# Fallback Accretion onto a Newborn Magnetar: short GRBs with Extended Emission

## TL;DR

This study investigates how fallback accretion onto a newborn magnetar influences the light curves of short gamma-ray bursts with extended emission, improving model fits and explaining observed features.

## Contribution

It integrates fallback accretion into the magnetar propeller model and demonstrates its effectiveness in fitting observed SGRB extended emission data.

## Key findings

- Fallback accretion improves light curve fits for SGRBs with extended emission.
- Model parameters align with magnetar and fallback accretion predictions.
- Fallback accretion explains variability, flares, and long plateaux in SGRB light curves.

## Abstract

There are a subset of short gamma-ray bursts (SGRBs) which exhibit a rebrightening in their high-energy light curves known as extended emission. These bursts have the potential to discern between various models proposed to describe SGRBs as any model needs to account for extended emission. In this paper, we combine fallback accretion into the magnetar propeller model and investigate the morphological changes fallback accretion has on model light curves and fit to the afterglows of 15 SGRBs exhibiting extended emission from the Swift archive. We have parameterised the fallback in terms of existing parameters within the propeller model and solved for the disc mass and angular frequency of the magnetar over time. We then apply a Markov chain Monte Carlo routine to produce fits to the data. We present fits to our extended emission SGRB sample that are morphologically and energetically consistent with the data provided by Swift BAT and XRT telescopes. The parameters derived from these fits are consistent with predictions for magnetar properties and fallback accretion models. Fallback accretion provides a noticeable improvement to the fits of the light curves of SGRBs with extended emission when compared to previous work and could play an important role in explaining features such as variability, flares and long dipole plateaux.