Signature of a spin-up magnetar from multi-band afterglow rebrightening of GRB 100814A

TL;DR

This paper proposes a magnetar spin-up model to explain complex multi-band afterglow rebrightening in GRB 100814A, successfully reproducing observed features that challenge standard external shock models.

Contribution

The study introduces a novel magnetar spin evolution model, including spin-up due to fall-back accretion, to explain late-time rebrightening in GRB afterglows.

Findings

Magnetar spin-up causes increased magnetic dipole radiation luminosity.

Model reproduces the optical and X-ray afterglow features of GRB 100814A.

Explains late-time rebrightening as energy injection from a spinning-up magnetar.

Abstract

In recent years, more and more gamma-ray bursts with late rebrightenings in multi-band afterglows unveil the late-time activities of the central engines. GRB 100814A is a special one among the well-sampled events, with complex temporal and spectral evolution. The single power-law shallow decay index of the optical light curve observed by GROND between 640 s and 10 ks is $\alpha_{\rm opt} = 0.57 \pm 0.02$, which apparently conflicts with the simple external shock model expectation. Especially, there is a remarkable rebrightening in the optical to near infrared bands at late time, challenging the external shock model with synchrotron emission coming from the interaction of the blast wave with the surrounding interstellar medium. In this paper, we invoke a magnetar with spin evolution to explain the complex multi-band afterglow emission of GRB 100814A. The initial shallow decay phase in optical bands and the plateau in X-ray can be explained as due to energy injection from a spin-down magnetar. At late time, with the falling of materials from the fall-back disk onto the central object of the burster, angular momentum of the accreted materials is transferred to the magnetar, which leads to a spin-up process. As a result, the magnetic dipole radiation luminosity will increase, resulting in the significant rebrightening of the optical afterglow. It is shown that the observed multi-band afterglow emission can be well reproduced by the model.