Unveiling the progenitors of a population of likely peculiar GRBs

Traditionally, gamma-ray bursts (GRBs) are classified as long and short GRBs, with $T_{90} = 2$ s being the threshold duration. Generally, long-duration GRBs (LGRBs, $T_{90}>2$ s) are associated with the collapse of massive stars, and short-duration (SGRBs, $T_{90}<2$ s) are associated with the compact binary mergers involving at least one neutron star. However, the existence of a population of so-called ``peculiar GRBs", i.e., LGRBs originating from mergers, or long Type I GRBs, and SGRBs originating from collapsars, or short Type II GRBs, have challenged the traditional paradigm of GRB classification. Finding more peculiar GRBs may help to give us more insight into this issue. In this work, we analyze the properties of machine learning identified long Type I GRBs and short Type II GRBs candidates, long GRBs-I and short GRBs-II (the so-called ``peculiar GRBs"). We find that long GRBs-I almost always exhibit properties similar to Type I, which suggests that the merger may indeed produce GRBs with $T_{90}>2$ s. Furthermore, according to the probability given by the redshift distribution, short GRBs-II almost exhibit properties similar to Type II. This suggests that the populations of short Type II GRBs are not scarce and that they are hidden in a large number of samples without redshifts, which is unfavorable to the interpretation that the jet progression leads to a missed main emission.
Comment: 20 pages, 7 figures, Accepted for publication in the ApJ