Sub-burst-resolved Classification and Characteristics of Double-trigger Gamma-Ray Bursts

Abstract Over the past two decades, Swift and Fermi have detected a handful of gamma-ray bursts (GRBs) that produced two independent onboard triggers separated by a quiescent interval long enough for the trigger system to reset (“double-trigger” events). Consistent sky localizations indicate that the two triggers originated from the same astrophysical transient. Here we examine whether standard prompt-emission diagnostics remain stable across sub-bursts in confirmed double-trigger events, focusing on three well-observed cases: GRB 091024A, GRB 110709B, and GRB 220627A. Treating each triggered episode as an individual sub-burst, we compare its duration ( T 90 ), hardness ratio, minimum variability timescale (MVT), spectral lag ( τ lag ), peak energy ( E p ), and energetics with those of typical long-duration GRBs (LGRBs) and with single-trigger LGRBs that exhibit extended quiescent periods. We find that all sub-bursts are phenomenologically consistent with the LGRB population across the adopted diagnostics. We therefore interpret confirmed double-trigger GRBs not as a physically distinct burst class but as observationally selected, clean realizations of long-quiescent, multiepisode long GRBs. These results suggest that LGRB central engines can undergo extended dormancy and subsequent reactivation.