A possible Macronova in the late afterglow of the `long-short' burst GRB 060614

TL;DR

This paper reports the discovery of an infrared bump in GRB 060614's afterglow, suggesting it originated from a compact binary merger and possibly produced heavy r-process elements, challenging the typical massive star origin of long GRBs.

Contribution

It provides evidence for a macronova in a long-duration GRB, indicating a merger origin rather than a supernova, which is a novel insight into GRB progenitors.

Findings

Infrared bump inconsistent with supernova emission.

Supports a merger origin for GRB 060614.

Indicates significant production of heavy r-process elements.

Abstract

Long-duration ($>2$ s) $\gamma$-ray bursts that are believed to originate from the death of massive stars are expected to be accompanied by supernovae. GRB 060614, that lasted 102 s, lacks a supernova-like emission down to very stringent limits and its physical origin is still debated. Here we report the discovery of near-infrared bump that is significantly above the regular decaying afterglow. This red bump is inconsistent with even the weakest known supernova. However, it can arise from a Li-Paczy\'{n}ski macronova $-$ the radioactive decay of debris following a compact binary merger. If this interpretation is correct GRB 060614 arose from a compact binary merger rather than from the death of a massive star and it was a site of a significant production of heavy r-process elements. The significant ejected mass favors a black hole-neutron star merger but a double neutron star merger cannot be ruled out.