Testing the Magnetar Model via Late Time Radio Observations of Two Macronova Candidates

TL;DR

This study used late-time radio observations of two macronova candidates to test the magnetar model, finding no radio emission and thus ruling out magnetars as the central engine in these cases.

Contribution

It provides the first observational constraints on the magnetar model for these specific macronova candidates through radio non-detections.

Findings

No radio emission detected from either candidate.

Strong upper limits set on ejecta energy and mass.

Results exclude magnetars as central engines in these events.

Abstract

Compact binary mergers may have already been observed as they are the leading model for short gamma-ray bursts (sGRBs). Radioactive decay within the ejecta from these mergers is expected to produce an infra-red flare, dubbed macronova (or kilonova), on a time scale of a week. Recently two such macronova candidates were identified in followup observations of sGRBs, strengthening the possibility that those indeed arise from mergers. The same ejecta will also produce a long term (months to years) radio emission due to its interaction with the surrounding ISM. In search for this emission, we observed the two macronova candidates, GRB 130603B and GRB 060614 with the Jansky very large array (VLA) and the Australia Telescope Compact Array (ATCA). Our observations resulted in null-detections, putting strong upper limits on the kinetic energy and mass of the ejecta. A possible outcome of a merger is a highly magnetized neutron star (a magnetar), which has been suggested as the central engine for GRBs. Such a magnetar will deposit a significant fraction of its energy into the ejecta leading to a brighter radio flare. Our results, therefore, rule out magnetars in these two events.