2025MNRAS.540L...6Q

Long gamma-ray bursts (LGRBs) are typically thought to result from the collapse of massive stars. None the less, recent observations of gamma-ray bursts (GRBs) 211211A and 230307A, coupled with the low-redshift excess of LGRB event rates relative to star formation rates, present significant challenges to the prevailing model. We re-examine the selection criteria for higher redshift-complete GRB samples and identify 280 Swift GRBs with peak flux over $2.6 \ \text{ph} \, \text{cm}^{-2} \, \text{s}^{-1}$. Assuming that all LGRBs with $z \ge 2$ originate from collapsars, we construct the GRB luminosity functions in three scenarios: no evolution, luminosity evolution, and density evolution. Our results indicate that a strong redshift evolution in luminosity ($\delta = 1.87^{+0.27}_{-0.31}$) or in density ($\delta = 1.10^{+0.21}_{-0.20}$) is necessary. The luminosity/density evolution model predicts 72.67/57.28 collapsar GRBs at $z < 2$, which can account for 67.29 per cent/53.04 per cent of the observed LGRBs. This suggests that a substantial portion of LGRBs at $z< 2$ may not be collapsar GRBs, which would challenge the universality of empirical GRB relations and affect their reliability in cosmological applications.