Evidence for magnetar formation in broad-lined type Ic supernovae 1998bw and 2002ap

TL;DR

This paper presents evidence that magnetars, rather than black holes, powered the light curves of two broad-lined type Ic supernovae, including one associated with a gamma-ray burst, based on analysis of their early and late-time luminosity decay patterns.

Contribution

It provides the first direct evidence linking magnetar formation to broad-lined type Ic supernovae, including those associated with gamma-ray bursts.

Findings

Early peak and late-time slow decay in light curves attributed to magnetar spin-down.

Intermediate linear decline caused by radioactive decay of 56Ni.

Both supernovae likely powered by magnetars with initial spin periods around 20 ms.

Abstract

Broad-lined type Ic supernovae (SNe Ic-BL) are peculiar stellar explosions that distinguish themselves from ordinary SNe. Some SNe Ic-BL are associated with long-duration (\gtrsim 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 been found for GRBs so far. Here we report the finding that the early peak (t \lesssim 50 days) and late-time (t \gtrsim 300 days) slow decay displayed in the light curves of both SNe 1998bw (associated with GRB 980425) and 2002ap (not GRB-associated) can be attributed to magnetar spin-down with initial rotation period P0 \sim 20 ms, while the intermediate-time (50 \lesssim t \lesssim 300 days) linear decline is caused by 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.