# Magnetars in Ultra-Long Gamma-Ray Bursts

## TL;DR

This paper investigates the role of magnetars in powering ultra-long gamma-ray bursts and associated supernovae, providing evidence for a high-energy, wide-jet magnetar model based on broadband observations and afterglow analysis.

## Contribution

It offers new insights into the energy source of ULGRB 111209A and Supernova 2011kl, supporting the magnetar spin-down model with observational evidence for a high-density environment and a wide jet.

## Key findings

- High-density circumburst environment suggested by observations
- No jet break observed in afterglow for three weeks
- A magnetar with ~1 ms spin period can power the burst and supernova

## Abstract

Supernova 2011kl, associated with the ultra-long gamma-ray burst (ULGRB) 111209A, exhibited a higher-than-normal peak luminosity, placing it in the parameter space between regular supernovae and super-luminous supernovae. Its light curve can only be matched by an abnormally high fraction of $^{56}$Ni that appears inconsistent with the observed spectrum, and as a result it has been suggested that the supernova, and by extension the gamma-ray burst, are powered by the spin-down of a highly magnetised millisecond pulsar, known as a magnetar. We investigate the broadband observations of ULGRB 111209A, and find two independent measures that suggest a high density circumburst environment. However, the light curve of the GRB afterglow shows no evidence of a jet break (the steep decline that would be expected as the jet slows due to the resistance of the external medium) out to three weeks after trigger, implying a wide jet. Combined with the high isotropic energy of the burst, this implies that only a magnetar with a spin period of ~ 1 ms or faster can provide enough energy to power both ULGRB 111209A and Supernova 2011kl.