Classification and Characteristics of Double-trigger Gamma-ray Bursts

Abstract

Over the past two decades, the \textit{Swift} and \textit{Fermi} missions have identified a rare class of ``double-trigger'' gamma-ray bursts (GRBs) that produce two independent trigger events. These events are characterized by a sufficiently long quiescent period during which the on-board trigger system can reset, resulting in the subsequent emission being recorded as a second independent event. Consistent sky localization confirms that both trigger events originated from the same astrophysical source. Here, we present a systematic classification and characteristics study of three such cases: GRB 091024A, GRB 110709B, and GRB 220627A. We investigate each trigger episode emission independently using standard classification diagnostics, including duration ($T_{90}$), hardness ratio, minimum variability timescale (MVT), spectral lag ($\tau_{\rm lag}$), peak energy ($E_{\rm p}$), and energetics. We compare these properties with those of typical long GRBs (LGRBs) and with single-trigger LGRBs that exhibit extended quiescent periods. Our analysis reveals that all sub-bursts from the three double-trigger events consistently lie within the LGRB classification region. These results indicate that double-trigger GRBs are not a physically distinct subclass, but rather products of LGRB central engines that undergo extended dormancy and subsequent reactivation.