detection of rare events characterized by extremely long-lived prompt emissions lasting & 10<sup>3</sup> s, named ultra-long GRBs (ULGRBs), represents an additional classification threat, since it is still unclear whether ULGRBs represent a distinct class of LGRBs [18], or whether they are the high-end tail of the *t*<sup>90</sup> distribution of LGRBs [19].

Finally, it has been reported the existence of extragalactic transient X-ray sources, dubbed X-ray flashes (XRFs), with spatial distribution, spectral, and temporal characteristics similar to LGRBs [20,21]. The distinguishing properties of XRFs are


In view of these hazy results, classifying GRBs through *t*<sup>90</sup> and HR criteria only turns out to be puzzling, since the measured *t*<sup>90</sup> varies with energy range. Thus, the definition of a novel GRB classification scheme requires multi-wavelength criteria to better understand the physical properties behind the GRB emission.

In this respect, attempts to recategorize GRBs from the popular long/short classes have been made in Ref. [22], introducing alternative classes of *Type I* and *Type II GRBs*. According to this scheme, the Type I class includes short/hard GRBs and SGRBEEs with no SN association, typically found in regions of their host galaxy with low star formation, and very likely originating in compact star mergers (see details in the next Section 2.4). On the other hand, the Type II class includes long and relatively soft GRBs with SN association, usually found in star forming regions within irregular host galaxies, and thus associated with young stellar populations and likely originating in the core-collapses of massive stars (again, see details in the next Section 2.4).

Though the above scheme seems to be promising, further research on this issue is still ongoing. Therefore, for historical reasons in Section 2.4, we pursue the description of the progenitor systems keeping the bimodal classification in LGRBs and SGRBs.
