Abstract:
Identification of gamma-ray burst (GRB) progenitors based on the duration of their prompt emission (T90) has
faced several roadblocks recently. Long-duration GRBs (with T90 > 2 s) have traditionally been thought to be
originating from the collapse of massive stars and the short-duration ones (with T90 < 2 s) from compact
binary mergers. However, recent observations of a long GRB associated with a kilonova (KN) and a short
GRB with supernova association demand a more detailed classification of the GRB population. In this Letter,
we focus on GRBs associated with KNe, believed to be originating from mergers of binaries involving neutron
stars (NSs). We make use of the GRB prompt-emission light curves of the Swift/BAT 2022 GRB catalog and
employ machine-learning algorithms to study the classification of GRB progenitors. Our analysis reveals that
there are five distinct clusters of GRBs, of which the KN-associated GRBs are located in two separate clusters,
indicating they may have been produced by different progenitors. We argue that these clusters may be due to
subclasses of binary neutron star and/or NS–black hole mergers. We also discuss the implications of these
findings for future gravitational-wave observations and how those observations may help in understanding
these clusters better.
