Fallback accretion on to a newborn magnetar: long GRBs with giant X-ray flares

TL;DR

This study models giant X-ray flares in long GRBs using a magnetar propeller model with fallback accretion, revealing energetic challenges and high efficiency requirements in explaining observed flare properties.

Contribution

It introduces a modified magnetar propeller model incorporating fallback accretion to explain giant X-ray flares in long GRBs, highlighting energetic constraints.

Findings

Most fits require near 100% propeller efficiency.

High energy in flares challenges magnetar energy budget.

Fallback accretion alone cannot fully account for observed flares.

Abstract

Flares in the X-ray afterglow of gamma-ray bursts (GRBs) share more characteristics with the prompt emission than the afterglow, such as pulse profile and contained fluence. As a result, they are believed to originate from late-time activity of the central engine and can be used to constrain the overall energy budget. In this paper, we collect a sample of $19$ long GRBs observed by \emph{Swift}-XRT that contain giant flares in their X-ray afterglows. We fit this sample with a version of the magnetar propeller model, modified to include fallback accretion. This model has already successfully reproduced extended emission in short GRBs. Our best fits provide a reasonable morphological match to the light curves. However, $16$ out of $19$ of the fits require efficiencies for the propeller mechanism that approach $100\%$. The high efficiency parameters are a direct result of the high energy contained in the flares and the extreme duration of the dipole component, which forces either slow spin periods or low magnetic fields. We find that even with the inclusion of significant fallback accretion, in all but a few cases it is energetically challenging to produce prompt emission, afterglow and giant flares within the constraints of the rotational energy budget of a magnetar.