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 ($τ_{\rm lag}$), 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.

Figures (7)

• Figure 1: Prompt $\gamma$-ray emission phase of GRB 091024A.a, The background-subtracted light curves of GRB 091024A in distinct energy bands (15-25 keV: green, 25-50 keV: magnetic, 50-100 keV: cyan, 100-350 keV: yellow, 15-350 keV: black), derived from Swift/BAT data. b,$T_{90}$-$f(f_{\rm eff})$ plot, where $f$ represents the ratio between the peak flux and the average background flux of a GRB (Methods). $f_{\rm eff}$ is the effective $f$ parameter, indicating how a long GRB can be disguised as a short GRB by arbitrarily lowering its flux level. The three distinct bursts of GRB 091024A are highlighted by the red ($G_1$), orange ($G_2$) and cyan ($G_3$) solid stars, respectively.
• Figure 2: GRB 091024A in traditional GRB classification scheme.a-f, The traditional GRB classification is illustrated based on the duration/hardness ratio diagram (a), the duration/MVT diagram (b), the Amati relation (c), the Norris relation (d), the duration/EHD diagram (e), and the duration/$\xi$ diagram (f). The Swift (or Fermi) GRB sample is represented by solid yellow and cyan points for the Type I (short) and Type II (long) burst populations, respectively. The three individual bursts of GRB 091024A are highlighted by red, orange and cyan stars. Elliptical dotted lines in different colors indicate the 1$\sigma$ and 2$\sigma$ regions of the bivariate normal distributions for the Type-I (short) and Type-II (long) burst populations. Bivariate distributions, using kernel density estimation, are shown in different shaded regions for the Type I (short) and Type II (long) burst populations. The traditional separation line ($t_{90}=2$ s) between short and long GRBs is indicated by a black dashed vertical line. Duration is plotted in the source frame for a, e, f and the observed frame for b.
• Figure 3: Same as Figure \ref{['fig:091024_lc']} but for GRB 110709B.
• Figure 4: Same as Figure \ref{['fig:091024A_Classification']} but for GRB 110709B.
• Figure 5: Same as Figure \ref{['fig:091024_lc']} but for GRB 220627A.