Magnetic burial in millisecond magnetars and late GRB afterglow signatures

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 re-activation. 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 re-emergence of the magnetar magnetic field, on a few years time-scales, 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 $\gtrsim 90$ per cent of the external dipole flux was initially buried, re-emerging on a time-scale $\tau _{B}=3-40$ yr and restoring a surface field $B\simeq (2-5)\times 10^{15}\,$ G; the late-time X-ray brightening is far better reproduced by this scenario than by models without burial.