2017ApJ...837..128W

Broad-lined type Ic supernovae (SNe Ic-BL) are peculiar stellar explosions that are distinct from ordinary SNe. Some SNe Ic-BL are associated with long-duration ( 2 s) gamma-ray bursts (GRBs). Black holes and magnetars are two types of compact objects that are hypothesized to be central engines of GRBs. In spite of decades of investigations, no direct evidence for the formation of black holes or magnetars has yet been found for GRBs. Here we report the finding that the early peak (t 50 days) and late-time (t 300 days) slow decay displayed in the light curves of SNe 1998bw (associated with GRB 980425) and 2002ap (not GRB-associated) can be attributed to magnetar spin-down with an initial rotation period P₀∼20 ms, while the intermediate-time (50 t 300 days) exponential decline is caused by the radioactive decay of ⁵⁶Ni. The connection between the early peak and late-time slow decline in the light curves is unexpected in alternative models. We thus suggest that GRB 980425 and SN 2002ap were powered by magnetars.