An Intertwined Short and Long GRB with 4-minute Separation

Abstract

Gamma-ray bursts (GRBs), among the most energetic transients in the Universe, are traditionally classified into long-duration GRBs (lasting more than two seconds) and short-duration GRBs (lasting less than two seconds)\cite{Kouveliotou1993}. Long-duration GRBs are typically associated with the core collapse of massive stars (Type II), whereas short-duration GRBs originate from the merger of compact binary systems (Type I)\cite{Woosley2006, Zhang2006Natur, Zhang2009b, Berger2014}. Owing to their distinct physical origins, the two classes exhibit markedly different observational properties, which serve as key diagnostic criteria for GRB classification\cite{Norris2000, Zhang2009b, Lv2010, Lv2014, Qin2013, Li2016, Minaev2020}. Here we report a peculiar gamma-ray burst, GRB 160425A, comprising a short-sharp duration burst ($G_1$) followed by a long-broad duration burst ($G_2$), separated by only four minutes. Strikingly, nearly all standard prompt-emission observational diagnostics, including pulse morphology\cite{Norris2005}, duration\cite{Kouveliotou1993}, hardness ratio \cite{Horvath2010, Goldstein2017}, minimum variability timescale\cite{Golkhou2014, Golkhou2015}, spectral properties \cite{Dezalay1992}, spectral lag\cite{Norris2000,Norris2006, Yi2006, Bernardini2015}, and established empirical correlations (the Amati and Norris relations \cite{Amati2002, Norris2000}), consistently categorize $G_1$ as a short-like (Type-I, merger-origin) GRB and $G_2$ as a long-like (Type-II, collapsar-origin) GRB. The coexistence of merger and collapsar signatures within a single event challenges existing progenitor frameworks, calling for a fundamental re-evaluation of GRB classification schemes and progenitor scenarios.

Figures (3)

• Figure 1: Prompt $\gamma$-ray emission phase of GRB 160425A.a, The background-subtracted light curves of GRB 160425A 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,$T_{90}$ analysis for the two individual bursts of GRB 160425A. The right panel displays the light curve across the full BAT energy range (15-350 keV), while the left panel shows the corresponding cumulative counts over time. c, GRB 160425A in the distribution of quiescent period $t_{\rm gap}$ in comparison with other significant quiescent gamma-ray bursts. d,$T_{90}$-$f(f_{\rm eff})$ 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-duration GRB can be disguised as a short-duration GRB by arbitrarily lowering its flux level. The two distinct bursts of GRB 160425A are highlighted by the red ($G_1$) and orange ($G_2$) solid stars, respectively.
• Figure 2: GRB classification scheme.a-b, The traditional GRB classification is illustrated based on the duration/hardness ratio diagram (a), and the duration/MVT diagram (b). The Swift GRB sample is represented by solid yellow and cyan points for the short-duration and long-duration burst populations, respectively. The two individual bursts of GRB 160425A are highlighted by red triangular and orange stars. Elliptical dotted lines in different colors indicate the 1$\sigma$ and 2$\sigma$ regions of the bivariate normal distributions for the short-duration and long-duration burst populations. The traditional separation line ($t_{90}=2$ s) between short-duration and long-duration GRBs is indicated by a black dashed vertical line. Duration is plotted in the observed frame for a and the source frame for b. Spectral lag analysis (c-e).c, Background-subtracted Swift light curves of GRB 160425A in different energy bands, used to calculate spectral lags. d-e, Energy-dependent spectral lag between the lowest energy band (15-25 keV) and higher energy band for the short-duration burst ($G_1$) and the long-duration burst ($G_2$) of GRB 160425A.
• Figure 3: Images of GRB 160425A and its host galaxy.Figure (a) shows the survey data from DESI Legacy Surveys DR10, ref.Dey2019AJ, where the region with a knot structure at the center represents the host galaxy of GRB 160425A. Figures (b) through (e) present observations of the central celestial region in Figure (a), captured by different optical telescopes at various times. Specifically, Figure (b) depicts observations by the GROND telescope on April 26, Figure (c) shows observations by the FORS telescope of the Very Large Telescope (VLT) on September 3, revealing three diffuse knots, and Figure (d) presents data from Swift-UVOT on April 25. Figure (e1) illustrates observations by the VLT on May 13, while Figure (e2) displays the same region as Figure (e1) with the background galaxies subtracted, revealing the optical image of the GRB. Figure (f) provides observations of soft X-rays by Swift-XRT on April 25, showcasing a larger field of view compared to the images mentioned above.