Magnetic Burial in Millisecond Magnetars and Late GRB Afterglow Signatures

TL;DR

This paper explores how magnetic burial and reemergence in millisecond magnetars influence late GRB afterglow signatures, providing a model that explains the observed X-ray excess in GW170817/GRB 170817A.

Contribution

It introduces a novel model linking magnetic burial and reemergence in magnetars to late-time GRB afterglow features, especially the X-ray excess observed years after the burst.

Findings

Re-emerging magnetic fields can explain late X-ray brightening.

Over 90% of the initial magnetic flux was buried during accretion.

Reemergence timescale is estimated between 3 and 40 years.

Abstract

Millisecond magnetars, one of the potential candidates for the central engine of Gamma-ray bursts (GRBs), can experience significant magnetic field enhancement shortly after their formation. In some cases, this evolution is further influenced by the accretion of stellar debris, which modifies the dipole magnetic field strength. During a hypercritical accretion phase that lasts seconds or longer after the progenitor explosion, a thin crust may form, submerging the magnetic field (the so-called magnetic burial scenario). Once hypercritical accretion ceases, the buried field can diffuse back through the crust, delaying the external dipole's reactivation. On the other hand, observations have shown that relativistic outflows ejected by these objects and decelerated by the circumburst environment cause a late and temporary emission known as afterglow. This work investigates how the submergence and subsequent reemergence of the magnetar magnetic field, on a few years timescales, affect the GRB afterglow dynamics. Specifically, we apply this phenomenological scenario to the late-time X-ray excess observed approximately three years post-burst in GW170817/GRB 170817A, exploring how the evolving magnetic field strength may contribute to this emission. Our modelling of GRB 170817A indicates that $\gtrsim90$ percent of the external dipole flux was initially buried, re-emerging on a timescale $\tau_{B}=3-40$ yr and restoring a surface field $B\simeq(2-5)\times10^{15}\,$G; the late-time X-ray brightening is far better reproduced by this scenario than by models without burial.