Interpreting the X-ray afterglows of gamma-ray bursts with radiative losses and millisecond magnetars

TL;DR

This paper develops an advanced model for X-ray afterglows of gamma-ray bursts, incorporating radiative losses and magnetar spin-down, which better explains observed data and predicts X-ray flares and plateau diversity.

Contribution

It introduces a comprehensive model including radiative losses and arbitrary braking index, improving understanding of magnetar spin-down effects on gamma-ray burst afterglows.

Findings

Model fits observed X-ray afterglow data better than previous models.

Predicts the occurrence of X-ray flares in gamma-ray bursts.

Measures the braking index, indicating gravitational-wave dominated spin-down.

Abstract

The spin-down energy of millisecond magnetars has been invoked to explain X-ray afterglow observations of a significant fraction of short and long gamma-ray bursts. Here, we extend models previously introduced in the literature, incorporating radiative losses with the spin down of a magnetar central engine through an arbitrary braking index. Combining this with a model for the tail of the prompt emission, we show that our model can better explain the data than millisecond-magnetar models without radiative losses or those that invoke spin down solely through vacuum dipole radiation. We find that our model predicts a subset of X-ray flares seen in some gamma-ray bursts. We can further explain the diversity of X-ray plateaus by altering the radiative efficiency and measure the braking index of newly-born millisecond magnetars. We measure the braking index of GRB061121 as $n=4.85^{+0.11}_{-0.15}$ suggesting the millisecond-magnetar born in this gamma-ray burst spins down predominantly through gravitational-wave emission.