An Intertwined Short and Long GRB with 4-minute Separation

Gamma-ray bursts (GRBs), among the most energetic events in the universe, are traditionally classified into long-duration GRBs, lasting over two seconds, and short-duration GRBs, lasting less than two seconds ¹ . Long GRBs are typically associated with the collapse of massive stars (Type II), while short GRBs arise from the merger of compact binary objects (Type I) ^2–5 . Due to their different origins, they exhibit distinct statistical properties in observations, which serve as valuable criteria for GRB classification ^{4, 6–11} . In this study, we report a peculiar gamma-ray burst, GRB 160425A, which consists of a short-sharp duration burst (G1) followed by a long-broad duration burst (G2) separated by only four minutes. In particular, multiple observational indicators, including duration, pulse shape ¹² , minimum variability ^{13, 14} , spectral index ¹⁵ , spectral lag ^{6, 16–18} , hardness ratio ^{19, 20} , EHD11 and ϵ 7 parameters, as well as the Amati and Norris correlations ^{6, 21} , all consistently categorize G1 as a Type-I merger-origin GRB and G2 as a Type-II collapsar-origin GRB. This coexistence of merger and collapsar signatures in a single event challenges existing progenitor models, necessitating a re-evaluation of GRB classification and progenitor scenarios.